Above: the abdomen of a male grasshopper. Like all other living
organisms, insects exist for one ultimate purpose: to reproduce. Or to put
it another way: they survive because they reproduce. Everything about
the insect serves this ultimate function, however, the reproductive organs
themselves reside in the abdomen.
The Insect Abdomen
In most insects the abdomen consists of about 11 segments, although often only the first 8 or 9 are obvious as
whole segments. Each abdominal segment usually consists of two armour-plated sections, the terga (sing. tergum)
on the top (dorsal) and the sterna (sing. sternum) beneath (ventral). These are connected by softer, elastic pleural
membranes that are usually not armour-plated (sclerotised) in the abdomen (as they are in the thorax). These
regions can be called pleura or pleurons, but the term 'pleurite' should only be used when the pleura or pleurons are
armour-plated (as they are in the thorax). Individual armour-plates are called sclerites and are made chiefly of
molecules of a polysaccharide (chain of sugar molecules) called chitin. Chitin has a hardness equivalent to copper
metal, but is more rigid and durable. Generally, in the abdomen the terga is composed of one main sclerite, as is the
sterna, however, in the thorax complications arise due to the leg and wing attachments and more plates may be
present in each thoracic segment. The armour plates of adjacent segments overlap and muscles inside each
segment can shorten or lengthen it (thanks to the flexible plural membranes) allowing a range of movements,
including telescoping of the segments. In bees, ants and wasps, the first abdominal segment is fused with the last
thoracic segment in front and there is a constriction between segments 1 and 2, so that the first abdominal segment
appears to be part of the waist and part of the thorax.
Typically there is one pair of spiracles per segment, except often the last couple, and these openings may be
situated on the tergites (cuticular plates of the terga), pleural membranes or sternites (cuticular plates of the sterna),
depending on species. These openings can often be controlled (opened or closed) and are the respiratory
openings, allowing oxygen into the gas-exchange tracheal system. This is a series of tubes, called tracheae
(singular trachea) reinforced by spirals of chitin, to stop them collapsing, and which branch repeatedly, the finer
branches lacking the spiral strengthening are called tracheoles. A fine tracheole may end inside every muscle cell
and every bundle of neurones. Textbooks will often state that the tracheae convey oxygen to the tissues by passive
diffusion, which is often true, but in larger and more active insects there may be a series of elastic air sacs at
intervals throughout the tracheal system and abdominal muscles may pump the abdomen, pumping the air sacs and
thus pumping air in and out of the tracheal system. This is why the abdomen of a bee or wasp appears to pulsate,
especially when it exerts itself. The tracheae leading from the two rows of spiracles generally merge into two main
lateral tracheal trunks, which then send out finer branches to the tissues. The presence of air in the tracheae makes
them appear silvery in fresh dissections. These tracheal tubes are quite tough and easily identified in dissections.
Tympanum and communicating by sound
Many insects communicate by sound. Grasshoppers, crickets and some others rub their hind legs against the tough
frontal margin (costal margin) of their forewings, a process called stridulation. The femur of the hindleg bears a
series of teeth, forming the stridulatory organ. In grasshoppers the first abdominal spiracle is modified to function
as an 'ear'. It consists of a large flat chitinous plate or membrane, called the tympanum, which vibrates in response
to sound, such as the song of a male stridulating to attract a female, with an air sac underneath (supplied with air by
a trachea). The air chamber amplifies the vibrations and groups of sensory neurones underneath the vibrating
membrane detect the vibrations. The whole organ is called a tympanal organ. Other insects have audio sensors in
their legs, which may be modified proprioceptors (sensors that detect movement of the limb) that may respond to
vibrations and/or sounds. Some termite soldiers communicate by rhythmically drumming their head against a
surface, whilst most ants stridulate by rubbing an area of their waste against a ridged plate on the front of the
adjacent abdomen. In the case of these social insects, these sounds serve to coordinate the activities of the nest,
rather than in courtship. Crickets are especially loud and stridulate by rubbing their legs and wings together, but
their stridulatory organs are so designed that they vibrate the wings at their natural frequency, creating a pure note.
This forced resonance of the wing probably explains why the sound produced can be so loud.
Cerci. Often the most obvious abdominal appendages is the pair of anal cerci, sticking out of the rear end of the
insect like two prongs. These are borne on the terminal segment (segment 11). The primary function of these is
sensory. Touching one of the cerci will typically induce an escape response - sending the insect running off at speed
away from the stimulus. Thus, the cerci act as an early warning system to alarm the insect against possible
predators sneeking up behind it. In some insects, like cockroaches and silverfish, the cerci may be very long. In
many insects the cerci are innervated by giant nerve fibres or axons, since larger diameter nerve axons conduct
signals faster. Stimulation of these giant axons will ellicit a rapid escape reflex. In some insects, the paraproct (see
diagram) may be innervated by similar fibres. In earwigs the cerci are modified into a pair of pincers (curved in the
male, straight in the female) which are used in defense, prey-capture, grooming and to help fold away the wings.
They are also used in battles between rival males fighting over a female. The males of maritime earwigs have
asymmetric pincers, one prong is longer and more curved, which makes it easier to grab and pin the rival male.
Reproductive appendages of the female. The genital ducts open, via genital openings, on the underside of segment
8 or 9. In the female these segments may have special appendages to assist with egg-laying (in mayflies and
stoneflies there are no such special appendages). Appendages from both these segments (probably
developmentally modified legs) may form an ovipositor. Two sets of valves (appendages from segments 8 and 9)
form the shaft of the ovipositor. These valves connect to basal sclerites called valvifers which articulate with the
abdominal terga. These valves interlock and can slide over one-another, moving the egg along inside the shaft,
assisted by spines or ridges on the inside surface of the shaft which grip the egg. Eggs are often laid inside an
excavated hole, inside soil or bark or another insect. In many insects which undergo complete metamorphosis,
transforming from a larva to an adult via a pupal stage (e.g. flies, moths and butterflies) lack this type of ovipositor,
formed of abdominal appendages. Instead, the terminal abdomenal segments are narrow and form a telescopic
tube-like ovipositor which can be extended as mobile probe, sometimes equipped with a pair of lobes on the end to
position the egg.
Reproductive appendages of the male. In most insects the male transfers sperm directly into the female's
reproductive system. This is accompanied by some sort of intromittent organ, often called a penis or aedeagus (I
prefer to use distinctive names to avoid the impression that the insect 'penis' resembles a mammalian penis,
because it is structurally and in mode of operation quite different). This opens on segment 9 and is accompanied by
appendages of segment 9 (external genitalia) called claspers, which serve to hold the female in position during
sperm transfer. There is an astonishing variety of forms of insect male geneitalia.
The Female Insect reproductive System
a common oviduct which enters a tubular chamber or vagina which opens to the outside via the female gonopore
(genital opening) on the underside of abdominal segment 8 or 9. In the vast majority of insects the male transfers
sperm directly into the female reproductive system, with the spermatozoa either free in fluid or encased in a capsule
(of various chemical composition) to form a sperm packet or spermatophore. The sperm are stored by the female
in the spermatheca sac, for later use: once she has mated she has enough sperm for life!
The spermatozoa may be deposited in the vagina (and then moved into the spermatheca) or directly into the
spermatheca. In moths, butterflies and some other insects the system differs - the female has two genital openings,
the opening of would have been the vagina is the ovipore and is used only for egg-laying, whilst sperm are
deposited into a separate copulatory opening, which leads into a sac called the bursa copulatrix, which receives
the spermatozoa before they are transferred to the spermatheca via a connecting duct called the seminal duct
(which may connect to the spermatheca indirectly via an ante-chamber called the vestibule).
A female insect will typically mate with 2 or 3 males at most, and the sperm from the most recent male to mate are
used preferentially. Mature eggs are passed down the oviduct and vagina from the ovaries by peristalsis (waves of
contraction). As they enter the vagina the spermatheca releases some sperm to fertilise the egg. One or two pairs
of accessory glands open into the vagina. The secretions from these glands include cement that coats the egg,
making it sticky to enable it to be attached to a surface when deposited outside. Instead, in aquatic insects a
gelatinous coat may be deposited around the egg. In the cockroach the eggs are temporarily retained in the lower
vagina, whilst the cement (which contains calcium oxalate) cements a number of eggs together into a bundle. The
accessory glands then secrete materials which form the egg case (ootheca) - the left accessory gland secretes
protein, which coats the egg bundle, and the right secretes quinone which tans and hardens the protein on contact
with air, forming the hard brown egg capsule. You may have seen female cockroaches carrying an egg capsule
attached to their rear end. Some cockroaches deposit these capsules at random, others plant them with more care.
Each ovary consists of from one (some aphids) to over 2000 tubes (some termite queens) called ovarioles. The
wall of each ovariole consists of a layer of epithelial cells, resting on a basement membrane, surrounded by
connective tissue. At the top each ovariole tapers into a terminal filament (the terminal filaments of the ovarioles
collectively forming the ovarial ligament which attaches to the body wall). Then there is a thicker elongated region
called the germarium, which contains the primordial germ cells or oogonia each of which matures into either a
haploid oocyte (egg cell, haploid meaning it contains half the normal number of chromosomes following a reduction
division (meiosis) of the oogonium) or a nurse cell (a diploid germline cell formed by mitosis of the oogonium). The
main body of the ovariole is the vitellarium, consisting of a chain of maturing oocytes, each embedded in a case of
epithelial cells (normal somatic diploid cells), forming a follicle around the developing oocyte. As the oocytes
mature they descend along the ovariole, so that the mature follicles are at the oviduct end and the less mature at
the germarium end. Each bulge in the ovariole contains one follicle, and you can see that the more mature (older)
follicles are larger at the oviduct end of the ovariole. The whole ovariole really is a chain-production line for
manufacturing eggs. Finally, each ovariole is connected to the oviduct by a small ovariole stalk. When the oldest
follicle is still not ready, access to this stalk is plugged by epithelial cells at the end of the vitellarium, but this plug
ruptures at ovulation. The mature oocyte escapes from the ruptured follicle that encased and nourished it. This
follicle then degenerates, making way for the next follicle.
There are three types of ovariole, found in different species: the first type contains no special nurse cells, the
follicular cells nourish the developing egg on their own; the second and third types produce special nutritive nurse
cells from the oogonia, as well as oocytes. In the second type, these nurse cells become interspersed in the follicle.
In the third type they remain in the germarium and send down nutritive cords to the developing oocytes.
The Male Insect Reproductive System
The male contains one pair of testes. Each testis consists of a number of tubular follicles, each consisting of a wall of
connective tissue sheath, which is often pigmented. In butterflies and moths the two testes are bound together in a
single connective tissue capsule.
Spermatozoan production. As in the female system, each tube is a chain-production line, but making sperm cells in
this case. The tip of each follicle contains densely packed primary germ cells or spermatgonia, forming a region
called the germarium (or zone of spermatogonia). Large nurse cells may also be present, which nourish the
developing spermatogonia. Further along the follicle is the zone of spermatocytes, are capsules of cells, called
cysts, each containing 64, 128 or 256 spermatocytes, all derived from a single spermatogonium by cell division. Next
follows the zone of maturation and reduction, in which each spermatocyte divides by meiosis (a reduction division in
which the number of chromosomes is halved) into 4 spermatids. Then follows the zone of maturation, in which the
spermatids, still encased in the cysts, develop into spermatozoa. Each grows a tail or flagellum by which the
spermatozoan can swim. The writhing of these flagella eventually rupture the cyst wall and the spermatozoa escape
into the vas deferens, via the stalk which connects each follicle to the vas deferens (this stalk is called the vas
efferens which can be seen in the diagram, but is not labeled). The spermatozoa are often still in bundles
(spermatodesms) where each bundle derived from one spermatocyte, held together by a gelatinous hyaline cap or a
cyst cell (which may nourish the spermatozoa) into which the heads of the spermatozoa are embedded. In some
insects as many as 16 spermatozoa are bundled together (not sure how it goes from the expected 4 to 16). The
sperm become activated in the vas deferens, but they remain bundled until they enter the female, where the caps
dissolve. The spermatozoa are stored in the seminal vesicles.
To summarise the sequence of sperm cell development is:
spermatogonium (2n) - spermatocyte (2n) - spermatid (n) - spermatozoan (n)
Where 2n indicates a diploid cell (containing the normal double set of paired chromosomes) and n a haploid cell
(containing only one set of chromosomes).
The seminal vesicles open into short ducts that unite to form a single ejaculatory duct, which is lined by cuticle and
has powerful muscular walls. The end of this ducts is often enclosed in an evagination of the body wall, forming an
intromittent organ (penis or aedeagus). A number of various accessory glands open into the ducts leading from
the seminal vesicles (which may themselves secrete various substances from the duct walls). These may secrete
chemicals which form the spermatophore capsule, in those insects that produce spermatophores.
It should be clear, at least in part, why insect reproduction is so efficient, when you see how their gonads function in
the mass-production of gametes: they are run much like conveyor belts in the chain-production line of a factory!
More information about spermatophores, eggs, courtship and mating can be found on the insect life-cycles page.
Click the thumbnail above
for an unlabeled version.
Click the thumbnail above
for an unlabeled version.
|Did You Know?
Unlike mammals, the gonads in insects have a minimal effect on reproductive behaviour and drive. A castrated
insect will mate just as frequently (though of course unsuccessfully) as one with fully functional gonads!
The females of some species of bees, ants and wasps have the ovipositor modified into a sting: a pointed barb which
injects venom. The egg in these cases is released from the base of the ovipositor shaft instead of from the tip. The
males cannot sting, although some male solitary wasps have a non-venemous pointed spine at the rear, an
extension of the last abdominal segment called a pseudosting, which may be capable of piercing skin.
Some ants use their stings to lay odour trails which recruits other workers to a food source. E.O. Wilson studied
trail-laying in one such ant, the fire ant Solenopsis saevissima (1962, Animal Behaviour 10: 134-147). When a worker
ant discovers a food source, especially if it is a nutritious one (they are much more likely to trail-lay if they find sugar
solution than simply water) the worker may trail lay on its way back to the nest. It does this by moving more slowly,
crouching close to the ground with the tip of its sting intermittently touching the ground, leaving an invisible streaky
trail of minute amounts of secretion from Dufour's gland in the abdomen. (Some ant species trail lay without using the
sting via other abdominal glands). If it encounters other workers on its way back then it may rush towards it and may
partly climb on top of it and shake its body up-and-down, lightly but vigorously. It would appear that no information
about the nature and direction of the food source is exchanged during these contacts, but that this behaviour simply
alerts the new worker to the trail. Workers can detect the trail by olfaction (sense of smell) within about 10 mm. Newly
recruited workers may locate the food source and lay more homeward trails to recruit still more workers. However,
each trail evaporates in a matter of minutes and so prevents excessive recruitment (ants will also not odour lay if the
food source is too crowded with workers for them to come into contact with it, so larger food sources recruit more
workers). If a trail has partially evaporated then an ant will follow it as far as it can and then head off in a random
direction and so may still locate the food source.