How insects eat

There are hundreds of thousands, even millions of different types of insects, displaying a vastly different size range, an incredible array of shapes and behaviours and inhabiting every conceivable terrestrial (land based) environment (insects live in fresh water and on the surface of the sea near land, but there are few marine insects and none that live at any depth). Air samples taken high altitudes, even many miles up, reveal insects there. Other insects live in trees, under the ground, in caves where it is permanently dark and they have evolved so that they no longer have eyes. There are fewer insects in very cold environments than in warm climates, but insects may be found in a great range of temperature conditions. In the coldest conditions they often over-winter as eggs or pupae.

With such an enormous variety of insects and habitats, and a huge range of food sources, they have needed to evolve a wide range of feeding mechanisms. We often divide insect pests into suckers and chewers for convenience, but this does not describe the great diversity which is revealed on closer inspection.

Plant feeders

I remember one argument with a particularly close minded Department of Agriculture extension scientist. He grew flushed red in the face and shouted and I thought he was going to stop the car and ask me to get out. He repeatedly stated that Portuguese millepedes were scavengers and fed only on dead plant material. His mind was not open to the opportunism which many insects will display. Yes, I do know that millepedes have more than six legs and are therefore not insects, but a there are several good points which can be made from this story which may be applied to true insects. It is the case that many insects and other animals, including millepedes, slugs (they are not insects either - they are molluscs) and earwigs prefer decaying plant material, but they will try a wide range of food. I have written in these pages about the millepedes which ate my strawberries (and strawberry plants). I calmly described to the irate scientist how, only days before, I had stretched out on the ground with my magnifying glass watching millepedes consume the clover plants in the back lawn (it was a herb lawn, containing no grasses) just metres from my back door. I watched for half an hour, noting carefully how their little mandibles chewed away at the edges of the leaves.

Many lessons could be drawn from this experience. I learnt from this episode that textbooks can very definitely be wrong, that scientists can be wrong, that even small organisms are complex enough in their behaviour to deviate sometimes from the norm and that careful observation is worth more than book learning or thoughtless acceptance of accepted knowledge from so called experts.

Mandibles, by the way, are one part of the biting equipment or mouthparts of insects. Take a magnifying glass (essential equipment for organic growers or gardeners of any kind) and examine the head of any chewing insect. Mandibles are the paired appendages immediately behind the mouth. They are hard and may have various structures which are like teeth or brushes. For most insects they are attached laterally to the head, meaning they move sideways. That is looking straight at the head from the front, they open and close with a sideways movement. In winged insects they may have a second point of articulation which allows them to move dorsally in a more powerful opening and closing motion. Mandibles are used for both cutting food away from the food source and grinding.

Maxillae are another mouthpart, located behind the mandibles. They operate only laterally (sideways) and are used for masticating (chewing) and pushing food into the esophagus.

Labium are the third mouthpart. They are also paired structures in front of the mandibles. They are like lips and are used to pass food back to the mandibles and maxillae rather than for chewing.

There may be a variety of other structures in this vicinity too, notably large organs called Palpus. These are thought to be only sensory organs and are not involved in eating.

Biting, chewing insects include adult and larvae of beetles (Coleoptera), larvae of moths and butterflies (Lepidoptera), larvae of sawflies (Hymenoptera-Symphyta) and the grasshopper group (Orthoptera)

Biting insects will however use their mouthparts in different ways. Locusts rarely seem to bite holes in leaves, instead they hold the leaf with the labrum and forelegs and attack it from the edge. Bean weevils and some caterpillars leave clear scalped out crescents in the edge of leaves, while other caterpillars attack the leaf surface from the top or bottom.

Houseflies have non functional mandibles and maxillae. The labium are formed into a proboscis with a spongelike end which will absorb liquids. If their food is not already liquid they exude a drop of saliva which dissolves the food outside of their body. They then sponge it up. This is why flies are so good at spreading disease - they dribble all over it leaving disease carrying fluids behind. Yuck!

Horseflies or marchflies have mandibles like sharp blades and maxillae which are formed into long probing styles which cut and tear the skin of their prey. The spongelike labium then mops up the blood and transfers it to the esophagus. Yum! This explains why they are attracted to open sores or wounds. The skin has been opened for them and they have to work less hard to access the body fluids they seek.

The bugs (Hemiptera) include stinkbugs, leafhoppers, aphids and San Jose scale. Nymphs and adult bugs use piercing mouthparts which can be stuck into plants (or animals) to suck up juices. Assassin bugs are an example of insect feeders. Mosquitoes, bedbugs, lice and fleas also have this type of feeding parts. In this case all three mouthparts have been formed into a hollow needle-like tube for sucking blood. They may also use saliva which contains anti-coagulant agents which prevent blood from clotting in the tube and blocking it. Again, this explains why they spread disease, first injecting some of their own fluids into the host, before starting to feed on the liquids which they need to survive.

Adult Lepidoptera (moths, butterflies) also have a siphoning tube or proboscis, which may be used for sucking up nectar and other fluids. Many adults of this group do not feed at all, only existing in this phase long enough to mate and lay eggs before they die. Some have very long proboscis. This may indicate a specialisation for feeding on tube or bell shaped flowers where the nectar source is well protected deep inside the flower parts.

Many bugs are very accurate with their proboscis, reaching into the interior of cells to suck up liquids. They can guide these probing parts around or through cells to select sugary or high nitrogen foods, or may use the stomata to gain access to the inner parts of leaves. In this way they reach the phloem, which transports sap through the plant. Well fed, healthy, organic plants may have a thicker cuticle and epidermis which offers some resistance to these bugs. Healthy well fed plants are also more turgid and harder to pierce. There is some suggestion that the bugs are unable to utilise whole proteins and are seeking free amino acids. This is sometimes used to explain why unhealthy plants seem to attract sucking bugs while good organic plants seem to display resistance to this type of pest.

Many insects feed primarily on decayed plant material or microbes. They may also specialise in eating lichens, algae, mosses, ferns or seed.

I love to watch insects feeding. In my notebook I record the variety of insects which assassin bugs will stick or the speed with which a preying mantis pulls its unfortunate prey apart, usually starting with the head parts. A pocket magnifying glass will greatly help, so will a little patience and a few moments taken out from the daily routine to wander quietly through the paddock, garden or orchard to look for insect visitors. You will learn a lot about the biodiversity of your particular patch and perhaps even learn that some six legged creatures are highly desirable beneficials rather than scary beasts to be trodden or stomped on.

I often set the following exercise for my students. Sit out in the garden or field, for at least ten minutes (some people find staying still in one place for this long very hard). Look down at the ground at a patch about one square metre in size. Take mental note or record on paper the variety of insects or creatures which crawl through or fly over that space during ten minutes. Try the same exercise in that spot in the early morning, midday or early evening, and in different seasons. You will probably learn a great deal and surprise yourself with the variety of life forms which you will observe.

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