|Insects - Glandular Systems
|Insects have complex glandular systems, consisting of a variety of
glandular cells and organs which produce a variety of secretions,
such as silk and venom. This article focuses on exocrine glands,
the gut lumen.
For a discussion of endocrine glands see our article on insect life
Insects (and other arthropods) have a large variety of glands producing a wide variety of secretions. Well
known are the defensive secretions that the bombardier beetle sprays from its rear - hydroquinone is mixed
with hydrogen peroxide (a powerful oxidising agent used in some rocket propellants) along with enzyme
catalysts and water. The heat generated produces steam which propels the hot liquid at an attacker. This
secretion is quite capable of killing other insects and is painful to human skin. Silk is another well-known
example, used in the construction, for example, of silk cases in chrysalises and in the silk threads which
many caterpillars use to escape predators or disperse themselves. Spiders of course use silk to capture
prey, by a variety of methods, and also as life-lines and 'parachutes' used in dispersal. (Note: spiders and
their allies are arthropods like insects, but they are not insects!). The remainder of this article will focus on
The study of insect secretions is surprisingly not well developed. Probably in the vast majority of cases the
functions of the secretions remain unknown. Considering how successful insects are and that they are
inspiring generations of robots, I really think more resourecs ought to go into research on their basic
|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.
The above model illustrates some of the salivary glands of a typical insect, such as an ant. These include
the large pair of labial glands situated in the thorax, and also often with a pair of divisions in the head
behind the brain (postcephalic glands). These are the main salivary glands of insects and release their
secretions into the labium (lower lip) of the insect. Both pairs of labial glands give out ducts which join to
form a single common duct which opens in the region of the mouth. When feeding a drop of saliva may
appear on the labium to moisten the food, as in the cockroach. The saliva may contain various secretions,
including digestive enzymes, such as amylase (to digest starch) in the cockroach.
In the honey-bee the labial glands of the thorax secrete fluid which may be used to build the comb, whilst
the postcephalic glands secrete a clear oil used for working wax as well as saliva for moistening food. In the
mason bee (Chalicodoma siculum) the postcephalic glands secrete hydrocarbons used to waterproof the
brood cells. The labial glands may also secrete pheromones. In many cases the functions of the secretions
Other salivary glands of the head include the pair of mandibular glands and (post)pharyngeal glands
which open into the pharynx of the foregut. There may also be circumoral glands situated around the
mouth. Insects also use saliva when grooming, in which case oily secretions may help waterproof the cuticle
and possibly have anti-microbial (antiseptic) activity. In young adult honey-bees the pharyngeal glands
secrete royal jelly, used to feed the young queens. In foraging adults the pharyngeal glands begin
secreting amylase and invertase (sucrase) for digesting sugars.
In some insects the labial glands (or in some cases an additional pair of them) are used to produce the
protein silk. Lepidopterans (moths and butterflies) and hymenopterans (wasps and their allies) produce silk
from their labial glands. Embiopterans ('webspinners' or 'lively wings') are a group of insects which produce
silk from tarsal glands in their feet, as do some dipterans (true flies). Some beetles (coleopterans) and
neuropterans (lacewings and their allies) secrete silk from their Malpighian tubules, structures
associated with the midgut of insects and which generally have an excretory function in removing
nitrogenous waste. The larvae of some cockroaches produce silk from the cerci (sensory projections at the
rear of the abdomen). Silk is extremely strong but very light, which has made it the object of research in the
development of body armour. However, it is also very flexible (which has hampered the aforementioned
research!). Silk is about 3 quarters the tough protein fibroin and one quarter the gelatinous protein sericin.
generally, one strand is produced by each labial gland and then cemented together into a ribbon once they
reach the common secretory duct of the labial glands.
The dermal glands of insects are modified epidermal cells which send fine secretory ducts through the
cuticle (the exoskeleton covering the epidermis). These secrete wax, mixtures of long-chain alcohols,
carboxylic acids and hydrocarbons, which waterproof the cuticle. Shellac (or lac) is a secretion of wax,
resins, sugars and pigment secreted by certain coccids (scale insects). Dissolved in ethanol, lac forms
liquid shellac, which is used as a wood finish, varnish and food glaze.
Venom is secreted by modified accessory glands and released through the sting (a modified ovipositor)
of certain Hymenoptera including stinging wasps, ants and bees. The accessory glands normally have a
role in reproduction. Dufour's gland also releases secretion into the ovipositor or sting. This may serve to
lubricate the ovipositor or to cement the egg to the wall of the comb cell. In some ants, the Dufour's gland
secretion is used in trail-laying, in which a worker who finds a suitable food source lays a trail with its sting
back to the nest, so that other workers may follow the trail to the food source. Bee venom contains the
protein melittin which causes cells to lyse (burst open).
- To read more about stings, read our article on the insect abdomen and insect reproduction.
Above: a cutaway model of an ant, showing some of the major
gland systems and other organ systems (the respiratory,
muscular and reproductive systems are not shown). The gut is
shown in green, the nervous system in blue. Insects have
complex anatomy and a variety of organs packed into their tiny
bodies. These organs do not just 'hang around' as suggested by
simplified models, but are held in place by ligaments and
diaphragms and buoyed up by the haemolymph which fills much
of the body space.
T. Katzav-Gozansky, V. Soroker, A. Ionescu, G. E. Robinson, and A. Hefetz, 2001. Task-related chemical
analysis of labial gland volatile secretion in worker honeybees (Apis mellifera ligustica). Journal of
Chemical Ecology 27: 919-926.
B. Holldobler and E.O. Wilson, 1994. Journey to the ants: a story of scientific exploration. Harvard
V.B. Wigglesworth, 1972. The principles of insect physiology (7th ed). Chapman and Hall.
H.E. Evans, 1984. Insect biology: a textbook of entomology. Addison Wesley.
Ants in the genus Formica secrete formic acid
acid up to 30 cm, or spraying it onto a wound
made by their biting mandibles.
Some ants produce an insecticidal venom
called iridomyrmecin. Some solitary wasps
secrete paralysing neurovenoms which
paralyse prey, such as spiders and caterpillars,
whilst keeping them alive (although the nervous
systems of the unfortunate victims may
degenerate) so that their hatchlings have fresh
Many insects secrete various foul-tasting and
poisonous substances to protect themselves
against predation. Some of these are
neurotoxins which may be sprayed or secreted
as a foam.
Ants have been described as 'batteries of walking glands'. The world of the ant is certainly largely olfactory
based, since their vision is much less acute than human vision, but their sense of smell is extraordinarily
sensitive. (Though it may not be true to say they can distinguish more odours, since the human nose,
insensitive as it is, can discern some 350 known odours).
The anatomy, location and function of insect glands is very diverse and in many cases the functions are
unknown. Many beetles have one or two types of antennal glands, opening via pores on the surface of the
antennae. These secretions are perhaps antimicrobial, perhaps spreading to other parts of the body
during grooming. Some possibly serve to lubricate the joints. Some may, like the secretions of other glands
in semi-social staphylinid beetles (rove beetles), have functions in communication.
Some rove beetles have defensive glands in their flexible abdomens, along with glands that are used to
stimulate ants to carry the rove beetle to their nest. First the beetle presents its appeasement gland to an
approaching ant. The ant licks the gland opening and then licks one of its adoption glands, also located in
the abdomen, then carries the beetle back to its nest. The beetles trick the ants, who treat them as their
own, and they may solicit food from the ants. Their larvae also trick the ants whilst feeding on the ant
larvae, whilst ants groom and take care of them! Rove beetles are sometimes semi-social, living in
communal burrows, and are, like the ants, bees and wasps, evolutionarily very advanced.