Insects are a diverse group, which is reflected in their diets; between them, they have exploited nearly every feeding opportunity.
There are two main ways that insects feed, those that bite their prey, killing them, and species that suck the juice from nectar or honeydew.
With over a million known species and at least another million awaiting discovery and description, insects are easily the world’s largest group of living organisms. They are also the most diverse group, apparent from their feeding mode and diet.
Chewing And Sucking Insects
Two of the largest feeding categories in insects are those that chew and those that suck.
Chewers have jaws that can bite and grasp food. These include the plant-feeding species, which leave holes providing clear evidence of their presence, and predators, which kill and rapidly dispose of their prey with bites.
Insects that are chewers will have mandibles for chewing and labium for seizing their prey.
Examples of chewing insects are grasshoppers, which attack vegetation, and dragonflies which hunt and kill. The emperor dragonfly is equipped with jaws that can bite. It can outfly most insects and overcomes its prey with a few bites.
The second category, the insects that suck, have a tubular proboscis which they use to extract liquid food from living plants, animals, rotting flesh, or dung.
These insects insert their mouthparts by pushing between crevices or piercing and sucking out the sap, juices, or body fluids.
Sucking insects normally have a stylet for piercing and a proboscis for sucking juices. Houseflies also have a sponge-like substance at the end of the sucking tube to mop up liquid food.
This feeding method leaves little trace and is evident in aphids, which feed on plants, and mosquitoes which feed on blood.
Differences Between Larvae And Adult Diets
Insects can be divided into two more categories according to their life cycles, which in turn affects their method of obtaining food.
Insects with an incomplete metamorphosis, in which the immature stages resemble small adults, and there is a gradual transition between youth and maturity.
They feed in the same way and have a similar diet throughout their lives. Aphid nymphs and aphid adults are both suckers, extracting nutrients from plants.
Insects with a complete metamorphosis, in which the transition is less gradual, normally have a different way of feeding when they are adults.
Some of these are pupae that do not feed, while other species may have adults that do not feed. Beetles, mostly chewers, and butterflies, mainly suckers, fall into this category, with the young feeding differently from the adult.
Sucking and chewing are the two main ways of obtaining food, and immature insects may or may not utilize the same resources and feed in the same way as adults.
One sucker with an incomplete life cycle is the froghopper, also known as the meadow spittlebug. The nymphs of this species cause small blobs of foam, commonly known as cuckoo spit, which can be found on vegetation in May and June.
Soon after hatching from the egg, the froghopper nymph pushes its stylet into a plant until it locates the xylem—the water-conducting vessels in plants.
It then extracts large quantities of water from the plant that pass through its alimentary system before being pumped before secretion. The nymph lives immersed in this foam which probably protects it from predators.
The plant’s xylem contains small amounts of nitrogenous nutrients in solution, and it is these that act as food for the froghopper nymph.
As the nymph grows, the foam blob becomes larger until the nymph eventually molts and emerges as an adult. It then leaves the foam, which quickly dries up. Adult froghoppers feed less often than the nymphs and will leave the plant.
The cabbage aphid is equipped with a sucking stylet that pushes into the plant’s phloem, where the energy and nutrients are located. The resources of the phloem are richer than the xylem, so aphids are able to extract large amounts of sugar.
This sugar solution passes through the aphid’s body and is excreted as honeydew, with the sugary stickiness found on the trees’ leaves in dry summers.
Cabbage aphids stick to cabbages and other brassicas. If there is a heavy infestation, it can be a considerable drain on the energy and nutrient reserves of a plant.
They remain on the same plant, only moving to another plant when the nutrient supply dries up.
The elephant hawk-moth is an insect with a complete metamorphosis, meaning that the larvae feed differently from the adults.
When fully grown, the caterpillars of the elephant hawk-moth are 9cm (3in) long and fat. By day they rest motionless at the base of rosebay willowherb stems and related plants, but at night they can be seen climbing the plants and consuming enormous quantities of leaves.
Elephant hawk-moth caterpillars must consume a vast amount as cabbages aren’t nutritious or particularly easy to digest.
In winter, the nutrients and energy acquired by the caterpillar during the previous summer are used to power the metabolic processes that turn the large caterpillar into a slim, pink and black moth.
Adults can be seen flying at dusk, and the elephant hawk-moth feeds by unwinding its coiled-up proboscis and inserting it into strongly scented, tubular flowers from which it laps up the sugary nectar.
It does this while hovering before the flower, only moving on to the next flower once the nectar is extracted.
Adults have different diets than larvae. The adults feed on nectar and juices from rotten fruits, while larvae are carnivorous, feeding on small invertebrates supplied by the worker wasps.
The seven-spot ladybird is a more familiar predator than the common wasp. In spring, the females lay little clusters of bright yellow eggs among aphid colonies.
Upon hatching, the ladybird larvae rush to the nearest aphids and start feasting. Provided there are enough aphids to consume, the larvae grow quickly and soon form orange and black pupae attached to leaves and stems.
If there are not enough aphids, the larvae attack each other in an act of cannibalism. Once they become adults, they continue to feed on aphids. Unlike when they are young, if there are not enough aphids, they will fly long distances to find a better supply.
The bird blowfly Protocalliphora sordida is a typical looking bluebottle. The adults feed on flower nectar, but their maggots are parasitic. Female flies lay their eggs in the lining of birds’ nests during the incubation period of the eggs.
The fly maggots hatch, usually at the same time as the nestlings, and will attach themselves using a sucking disc to the bellies of the young birds. The maggots pierce with mouth hooks and suck the blood, growing rapidly, but spend most of the time hidden among nest material. As many as one hundred maggots may exist in a single nest.
Froghopper, aphid, moth, wasp, ladybird, and parasitic bluebottles are just examples of how insects find and utilize food and illustrate the remarkable diversity within the insect class.
Adults of the common wasp feed on nectar and other liquid food, such as the juices from rotting fruit or honeydew produced by aphids. Unlike the elephant hawk-moth, however, they also have to obtain food for their larvae which develop in the nest.
Once a wasp colony is well-established, it contains thousands of cells, each with a larva that the workers must feed. The workers are fierce predators and scavengers and will attack and kill caterpillars, flies, butterflies, and spiders.