How Not to Be Eaten: The Insects Fight Back

Michelle Harvey gets caught up in the evolutionary arms race of six-legged predators and prey

February 23, 2012

The thought of a common cockroach scurrying across the kitchen floor strikes fear into the hearts of some of the strongest people I know. The knowledge that this brilliant beast can flee at a speed of 2.9 miles per hour and contains a light sensor in its rear end makes it a veritable vision from the realms of science fiction. Diversity and adaptability of this magnitude have equipped insects to become one of the most successful groups on Earth.

As US entomologist Gilbert Waldbauer observes, the insects compose some 75 per cent of the total animal species currently found on land and in aquatic environments. In a single acre of land they outweigh and outnumber all other animals, large vertebrates included. It is no surprise that such a taxonomically rich and abundant group have highly evolved mechanisms that enable them to avoid predation and proliferate. Many insects produce hundreds of offspring in their lifetime, yet each female insect needs produce only two offspring that survive to reproduce, in order to maintain the insect population at stable numbers. Given that most females produce countless offspring, insects must form an important food source for other organisms, or their numbers would spiral out of control.

Insects are wonderfully condensed sources of nutrition. As Waldbauer notes, eating other insects allows non-plant-feeding insects to draw, second-hand, on the energy of the Sun. Humans have long recognised the fantastic source of nutrition offered by insects, and they likewise serve as an appetising meal for birds, reptiles, frogs, mammals and fellow arthropods. This does not mean they willingly become someone else's lunch, though, and Waldbauer leads the reader on a fascinating journey through the escape and defence mechanisms of our six-legged friends. Written in an engaging style suited to anyone interested in science, biodiversity, behaviour and adaptation, How Not to Be Eaten offers an interesting look at some of the most bizarre insects in the world, alongside largely unknown, fascinating morsels on some of the more familiar ones.

Many insects feed on fellow insects and so have evolved useful feeding and hunting methods against their potential meals. Robber flies catch their insect prey mid-flight, inject them with poisonous secretions to liquefy their prey's internal tissues, and then suck their conquest dry. Ant lions are likewise effective predators on insect relatives - their larvae dig conical pits and lie in wait at the bottom, with only their enormous jaws exposed, awaiting the descent of an unwitting creature.

In order to evade predation, many insects opt simply to hide. Insects such as cockroaches display nocturnal behaviour and avoid light; others will hide their young when they are at a vulnerable stage of development - relatively immobile with soft, defenceless bodies - from organisms seeking an easy meal. Waldbauer's chosen example is the mason bee, Osmia bicolor, whose female deposits her eggs in empty land snail shells provisioned with "bee bread" (pollen and honey) and covers them with a protective roof of grass and twigs.

Insects are endlessly skilled at concealment, with stick insects in particular known for their ability to mimic vegetation. Waldbauer also details the complex case of the famed peppered moth, Biston betularia, that displayed visible change to a darker form in response to the Industrial Revolution, to better conceal itself against tree trunks darkened by air pollution. After the passing of the Clean Air Act of 1956, natural selection resulted in reversion to the lighter coloration. Insects' adaptability has been key to their persistence and success.

Waldbauer convincingly establishes here that insects form a critical part of the intricate food web of life, and human interference in this web can have disastrous effects. Insects are beneficial as biological control agents, and none of their roles is more important than keeping their fellow insects in check. Cottony cushion scales from Australia found their way into Californian orange groves in 1887, threatening economic disaster. The introduction of a natural enemy from Australia, the vedalia beetle, to eat these scales resulted in the reduction of scale populations in just two years, leading to the peaceful coexistence of the two species at low densities. But indiscriminate spraying of the insecticide DDT, starting in 1945, reduced the numbers of the vedalia beetle but not the scales, and the balance of predators and prey was again disrupted.

A chunk of Waldbauer's book is devoted to examining how non-insect predators have developed elaborate mechanisms for successfully capturing six-legged prey. The robin-sized burrowing owl exploits the food preferences of its prey by scattering dung at the entrance to its burrow as bait for the dung beetles on which it feeds. Other birds, such as woodpecker finches, use tools in the form of twigs and cactus spines to extract insects from crevices and holes in trees. Some birds have been reported to prey on stinging insects by squeezing their abdomens against perches to remove the venom from the sting gland before eating them. It is this tendency for predators to try to overcome an insect's defences that keeps the evolutionary arms race going, with prey and predator in a battle to outwit and overcome, with natural selection crowning the victor.

A swarm of insects may be the stuff of nightmare for us, but for some insects it is a way of meeting the perfect mate (think of it as speed dating for insects). Waldbauer explains the importance of grouping together as the dilution effect, whereby the chances of becoming prey are distinctly reduced in a group, because there's at least a fair chance that your neighbour will become someone's breakfast instead of you. Insects such as cockroaches use pheromones to locate each other and shelter together, raising their own chances of survival. Insects also cluster together for defensive purposes to collectively disseminate pheromones to warn of the presence of a threat.

But as with human warfare, it's not always a simple choice of running and hiding, or passively blending in. Sometimes defence is required and, as the author shows, insects are masters of chemical and morphological warfare. A number of species are able to spray predators with chemical repellents that can burn their mouth parts or glue them together. The spectacularly well-equipped bombardier beetle has abdominal glands containing compartmentalised hydrogen peroxide and hydroquinones. When the beetle is threatened, the substances are mixed in a single chamber with enzymes; this produces a large amount of heat and irritating quinones which are blasted from its abdomen in a directed attack. Like humans, insects use bright reds, oranges and yellows to warn predators of danger, flashing bright warnings on their hind wings at potential attackers. Predators quickly learn that insects with such colours may be distasteful or inflict pain and will avoid them. And, just like human society, the insect world has its share of cheats that morphologically mimic their distasteful cousins, despite their own palatability, and gain inferred protection by looking dangerous.

In the book's epilogue, Waldbauer describes the relationship between predator and prey population numbers as a "seesawing dynamic equilibrium". Like any supply and demand situation, predator numbers grow in the presence of high numbers of prey, and as prey numbers fall, predator populations also diminish. It is this delicate equilibrium, alongside an evolutionary arms race, that makes insects - often dismissed as simple, invisible and expendable - one of the most elaborate and fascinating groups of organisms on Earth. Waldbauer's book presents predation on insects and their corresponding defences in an entertaining light that will leave you gazing in wonder at that cockroach. For at least a few seconds, anyway, before you squash it.

The Author

Connecticut-born Gilbert Waldbauer is a prolific popular-science author, emeritus professor of entomology and a member of the American Birding Association.

As a schoolboy, he and a friend accompanied a biology teacher on weekend birding expeditions to nearby wildlife hot spots, longer trips being precluded by wartime rationing of petrol. It was on one such outing that Waldbauer saw his first yellow-crowned night heron as he squinted through a 3-power monocular - "actually half of a pair of ancient field glasses that I bought for 50 cents in a pawn shop".

After a spell in the US Army in 1946-47 earned him the chance to attend university under the GI Bill, Waldbauer began undergraduate studies in entomology at the University of Massachusetts at Amherst. He signed up for the one ornithology class on offer, and that led to his decision to pursue a career researching the interrelationships between birds and insects.

By 1953, he was a teaching assistant in entomology at the University of Illinois, Urbana-Campaign, and he remained at the institution until his retirement in 1995.

How Not to Be Eaten: The Insects Fight Back

By Gilbert Waldbauer

University of California Press

240pp, £19.95

ISBN 9780520269125

Published 13 February 2012

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