Above: the life-cycle of the rove beetle Aleochara bilineata (based in part on Wadsworth's 1915 classic account of this insect).
The adult of this insect is about 5-6 mm long (occasionally much smaller individuals form when several occupy the same host
or when the host is unusually small). Insects have an amazing variety of life-cycles in which the various stages are adapted for
different purposes, whether for exploiting a different food source (such as different host plants in aphids) or even different
habitats, such as dragonflies in which the nymphs are aquatic. Prepare to be amazed!
Insects exhibit three different principal types of life-cycle. Most insects lay eggs (though a few give birth to live young) which
are deposited in soil or attached to vegetation or some appropriate food source. In some insects, the egg hatches into a
nymph that resembles the adult, differing perhaps slightly in form, but much smaller; these are the hemimetabolous insects.
Holometabolous insects are those whose eggs hatch into a larva that differs greatly from the adult (imago) in form. A third
group include those in which the young stages do not differ appreciably from the adult, except in size, development of the
reproductive organs and minor changes in form. These are the ametabolous insects.
The tough exoskeleton (cuticle) of insects restricts their growth. To overcome this restriction, insects shed their skin every so
often, or moult (molt). The number of moults is precise and adults do not moult. When an insect moults, its new skin is soft
and white at first, this is an insect's most vulnerable period. The insect can then expand itself like an inflatable toy by taking in
air or water. The new cuticle subsequently darkens and hardens, and the newly formed spaces, that resulted from inflation,
are filled with blood and are slowly replaced by tissues, until the next moult. We talk of instars, where the first form, which
hatches from the egg, is the first instar larva or nymph. This moults into the second instar larva or nymph, and so on, until
the adult stage is reached.
In ametabous insects, the first instar changes little in form as it molts through the various instars to the adult stage.
Obviously size increases, and the reproductive organs are also immature in the early stages and there may be slight changes
in form. Otherwise the young look like miniature adults. This form of development occurs in the more archaic insects - the
wingless insects or Apterygota (apterygotes, pteron = wing) such as silverfish and many of the strange and tiny soil dwelling
bristletails. These insects are archaic or 'primitive' meaning they more closely resemble early fossil forms ('primitive' in this
context does not necessarily imply a lack of complexity or sophistication).
In hemimetabolous insects, the nymph undergoes a gradual metamorphosis (incomplete or direct metamorphosis) into
the adult form, appearing more like the adult with each moult, but these changes are slight, since the first instar nymph
resembled the adult anyway, and the most obvious change is an increase in size. The silverfish that you find in your bathroom
or under your fridge is a hemimetabolous insect - you may have spotted tiny ones only 1-2 mm in length which are actually
young nymphs. Later nymphs that look very much like the imago are often called juveniles. In these insects, the larval tissues
gradually transform into the adult tissues.
In holometabolous insects, there are several instars of larvae (an exact number that varies with species) before the final
instar larva moults into the pupa stage. Each larva characteristically looks very different indeed from the adult, and they may
even look very different from one another. The pupa is characteristically immotile and inactive (though some are capable of
vigorous movements) and the classical example is that of the Lepidoptera (butterflies and moths) which is encased in a
chrysalis of secreted chitin or a cocoon of silk that the last instar larva spun around itself and is often found fastened to the
underside of a leaf upon which the larva fed. Flies also have characteristic pupae. If you leave a piece of meat outside for a
while, allowing carrion flies like blowflies to lay their eggs on it, and then place it in a box with some moist cotton wool to stop it
drying out, then you will observe the larvae (maggots, as fly larvae are called) hatch, feed and grow, turning the meat into a
liquid soup which the larvae ingest and finally turning into pupae when their food is exhausted. The pupae of these flies are
reddish cylindrical objects that are very rigid and tough an quite inert. Despite their apparent inertness, however, pupae are
metabolically very active, using up food reserves stored by the feeding larva. They use this energy to massively re-arrange
their tissues, which may liquefy at some stage, before reforming as adult tissues. This pupation process is a complete
metamorphosis (indirect metamorphosis) and when ready the adult insect will emerge from the pupa, typically by gnawing a
hole through it, pulling itself out and allowing its wings to expand with blood and its cuticle to darken and harden. Butterflies
are the best known examples of this, changing from leaf-eating caterpillars into adult butterflies, but flies (Diptera) do this too,
as do beetles (Coleoptera) and Hymenoptera.
Having larvae that differ radically from the adult enables insects to exploit a variety of resources. Typically the larvae and the
adults (if they feed at all) consume different foods, thereby avoiding direct competition with one another and exploiting more
resources. A classic example is the dragonfly, whose larva lives in freshwater and is a ferocious predator that swims by jet
propulsion (expelling water from its anus) and catching tadpoles, small fish and other insects with its protrusible jaws
(reminiscent of the 'Alien' of sci-fi). The adults live mostly in the air, where they catch other insects to eat, mate on the wing
and even lay eggs whilst hovering, and only occasionally landing to rest. Talking of aliens brings me on to another type of
insect, of the holometabolous type - parasitoids.
Parasitoids - real-life 'Aliens'!
The rove beetle Aleochara has a particularly strange life cycle. The adults are ferocious predators, eating fly maggots and the
like, whilst the larvae are parasitic (properly parasitoidic) on fly pupae. Each species of Aleochara is quite specific to the kind
of pupae its larvae will infect. Aleochara curtula infects the pupae of the carrion fly Calliphora and the adults will form societies
living on a single animal carcass. The dominant male will keep a harem of females on his carcass and ward off male rivals.
Some males, however, cheat by mimicing females (they smell the same as females and tend to be smaller than other males)
and mating with females in the harem when the dominant male isn't looking! The adults will eat maggots of the carrion flies that
feed on the carcass. Aleochara bipustulata and Aleochara bilineata (whose life-cycle is shown above) feed on certain
root-flies of the genus Delia, such as the onion-root fly, Delia antiqua, and the cabbage-root fly, Delia radicum. These flies lay
their eggs in the soil around their choice of plants (onions for Delia antiqua, cabbage, suede (rutabaga or yellow turnip) and
turnips for Delia radicum). The adult Aleochara also lay their eggs in the soil around these plants, especially plants that are
infected by the root-flies. These insects do not appear to be as social as Aleochara curtula, though they do tend to form
communal burrows around the infected plants and they eat root-fly maggots and other suitable items (but can be kept in the
laboratory by feeding them cat biscuits!).
Each adult female Aleochara bilineata lays about 10 eggs each day, and lives for 2-3 months, and so lays a total of about
500-600 eggs in her lifetime. The eggs hatch into first instar larvae. These larvae are reasonably well-developed with quite
powerful legs, strong jaws and well developed antennae and sensory bristles. They also have two rudimentary eyes, which
enable them to sense the direction light is coming from - they avoid light, burrowing down into the soil. They locate a root-fly
pupa in the soil and gnaw a hole in the pupal case, enter, seal the hole and then slowly devour the fly pupa developing within!
Above: the first instar larva of Aleochara bilineata. Notice the small eyes, the sensory hairs that are especially
well-developed on the head and tail. Notice also the antennae, and the row of protuberances near the bottom margin of the
abdomen, each of these is surmounted by a spiracle pore for respiration. The legs are quite well developed and the
cuticle, especially in the head, is brown and quite rigid, which indicates that strong muscles attach to it. This individual has
been feeding, which is why its abdomen is quite full and bloated. This larva is free-living and the strong cuticle gives it then
it will die.
The larvae will try to avoid infecting a pupa that already contains another Aleochara larva (a process called
superparasitisation), unless pupae are in very short supply, in which case two or more larvae may enter the same pupa
and compete for resources, though usually only one will survive, it will be much reduced in size due its loss of food to its
rivals. If they are forced to parasitise an occupied pupa, then they prefer to select one that does not contain one of their
Whilst all this is going on, the fly pupa looks normal to a casual observer. The first instar larva will feed and then moult into
the second instar larva, which grows and moults into the third instar larva, which is the final larval stage. The second and
third instar larvae look identical, except that the third instar larva is much bigger. They have soft and white cuticles and
small legs - they don't need to move much and they don't need much protection against dehydration or predation, since
they are protected inside the tough pupal case of the fly. Their sensory bristles become much reduced. They lack the long
sensory hairs on the tail, which alert the first instar larva if anything tries to creep up on it. They lack the pair of eyes
present in the third-instar larva and their jaws are weaker, since they do not need to gnaw through the tough pupal case,
but simply feed on the soft liquefying tissues of the fly pupa. Their antenna are much reduced. They have what appears to
be an ocellus, a type of eye, in the middle, on top of the head. Ocelli form poor images and serve only to monitor light
levels. They help insects determine day length and the time of year. The Aleochara larvae will overwinter inside the host fly
pupa if necessary, before completing the life-cycle.
Above and below: the second instar larva of Aleochara. The cuticle is soft and white and the powers of locomotion and
sensors are much reduced. The body is bloated with food. Some spiracles are visible (the others were hidden by 'crud'
since these insects are messy eaters, and practically swim in the liquid tissues of their host). This reduction in systems is
common in parasitic creatures and is called degeneration.
The third instar larva transforms into the pupa. The pupa is soft and white and ensheathed in a soft, transparent
membrane. It has no need to form a tough cocoon, chrysalis or pupal case, since it is still enclosed in the host fly
puparium (pupal case). When ready it will emerge by gnawing a hole in the pupal case and escaping as an adult
beetle - so, instead of the usual fly emerging from the pupa, an alien beetle will emerge instead!
Although the adults are predators and continue to feed on maggots, Aleochara needs to consume only one prey
insect to complete its life-cycle, and it will consume one insect and consume it entirely. The larva is therefore
described as a parasitoid. A parasitoid is a special kind of parasite. A parasite lives on or in a host and feed upon it,
but often without killing its host, indeed a parasite benefits by NOT killing its host. Predators benefit by completely
destroying and consuming multiple prey items. A parasitoid is half-way between a predator and a parasite - it needs
to kill and consume a single host to complete its development.
Parasitoids have featured famously in science fiction. One of the first to do so (if not the first) was the Wirrrn - a larger
than man-sized alien insectoid creature that infected humans that slept in suspended animation on board the Nova
Beacon space ark in Dr Who. Upon waking they found one of their crew missing and a strange presence growing on
board the space station! The alien from the film of the same name was also a parasitoid. These parasitoids fed on
humans, on Earth they don't do this, but to a fly, the horror is real!
One question remains - how does Aleochara locate its host?
The first part of this question concerns how the adults locate onion and cabbage plants, but we shall look at that in a
section on insect behaviour. Here we shall look at how the first instar larva locates a fly pupa. Studies in the literature
suggest that the larva executes a random search and recognises a suitable host by touch when it bumps into it by
chance. However, below we present an alternative model. First, we shall look what sensors the first instar larva has on
its antennae. a diagram of the antenna is shown below:
and 3) and two segments that may be true segments or simply outgrowths and so are labelled as pseudosegments (PS1
and PS2). Segment 3 forms what is called the outer lobe (OL) and a cone forms the inner lobe (IL). Segment 2 bears two
hairs that are evidently touch mechanoreceptors with the characteristic structure of trichoid sensilla with a dendrite
attached to flexible sockets in the base and poreless hairs with no dendrites in their lumens. The outer lobe bears three
large hairs (about 60 micrometres long) that are very long in proportion to the antenna. These hairs have been assumed
to have only a tactile role, but they have the structure of dual mechano-gustatory receptors, with a dendrite in the flexible
socket and one or two sheathed dendrites running the length of the hair inside the hair lumen. It is not known whether
these hairs end in a terminal pore, but there are no pores along the length of the hairs and the tips can appear bulbous in
scanning electron microscopy, perhaps due to exudate from a terminal pore or due to some associated structure. These
hairs may be used to probe potential host pupae and may pick up sensory cues indicating whether or not the pupa is
already infected, and if so, whether or not it is infected by close kin. The outer lobe also bears three small trichoid sensilla
(S) which also contain dendrites and so appear to be dual mechano-gustatory receptors, and a blunt peg (P).
The peg (P) and the inner lobe are grooved. There is no detailed information regarding the structure of the small peg, but
the inner lobe has the structure characteristic of a large and complex olfactory sensor, strikingly similar to those on the
antennae of larva of the beetle Ctenicora destructor and the housefly Musca domestica. These organs are grooved, with
pores running along the grooves. The pores lead into a large lymph filled cavity with an outer cortex of dendritic branches
emanating from multiple primary dendrites. The inner lobe has a volume of about 1000 cubic micrometres and is much
larger (by about 350-fold in volume) and more complex than any olfactory organ found on the adult antenna, although the
adult's antenna has many more (about 1500) smaller olfactory sensilla. Thus, it appears that the larva has a
well-developed sense of smell, but what does it use it for? One clue comes from examining the antenna of the second
instar larva, shown below:
Above: the antenna of the 2nd-instar larva of Aleochara bilineata. The scale bar is 20 micrometres. Notice that the three
segments remain, but that the sensors are all much reduced. The trichoid sensilla are shorter, with the three longest
reduced to about 11 micrometres in length (a 5.5-fold reduction in length) and the three shorter sensilla reduced 2.5-fold
to 4 micrometres in length. The olfactory inner lobe is also much reduced to about 40% of the volume of that in the first
instar. This reduction indicates that the functional requirements of these sensors are reduced once the larva is inside its
Thus we can conclude that chemoreceptors, including olfactory sensors, as well as touch sensors are very important to
the first instar larva. Olfactory sensors may be used to avoid predators, or to ensure that the insect does not stray too far
from the host plant, or they may be used to sniff-out host pupae - we don't know! They are not used to locate food other
than a host pupa, since the larvae do not feed until they enter their host. There presence, however, forces us to rethink
the accepted wisdom that the larvae locate hosts purely by random searching and verify their suitability by touch alone.
Why then do the larvae appear to make random search movements? This may be an artefact of the experiment. It is not
realistic to place pupae in fresh sand or soil and then add larvae and expect them to locate the pupae in the same way
that they would do so in nature, simply because as the root-fly maggots get ready to pupate, they leave the vicinity of the
roots and burrow down deeper into the soil away from the plant. It would, therefore, be expected that the larvae would
leave a trail in the soil and maybe the Aleochara larvae can sniff-out the odours in this trail? Removing the pupae and
transplanting them will abolish such trail cues and then the larvae may have to resort to random searching. More research
is needed to resolve these issues.
Above: a longitudinal section through the inner lobe of the 1st-instar larva of Aleochara bilineata. (Unpublished data,
courtesy of C. Skilbeck, J. C. K. Brown and M. Anderson, 1996). Note the pores (P) in the bottom of the grooves and the
cuticular struts (C) visible as this section is just beneath the surface. Also note the extracellular flat membranous
vesicles, historically called dictyosomes, but since this descriptive term is now specifically to certain Golgi bodies, it is
better to call these extracellular membranous discs. The scale bar is 500 nanometres.
Above: longitudinal sections through the inner lobe, which are more medial (deeper into the structure). Top: a
dendritic branch with a swelling (S) can be seen just beneath the surface of the cuticle (such beaded dendrites are
characteristic of these types of sense organs in insect larvae). Note the cuticular struts (C), with pores in-between,
and bundles of darkly staining lipoidal vesicles (V). Scale bar = 500 nm. Bottom: a large dendrite (D) gives rise to
nanometres. (Unpublished data, courtesy of C. Skilbeck, J. C. K. Brown and M. Anderson, 1996).
Aleochara bilineata is properly referred to as a predator-parasitoid. More classical parasitoids include many wasps,
and some flies, that do not feed on animal tissue as adults (they are said to be protelean). Wasps are holometabolous
insects of the order Hymenoptera. Although the wasps people are more familiar with are not parasitoids, forming
colonies or nests defended by the stinging female drones, many wasps are parasitoids. These are seen by many, but
known by few. Ever see a small all-black and often tiny wasp-like creature (with the wasp-like waist) with a long 'sting'
protruding from its body? Well, these are female parasitoid wasps that lead a solitary existence and whose 'sting' is
not a sting at all but the ovipositor (egg-laying tube). Typically, the ovipositor is equipped with sensors (sensilla) that
appear to have a gustatory (taste or contact chemorecptive) function, in addition to mechano-sensors and
heat-sensors. Probably the ovipositor can taste when it contacts a suitable host into which to deposit the eggs.
The female typically lays her eggs in the larvae (or pupae or rarely adult) of certain specific host (holometabolous)
insect. For example, the parasitoid wasp Trybliographa rapae is also a parasiotoid of cabbage root fly (Delia radicum)
and a few other species of the Delia genus. These wasps use the sense of smell (olfaction) on their antennae to
locate the general vicinity of their host. For example, they may respond to the odours of the plant (e.g. cabbages)
upon which their host feeds, and in particular the odours released by a plant that is being attacked. Plants will often
release stress chemicals (alarm pheromones) to alert predators and parasitoids to the presence of attacking insects.
Having found a suitably infected plant, whereas Aleochara bilineata will lay eggs in the surrounding soil and let the
first instar larva locate the host pupa, the wasps will locate an individual host larva and lay their eggs directly on or
inside it. (This is where taste sensors on the ovipositor, antennae and feet of the wasp will come in handy). The eggs
hatch and the wasp larvae feed upon the tissues of the host larva (a maggot if the host is a fly, but a caterpillar for a
lepidopteran host) and typically arrest its development, so that the host never pupates, and then the wasps emerge to
pupate, attached to the outer surface of the dead host (inside cocoons). Superparasitoidism, in which many larvae
develop within a single host, is typical in these wasps. Sometimes a single egg will develop into several individuals
(the embryo divides into many embryos) - a form of asexual reproduction called polyembryony.
Superinfection - superparasitism resulting from two or more different wasps laying eggs in the same host is largely
avoided. The first female to lay her eggs must mark the host in some way, perhaps with a pheromone, that alerts
other interested wasps that the host is already taken. Parasitoids tend to avoid already infected hosts so as to avoid
competition for food for their larvae, unless they are desparate and can find no other host.
The Host - Parasitoid Arms Race
Spines. Some insect larvae protect themselves by developing large, often branched, spines all over their body.
These spines make it difficult for a parasitoid wasp to get close enough to lay eggs, though of course some wasps
have developed especially long ovipositors to overcome this obstacle!
Encapsulation. Some insects also react to eggs deposited inside their body cavities (the 'blood-filled' or strictly
haemolymph-filled haemocoel) by recognising it and encapsulating it. Haemolymph cells surround the egg and form a
tough capsule around it and kill the egg (by altering its chemical environment).
Controlling Sex Ratios - How female wasps can choice the gender of their offspring!
In humans, as you may know, sex is controlled genetically by a pair of sex-chromosomes: males carry an X and a Y
(XY), females two X (XX) chromosomes. The unfertilised egg is thus always X and each sperm can carry either an X or
a Y. Which type of sperm reaches the egg first and fertilises it determines the gender of the baby (usually, sometimes
things interfere with this process).
Hymenoptera (bees and wasps) like our parasitoid wasps, can control their sex ratios. The female determines the
gender of her own offspring during egg-laying. Males are produced from unfertilised haploid eggs (carrying just one
set of (maternal) chromosomes) while females develop from fertilised diploid eggs (carrying two sets of chromosomes,
one paternal and one maternal, as in humans). The female stores sperm from any males that mate with her in a
spermatheca sac. She can decide whether or not to mix sperm with the egg as it passes down the oviduct. if she
does so, then the egg will be fertilised and develop into a female. Of course, she does not choice in the way a human
might, but reacts instinctively according to various stimuli.
|Did you know?
The male rove beetle Aleochara tristis has an intromittent organ (or 'penis', called the flagellum) that is twice his body
length. When retracted into the male this organ is coiled into nine loops. During mating it is inserted into the long
coiled spermathecal duct of the female, where it guides delivery of the spermatophore (a package of sperm).
Retracting such a long organ posses problems as it is under tension. Simply pulling it out will cause it to coil up into a
tangle and become knotted and of no further use. To prevent this the male partly withdraws the organ, then turns
around, with his back toward the female, and positions it over his shoulder (he shoulders it) and then pulls it out in an
orderly fashion, allowing it to retract back inside the male and recoil without tangling! This is an example of
exaggerated genitalia and a behavioural adaptation to deal with this peculiar characteristic.
|Did you know?
The tiny fruit fly, Drosophila, which is only some 2-4 mm in length, has the longest sperm cells of any known organism
on Earth. Each spermatozoan is up to 58 mm long - that is 10 to 20 times as long as the adult male himself!
The final instar larva molts into the pupa, which may be enclosed in a cocoon of silk, spun by the larva prior to
pupation, or may develop its own hardened cuticle. In many flies the last-instar larva cuticle contracts into a
barrel-shape, tans and hardens to form a case called the puparium. The actual pupa is inside this puparium. The
larva becomes quiescent immediately prior to pupation - the so-called prepupa stage. The pupa is typically immotile,
though in some forms it is capable of certain movements in response to stimuli. However, the pupa is metabolically
very active and not dormant (unless it enters a quiescent state to overwinter). Tissues and cells are rearranged to
form the adult organs. Much of the interior takes on a liquid consistency. Some adult organs develop from larval ones
by modification, for example the Malpighian tubules (excretory organs) of Lepidoptera (moths and butterflies). Others
develop from discs of cells that grow in the larva, imaginal discs, which are often invaginations of the epidermis. In the
pupa the cells in these discs differentiate and form the adult organs. In flies like Drosophila (fruit fly) there are three
pairs of leg discs, one pair of wing discs, one pair of eye discs, one pair of antennal discs and an unpaired genital
disc. The imaginal discs of the Caliphora blowfly can be observed in a dissection of the maggot, as shown in the
Growing is difficult when one is confined by a suite of armour, which is essentially what the cuticle of an insect
resembles (the chitin that makes up the cuticle has a hardness similar to that of a copper alloy). Insects get around
this by periodically molting, usually a set number of times, briefly exposing the soft white body beneath and then the
insect takes in air (or water) to swell its soft body, whilst its new cuticle hardens. Before the next molt it will then
replace this air with fluid and cells as it grows. Molting can be divided into seven stages:
1. Apolysis. The epidermis (hypodermis), which lies immediately beneath the cuticle (and which
secreted it) detaches from the old cuticle, a process called apolysis. The resulting 'space' between the
epidermis and the cuticle becomes filled with molting gel, beneath which it begins to secrete a new
cuticle, which is soft at first. The insect cuticle consists of several basic layers - the outer epicuticle,
underlying exocuticle and innermost endocuticle.
2. New epicuticle secretion. The outermost layer of the new cuticle, the thin epicuticle, is secreted
by the epidermal cells. This consists of an outer layer of cuticulin, deposited first, and an inner layer
of protein, deposited second.
3. Procuticle deposition. The new immature cuticle is secreted as chitin microfibrils (chitin is a
polysaccharide carbohydrate). Enzymes in the molting gel digests the inner layers (endocuticle) of the
old cuticle, recycling some components.
4. Ecdysis or shedding of the old cuticle occurs. The old cuticle (weakened by the action of the
molting gel enzymes, splits along a middorsal suture (a line of weakness running along the back). The
cast 'skin' consists of the old epicuticle and underlying exocuticle 9the endocuticle having been
recycled in stage 3).
5. Expansion of the insect body, within its soft and white new cuticle as the insect swallows air. The
new cuticle formed so far contains wrinkles to accommodate the stretching and the proteins within it
slide past one another, allowing the new 'skin' to stretch without elastic recoil. At this stage the insect is
terribly vulnerable and molting usually takes place in some dark concealed place.
6. Hardening and darkening (tanning) of the new cuticle. Cross-links form between the protein fibrils
in the cuticle, giving it strength. The cuticle also darkens. In this way the new exocuticle is formed.
7. Endocuticle secretion. For several days after ecdysis, more chitin and protein are secreted,
adding new layers to the inside of the cuticle, forming the innermost endocuticle.
Finding a mate - the key to identification
Insects are very abundant, with an estimated 10 billion insects per square kilometre, and occur in a phenomenal
variety of species, many of which are closely related and superficially similar. Finding a mate of the right species is
crucial but very challenging! Naturally insects have evolved ways of solving this problem. In the insect world males
may be attracted to females or females to males. The male may be attracted to light produced by the female
abdomen (Lampyrid beetles). Females may be attracted to swarms of males dancing in flight (e.g. danceflies or
assassin flies), or to males calling by stridulation (crickets and grasshoppers). In some crickets, carefully crafted
burrows can amplify this sound to a deafening amplitude. The sexes may communicate by passing vibrations along
vegetation or by a kind of semaphore (sign language). Male mosquitoes are attracted to the sound of flying females.
Colouration and patterning may differ between the sexes, including sometimes ultraviolet colours invisible to human
eyes. Male houseflies (Musca dometsica) have larger eyes than the females and locate females by sight; indeed they
will investigate black dots of the right general size. Some insects have designated meeting places where individuals
gather and scramble for mates. The soldier beetle, for example, so-named because of its straight long and bright red
body, will gather on the flower-heads of umbellifers (such as hog-weed). The red beetles really stand out on these tall
and broad bright white flower-clusters.
However, the most important means of recognition is probably smell. Males and/or females may emit species-specific
pheromones. The female silkworm moth, Bombyx mori, produces tiny amounts of the pheromone bombykol. This
was the first pheromone to be isolated and the antennae of the male are extremely sensitive to it! The female
spreads out a pheromone plume as she flies and a male crossing the plume downwind can pick-up the scent several
miles away! The male will then fly upstream in a zig-zag pattern, until he finds the female. (See antennae and
sensilla). In some insects a male will even attempt to mate with a piece of blotting paper soaked in the correct
Once receptive partners meet courtship ensues. When it comes to physical contact during mating, taste is an
important final check, the mouthparts and antennae being used for this purpose. Despite all these precautions,
mistakes do occasionally happen. An attempt to mate may occur between closely related species with similar
pheronomes, and sometimes between members of the same sex. Insects, however, have one final check - males
have very diverse intromittent organs, depending on species, designed to fit neatly into the equally diverse female
parts. This is the lock-and-key model of insect genitalia - if the species is right then the male organ will fit the
female's receptive organs, like a key in a lock. This may be why some insects (like Aleochara tristis, see the first blue
box at the top of the page <anchor this link> ) have such massively exaggerated genitalia!
Molting has to be coordinated and controlled. All the epidermal cells must secrete new cuticle simultaneously and the
larva has to molt into the correct subsequent life-stage (another larval instar, pupa or adult). This is achieved by the
properly called haemolymph).
|Did you know?
The toughness of insects made it easier for experimenters to work out the hormones controlling molting. Cut the
head off a cockroach and its body will continue to function for days, grooming itself and responding to touch. Parts
of pupae may continue to live for months! Cutting a caterpillar in two enables an experimenter to determine which
half pupates! Removing a caterpillar's brain enables one to determine the role of the brain in pupation!
sufficiently on blood, with ecdysis usually occurring 10 days after a blood meal. If Rhodnius is decapitated
immediately after feeding, the headless body can live for months but does not molt. If, however, the head is
removed 5 days after feeding, then the insect molts after 10 days as normal. This suggests that the brain is
secreting some hormone. Hormones are chemical signals that are slowly released and slow to act. Only if enough
hormone is secreted will molting occur. Once this has occurred the brain is no longer required for molting.
The hormone secreted by the brain is called prothoracicotropic hormone (PTTH). This is secreted by a
neurohaemal system (in many ways similar to the hypothalamus-pituitary axis in mammals) that is, the chemical is
produced inside the cell bodies of specific neurones (nerve cells) in the brain, transported along the cell axons and
then secreted into the haemolymph. This is similar to how neurones work in synapses - vesicles carrying a
neurotransmitter chemical, synthesised in the cell body, travel down the axon to the synaptic knob, to be released
when required into the synaptic space between the neuron sending the signal and a recipient neurone. The
neurotransmitter signal diffuses across this space (synaptic cleft) and binds to receptor molecules on the recipient
neurone, activating it. This is one way by which a signal is passed from one nerve cell to another. In a neurohaemal
system, there is no target neurone, instead the vesicles discharge their cargo, a hormone rather than a
neurotransmitter, into the blood (or haemolymph in an insect).
Insects typically contain a few tens of neuorhaemal neurons in the brain. One pair of these send their axons to a
special pair of endocrine glands, called the corpora cardiaca (singular: corpus cardiacum). These glands are
situated beneath the brain. The axons cross-over, so that the neuron whose cell body is in the left side of the brain
innervates the corpus cardiacum on the right side of the head. PTTH manufactured in the cell bodies is transported
inside vesicles down these axons, to the neuron endings in the corpora cardiaca. When suitably stimulated, e.g. by
feeding to satiation, the vesicles in these endings fuse with the neuron cell membrane, releasing the PTTH into the
haemolymph which bathes the brain. This PTTH enters the circulating haemolymph and when it reaches the target
organ, the pair of prothoracic glands (or ecdysial glands, situated at the back of the head or at the front of the
thorax) the PTTH molecules bind to receptor molecules in the prothoracic gland cells, triggering a signalling
cascade within these cells and activating them to secrete molting hormone (MH or ecdysone, though there is a
family of ecdysones, with 'ecdysone' being one major molting hormone) into the haemolymph. This hormone then
binds receptors in the target cells, which include epidermal cells, triggering ecdysis.
Above: a section through the insect brain, showing the related neurohaemal systems that regulate
molting and development. The insect brain consists of the supraoesophageal ganglion (a group of
structures) situated dorsal to the oesophagus of the gut, and the suboesophageal ganglion, situated
beneath the gut. A pair of nerve trunks, the circumoesophageal connectives (commisures) conects
'brain'). The axons of the neurosecretory cells are packed with granule-like vesicles that carry the
hormone to the gland. For example, the axons and cell bodies of the neurosecretory cells connecting to
the corpora cardiaca are a shiny blue, as the granules scatter light according to wavelength (Tyndall
blue produced by Tyndall scattering) in a similar way to the scattering of sunlight that makes the sky
Another pair of neurohaemal glands, the corpora allata (singular: corpus allatum) release juvenile
hormone (JH) into the hameolymph. This hormone suppresses sexual maturation, maintaining juvenile
features with each successive molt, that is it inhibits metamorphosis.
Ecdysone is a steroid hormone derived from cholesterol, and is not unlike the mammalian sex hormones
(testorone and oestrogens) and being lipid-soluble it crosses the target cell membrane by passive
diffusion and then binds to its receptor inside the cytoplasm. The binding of ecdysone to its receptor
triggers a cascade of events which leads to gene transcription and the synthesis of new proteins and
enzymes. Ecdysone functions more generally as a sex hormone and, for example, triggers sperm
production in the testes, whilst JH inhibits sperm production.
Molting is accompanied by quite complex, but very specific, behavioural patterns. It requires a lot of
energy, especially when the adult emerges from the pupa. This may involve rhythmic rotations of the
abdomen, designed to loosen attachment to the old cuticle, peristaltic muscular contractions to help the
insect wriggle free and the wings maybe shrugged to pull them free. Molting is quite a strenuous activity
the cuticle extends partway into the gut at both ends (into the foregut and hindgut) and into the spiracles
and along the tracheae. All these old cuticular structures must be pulled out. These behavioural
changes are triggered by eclosion hormone. (Eclosion is the act of emerging from the old cuticle).
Eclosion hormone (EH) is secreted by the brain in response to light or perhaps the time of day (molting
is safest when carried out in the dark or at night).
A number of other insect hormones, which regulate development, have been discovered. For example,
diapause hormone (DH) can trigger overwintering dormancy in pupae and eggs. Vertebrate biologists,
unacquainted with detailed invertebrate zoology, might be forgiven for thinking that insects are simple.
They certainly have fewer cells than large vertebrates (and probably fewer cell types) and their nervous
systems are not as complex as those of mammals, but they are far from simple. Do not be deceived by
their small size. In small insects, some organs consist of one to a few cells, but nevertheless the cells of
the insect body require complex regulation. Not only do insects have complex nervous systems, but they
appear to have complex hormonal systems as well. Their apparent simplicity is exaggerated by the lack
of detailed studies on insects, compared with vertebrates. Insect biology is also greatly complicated by
the enormous diversity of insects.
Returning to the seven stages of molting, the following pattern can be generalised (details may differ
depending on species): stages 1-3 are triggered by ecdysone; stage 4 (ecdysis or eclosion) is triggered
by eclosion hormone. the remaining stages (5-7) of cuticle expanison, hardening and darkening and
endocuticle deposition are controlled by another horome, bursicon.
Insects are important subjects of study in its own right. Humans share the earth with some of the most
sophisticated and marvellous machines imaginable, and it would be silly for humanity to ignore such
engineering marvels. All life-forms are fascinating subjects of study and should be studied because they
exist and because humans ought to appreciate the real world around them. Furthermore, robotocists
are increasingly finding ways to mimic invertebrate biology. Robots have been developed based on
insects and jellyfish, for example, making use of their clever nervous systems and modes of locomotion.
We avoid politics on Cronodon, but I will say the following. Unfortunately, those who actually do and did
the basic biology research, so useful to modern engineering, are rarely acknowledged. Invertebrate
biology is increasingly sidelined, whilst engineers (or their bosses) have their praises sung, but in the
end, the greatest debt is owed to Mother Nature herself. She is the greatest of all engineers.
|Did you know?
Some plants defend themselves against grazing insects by producing chemicals that mimic insect hormones. The
resulting hormonal imbalance that results in an insect that eats too much foliage can be fatal! Of course, some
insects have developed immunity to these hormone mimics - another arms race!
Courtship, Beating Rivals and Mating
In many insects the male offers the female food as part of courtship. This food may consist of secretions from the
male, hardened saliva, or a dead insect which may be stolen from a spider's web or caught and killed. The female
will eat her food during mating and may leave prematurely if the food runs out. On the other hand, the male may
take back the food after mating and reuse it to give to other females. Some insects may wrap their parcel in a silk
cocoon, making it easy to deceive - sometimes instead of food the parcel contains only a seed or other inedible
matter, or may even be empty! After all, the name of the game is survival of the fittest and Nature has no respect for
Scorpionflies may steal an insect from a spider's web and then they will sit on a perch with it, emitting pheromone to
attract a female. If no insect can be found, then a pillar of dried saliva will be offered as a replacement. The female
will assess the size of the gift before accepting or rejecting the male. In addition to being a measure of the male's
fitness (and hence the likelihood of him possessing fit genes to pass on to the female's offspring) it also will help
offset the cost of producing and laying eggs.
In a number of insect species, the males will fight-off rivals. These males typically have exaggerated appendages or
protuberances on their heads for butting their rivals in a trial of strength. Size will also be important, the stalked eyes
of certain flies may be used to gauge size with smaller males tending to yield to larger ones (or those perceived to
be larger, which is one reason why these exaggerated parts may evolve - to make the male look bigger). Rhinoceros
beetles and stag beetles, whose larvae eat deadwood, will joust over a fallen log. Each male will use his large jaws
(stag beetles) or horns (rhinoceros beetles) to try and throw the opponent off the log. The victor hopes that a female
will lay eggs in his log. Male dragonflies will take up a prominent perch near water and try to drive off other males in
The rove beetle Aleochara curtula is a predator-parasitoid of certain carrion flies (Calliphora) and males will fight for
dominance of an animal corpse, the dominant male maintaining a harem of females on the same corpse. Some
males are produced which are rather 'effeminate', they are too small to win by trial of strength, and they produce
female pheromones. The dominant male is deceived by thinking that these small males are females and will accept
them into his harem and will mate with them. However, the effimate males will mate with his harem when his back is
The females of some insect species mate only once. Sperm in female insects is stored in a special chamber, the
spermatheca (or receptaculum seminis, seminal receptacle) although the sperm may be initially deposited elsewhere
in the female tract, and fertilise the eggs as they travel through the oviduct when being deposited. One mating may
provide enough sperm for all the female's eggs, even if these are laid over several months. Others do mate more
than once, which poses a problem - how does the male know which male's sperm will be used? The trend is first in,
last out. The last male to mate is more likely to win the race. Some even have special structures on the tips of their
intromittent organs, such as a spoon-shaped structure used to scoop out any sperm already present in the female,
replacing it with his own (e.g. damselflies). Others have inflatable swellings on the tip, used to compress down any
pre-existing sperm, ensuring they are pushed to the back of the queue. Some males will secrete a gelatinous plug to
seal-off the female reproductive openings until egg-laying (a kind of 'chastity belt'). Others will deposit a scent on the
female which makes her unattractive to other males. A Xylocoris male will rape other males, filling their reproductive
tracts with his own sperm, so that when these males mate with a female, it is the rapist's sperm which gets deposited!
The transfer of the sperm to the female differs greatly. In some the intromittent organs simply deposits sperm, in
seminal fluid, inside the female. In others the sperm are packaged inside parcels called spermatophores. A
spermatophore may simply be a ball of sperm that has dried on the outside, or it may be enclosed in a silk cocoon or
a cuticular or proteinaceous structure. Inside the female, the packaging either dissolves or is digested by enzymes
secreted by the female. In some species the spermatophore is deposited outside, the female then picks it up and
places it inside herself, in others the intromittent organ of the male deposits it. Earwigs have a pair of intromittent
organs. Typically, and insect intromittent organ is a chitinous chute down which the sperm or spermatophores travel.
Dragonfly males have two sets of genitals. The primary genital opening, in the tip of the abdomen, places the sperm
into the secondary genitalia, which are situated at the base of the abdomen, underneath the second segment. This
frees the claspers in the tip of the abdomen, which will then grasp the head of the female. The female will then bend
her abdomen, using the tip to collect the sperm from the secondary genitalia, the mating couple forming what is
called a 'wheel' at this stage. It is the secondary genitalia that function as the intromittent organs and which contain
the spoon-shaped structure to scoop out the sperm of rivals.
Machilis is an archaic and wingless insect, an evolutionary relic from a lineage that never developed wings or flight
and although it has six walking legs, like all insects, its palps are very leg-like and a pair of rudimentary legs exist on
each antennal segment. Courtship in this insect is interesting: the male spins out fine threads of silk to which he
attaches several spermatophores. during a 'mating dance' he will guide the female into position so that she walks
along the silken thread and as her genital opening passes over each spermatophore it is engulfed and taken inside.
Some males will remain with the female after mating, guarding her to ensure no other males gain access to her and
displace his sperm. (Note that this is not always in the female's best interest.) Dragonfly males will accompany the
female while she is laying eggs, depositing them by dipping her ovipositor (at the tip of her tail) in a pond whilst in
flight, ensuring the job gets done. (Similarly in damselflies, the male will accompany the female as she lays eggs in
The cost of producing and laying eggs is high for the female, but the male makes more of a contribution than is
apparent. We have seen that males may supply food to the female during mating, and in addition a large part of the
spermatheca may be digested and absorbed as nourishment by the female. Excess spermatozoa that are not used
may also be absorbed and assimilated by the female.
Insects provide care for their young by laying eggs in suitable places, often on or near to food. Carrion flies lay their
the food plant of the larvae and dragonflies lay their eggs in the water where the carnivorous larvae will live and
hunt their own food. Many insects provide no more parental care than this, abandoning the eggs once they are
deposited. The tropical fungus beetle, Pselaphicus giganteus lays its eggs on a rotting log and guards them. It's
babies feed on a specific species of fungus and the mother remains with the youngsters to lead them from fungus to
fungus. This specialist food source is sporadic and difficult to locate so it makes sense for the mother to do this.
Insects may adapt one of two extreme strategies - either depositing lots of eggs and leaving the young to fend for
themselves, or having fewer young but taking better care of them to ensure that a higher fraction survive. The
females of some shieldbugs (stinkbugs) protect their eggs, covering them with their bodies and fending off
parasitoids, ants and other predators, and also protect their nymphs. In solitary wasps, the female often provides a
secure nest for her young, stocking it with food (paralysed insects or spiders that stay fresh). Sometimes the female
wasp will bring food to the growing larva over a period of several days. Wasps may form communal nests, in which
several females (often closely related sisters or mothers and daughters) prepare their own cells for their offspring.
Fully social wasps and other social insects are discussed in the next section.
Although it is often the female who is lumbered with the eggs and their care, this is not always the case. Sometimes
the males play the nurturing role. In some water bugs, the female lays the eggs onto the male's back and the male
is then burdened with their protection. The males of some assassin bugs also protect the eggs. Dung beetles roll up
balls of dung and bury them as food for the hatchlings, and often both males and females participate in these
preparations. The female may remain in the burrow to keep the dung balls clean. Carrion beetles may bury a
corpse and roll up a ball of flesh in which the eggs are laid.
Social Hymenoptera (bees, wasps and ants) exhibit haplodiploidy. The queen is diploid (meaning she has two sets
of n chromosomes, 2n, one paternal and the other maternal) whilst the male (king) is haploid (possessing just one
set of n chromosomes, n, inherited from its mother). When the queen lays eggs, those that are fertilised become
sterile female workers, and are diploid (2n, carrying maternal and paternal chromosomes). Those that are not
fertilised develop into fertile haploid (n) males. In this sense, production of the males is asexual. The workers are all
sisters. Some of the queen's daughters will be chosen to become new fertile queens, by feeding them a special diet
(such as royal jelly in bees).
Kin selection looks at things from the point of view of the genes. All that matters in nature is survival and survival is
achieved through reproduction. We can see this from the genes point of view (the selfish gene hypothesis) in which
case what really matters is not survival of individual bodies, but rather survival of the genetic information, existing as
multiple copies of the same gene. This means that genes which cause an individual to favour the survival of its
offspring, and to a lesser extent its close relatives, is more likely to persist, since at least one of its copies is more
likely to survive. This explains the evolution of altruism, such as when a mother dies to ensure the survival of her
offspring who carry many of her genes. The process is not infallible, but statistically such genes are favoured. Since
an animal cannot see whether or not another individual carries its own genes, the tendency is for altruism to occur
between individuals that resemble one-another, since they are likely to have more genes in common. In a nest of
bees, since all the workers are sisters, they are quite closely related genetically, having all the same paternal genes
and sharing 50% of the maternal genes. In total the sisters share 75% of their genes in common (the coefficient of
relatedness is 3/4). The Selfish Gene Hypothesis explains why a bee will die to save several of her sisters, and why
most of the daughters forfeit their reproductive potential to ensure the survival of the chosen sisters who become
Social insects exhibit polymorphism, producing a number of different forms or castes. This is most pronounced in
ants and termites which may produce a variety of worker and soldier castes for different tasks (see insect society).
Asexual reproduction is part of the life-cycle for many insects. We have already seen that polyembryony can occur
in some parasitoids and that social hymenopterans reproduce by haplodiploidy, in which the males are produced
Polyembryony is the production of more than one embryo from a single egg, in which case the siblings are
genetically identical. This occurs by fission or splitting of the embryo (the equivalent effect in humans gives rise to
identical twins). In some insects this occurs sporadically in some eggs and so is atypical. In insects it may also be
abnormal, induced by certain chemicals in the mother's diet. In some parasitoids as many as 150 embryos may be
produced from a single egg, the exact number depending upon the size of the host - more embryos being produced
in a larger host.
Parthenogenesis (virgin birth) is the development of the egg cell into a new organism without fertilisation. The
offspring produced is, like the egg, haploid. It may occur sporadically or abnormally, or it may be part of the normal
development of some insects. The eggs may give rise solely to females, or solely to males, or to a mixture of both.
Warramaba virgo is a parthenogenetic grasshopper in which all the individuals are females produced
parthenogenetically (obligatory and constant parthenogenesis). In stick insects (phasmids) females also produce
other females parthenogenetically, but occasionally males are observed, which are sometimes sterile, but
sometimes fertile, as in Carausius morosus. In phasmids, however, the normal condition is diploid, although
sometimes haploid generations occur which later revert to the more stable diploid form. In cyclical parthenogenesis
there is an alternation of generations, with one or more parthenogenetic all-female generations occurring
alternately with a bisexual generation of males and females, also produced parthenogenetically. In some
parthenogenetic insects males are very rarely observed, and sometimes never, which raises the question do males
every occur in these species or are they extremely rare?
In this phenomenon (also called paedogenesis or neoteny) the larvae become reproductively mature. The larvae
usually reproduce parthenogentically by producing eggs or live young (viviparity) when the eggs develop inside the
larva. Many of the wingless aphids contain developing embryos whilst still in the nymph stage themselves.
Some insects have very complex life-cycles involving both asexual and sexual reproduction. Aphids are an excellent
example. Aphid life cycles are often extremely complex, involving an alternation of forms that differ morphologically
(in form) and physiologically (a phenomenomn called polymorphism). In addition they exhibit asexual reproduction
by parthenogenesis, though there is often a sexual stage. You may have noticed that aphids (greenfly and blackfly)
have a variety of forms, some wingless and others with wings. Some morphs (forms) are winged, others wingless. As
an exmaple, the black bean aphid life-cycle is illustrated below:
The black bean aphid (bean aphid) alternates between two plant hosts, upon which it feeds parasitically by
drinking the sap through its piercing stylet. The secondary host is occupied during summer and winged migratory
forms migrate between the primary and secondary hosts in spring and autumn (fall).
Fundatrix: this female and wingless aphid hatches from eggs, on the primary host, in the Spring and founds a
new colony by a form of parthenogenesis in which only females are produced (thelytokous parthenogenesis).
She is also known as the stem-mother. The daughters are genetic clones of the mother and each bears a pair of
female X sex-chromosomes. These daughters are produced by thelytokous parthenogenesis. Some of these
daughters will be winged fundatrigeniae (singular fundatrigenia) which migrate to the secondary host. These
give rise, also by parthenogenesis, to virginoparae, which produce all the other forms by parthenogenesis. All
the stages are female, except for the winged males (which contain one X chromosome as the only
sex-chromosome, X0) which are produced by a special form of meiosis.
The males fertilise wingless female oviparae, which lay the eggs. The eggs are diploid and develop into the
diploid fundatrix females, completing the cycle. Fertilisation does apparently occur, though only sperm containing
an X chromosome are produced (no sperm with no sex chromosomes result). Contrast this to the parthenogenetic
production of males in wasps, which are haploid. All the parthenogenetic offspring in the bean aphid case are
diploid. Aphid males are diploid, but are produced by a special meiosis in which only the number of
X-chromosomes is halved (Moran, 1992, Annu. Rev. Entomol. 37: 321-48).
There are many other terms describing the forms produced in the various aphid species life cycles. Virginparae
(singular virginopara) produce parthenogentic forms, including the sexuparae. Sexuparae produce sexual forms,
gynoparae produce only females and androparae only males, again by parthenogenesis. Sexuales (singular
sexualis) are the sexual forms, namely the oviparae that produce eggs, and the males. The migrantes (singular
migrans) are the dispersal forms which migrate to the secondary host in spring and to the primary host in autumn.
The virgos (singular virgo) are any form that reproduces by parthenogenesis. These are just some of the terms
used in aphidology (the study of aphids) to describe aphid life-stages. Evidence suggests that more winged forms
are formed when the host plant is of a lower nutritional value, allowing dispersal to new food plants. Temperature
and day-length may also determine when the winged dispersal stages are produced.
Some aphids produce additional morphs. For example, some aphids have a soldier class which defends the
Asexual reproduction produces offspring that are genetically identical (except for the occasional mutation) to their
parent. It is simple and fast, requiring only a single individual to found a population. It is also useful in a stable and
predictable environment in which the cloned form is well adapted. When a fundatrix founds a new colony in spring,
these conditions are met. The season is early, so there are few predators around, and the fundatrix could be the
only occupier of a host plant. She is adapted for rapid production. Having short legs, short antennae and a soft
body makes her vulnerable to predators, but as few are around this is less of a problem. She simply reproduces
rapidly by parthenogenesis. In aphids the young may carry young of their own before they are even born! Thus
founding a colony can be very rapid indeed! (As any gardener will know!).
Sexual reproduction mixes genes together and introduces new variation into a population. The winged male will
likely be a member of a different clone (although our life cycle diagram implies he belongs to the same clone).
Sexual reproduction creates a more varied population, which gives the population plasticity in responding to
changes in the environment. Most insects retain a sexual stage, though some apparently rely on asexual
reproduction completely (although even these populations may occasionally reproduce sexually, perhaps under
- Pupariation occurs in many flies, including blow flies and houseflies. The cuticle of the last larval instar
contracts into a barrel-shape, hardens and tans into the pupal case or puparium, thus the last larval skin is
not shed but encloses the pupa. The pupa actually lies within the puparium.
- In many fly (Diptera) larvae, the arrangement of the neuroendocrine glands differs - the corpora cardiaca,
corpora allata a,and prothoracic glands re fused together to form the ring gland, or Weismann's ring.
The Insect Egg
Most insects lay eggs, they are oviparous. However, some do give birth to live young by retaining the developing
egg inside the maternal body, such as happens in aphids reproducing by parthenogenesis, and giving birth to live
young (they are viviparous). If conditions are unfavourable, then eggs may enter a resting or quiescent stage,
until conditions become favourable again. Other eggs will enter a dormant state to overwinter, regardless of the
immediate suitability of conditions, a phenomenon called diapause.
The egg is typically fertilised just prior to laying. At fertilisation a sperm will enter the egg through a pore, called
the micropyle, and fertilise it. The fertilised nucleus sits inside a mesh of cytoplasm enclosing globules of yolk.
When laid the egg is enclosed in two membranes, the chorion (egg-shell) and the vitelline membrane. The
chorion develops from maternal ovary cells that surround and nourish the developing egg (follicular cells) and
consists of some seven layers of more-or-less tanned proteins. The tanning hardens the egg and protects it
against sunlight. The egg is also water-repellent as a covering of wax or similar material may be deposited over
the chorion. This water-proofing also reduces the risk of the egg drying out, as well as reducing the risk of an egg
drowning. Some eggs require moisture, absorbing it from the surrounding soil (hydropy), for example in
Aleochara bilineata the egg increases in volume by a factor of about 1.68 and the larger the hatchling, the better
chance it is thought to have of surviving. Such eggs are usually soft-shelled, though in Aleochara bilineata the
shell is rigid and swelling causes the inner lining of the shell to fracture. Hydropic eggs may also require nutrients
and oxygen to complete their growth and this is a way of allowing the mother to invest less in each egg and so lay
more smaller eggs that complete their own development. In an insect like Aleochara bilineata, this is
advantageous, since the first instar larva must locate its host and this is partly a hit-and-miss affair. Some insects
attach their eggs to leaves via stalks and the stalks take up water from the host plant, storing it in a bladder and
replacing water lost to the dry air.
Covering the chorion, on the outermost surface of the egg, is a layer of cement, secreted by the mother during
egg-laying (oviposition), which fastens the egg to a surface. The cement may form an egg case or ootheca
around a cluster of eggs, as in cockroaches and mantids. In the mosquioto, Culex, the eggs form egg rafts,
though these are held together by the surface tension of the water on which they are deposited, rather than by
cement. Some cockroaches merely deposit their oothecae apparently willy-nilly, whilst others bury them in sand
and some retain them inside the female body, supplying water and nutrients to the developing embryos and then
giving birth to live young.
The vitelline membrane is inside the chorion and is derived from the cell wall of the ovum (egg cell) itself.
(Sometimes it is partly or wholly maternal in origin). This may be a thin delicate membrane that disappears when
development of the embryo begins, or it may persist as a tough inner shell. Later in embryonic development, a
layer of epithelial cells may form beneath the vitelline membrane and secrete their own cuticle, forming a
secondary vitelline membrane.
The egg requires oxygen - it respires. If the chorion is thin enough, then oxygen may simply diffuse across the
egg surface, however when strength and thickness of the shell is required, the shell may contain a layer of air
spaces which connect to the outside. (Typically the openings are at one end of the egg, sometimes as a tuft of
filaments). The ootheca, if present, may contain elaborately moulded ducts too supply air to the eggs inside. The
eggs of flies (Diptera) may have horns which can obtain oxygen from the air or from water (acting as gills or
plastrons, see aquatic insects).
Embryonic development and hatching
The egg is polarised, with the fertilisation nucleus at one pole. Its development may be quite determined (as in
nematodes) for example in gall midges. In this case the fate of each region of the egg is determined before the
egg undergoes cell division - different parts of the egg are pre-destined to become different parts of the embryo.
This is determinate development. Such an egg exhibits mosaic development (each patch of the egg having
a different destiny).
In other insects, the fate of the various egg parts is less determined and even after the nucleus has divided to
form many nuclei, the fate of each nucleus is not finally determined by its position. (Momentarily centrifuging the
eggs to pull all the nuclei to one pole results in normal embryos being formed). This is indeterminate
development. The egg is centrolecithal, meaning that the yolk is concentrated in the centre of the egg (with
the nucleus embedded in it) and the rest of the cytoplasm concentrated in a layer beneath the egg shell. The
mode of cleavage is peculiar and aberrant. One side of the egg develops into the embryo, surrounded by the
fluid-filled amniotic cavity which forms between the embryo and egg membranes.
The egg has a detachable lid or cap at one end to allow the young insect to emerge, head first. As hatching is
about to begin, the insect swallows the amniotic fluid and so swells to fill the entire egg. It may also follow air that
subsequently diffuses into the egg, increasing in volume and pushing on the egg cap by muscular action, or by
pumping blood to the head to push the cap off. In some insects, such as the blowfly Calliphora, the chorion
absorbs moisture from humid air, weakening the lid. Hatching spines may be used to push against the egg shell
orb to cut a hole in it. Moths and butterflies (Lepidoptera) gnaw their way out using their jaws. Now the cycle is
In the tsetse fly the eggs hatch inside the uterus of the mother where the developing larvae are retained and
nourished by nutritive secretion from special 'milk glands'. The mature larva is laid and then immediately pupates.
This type of viviparity is known as adenotrophic viviparity and occurs only in some flies.
|Did you know?
Research has shown that the silk cocoons of hornet pupae (Vespa orientalis) store excess heat energy when
warm, in the form of circulating electrons (electric currents) and that should the pupa cool then some of this
electron energy is converted into heat. (A thermoelectric effect). This helps maintain the pupa at its optimum
developmental temperature (28-32 degrees C). The adults will also regulate the temperature of the pupae in the
nest by fanning with their wings and attaching water droplets for cooling and by blowing warm air into them from
their tracheal openings when they are too cool. If the optimum temperature is not maintained, then malformed
adults are produced.
This article summarises information from our own research and a large number of publications studied over the
years, however, below is a list of some of the references for suggested reading:
1. Ishay, J.S. and Barenholz-Paniry, V. 1995. Thermoelectric Effect in Hornet (Vespa orientalis) Silk and
Thermoregulation in a Hornet’s Nest. J. Insect Physiol. 41: 753-759.
2. Marie-Josée Gauvin, Guy Boivin and Jean-Pierre Nénon, 2001. Hydropy and ultrastructure of egg envelopes
in Aleochara bilineata (Coleoptera, Staphylinidae). Zoomorphology 120:171–175.
3. Evans, H.E. Insect biology: A textbook of entomology, 1984. Addison-Wesley Publishing Co.
4. Wigglesworth, V.B. 1972. The principles of insect physiology, 7th ed. Chapman and Hall.
5. O'Toole, C. 1995. Alien Empire: an exploration of the lives of insects. BBC books.