With some 400,000 species, beetles are among the largest and most successful groups of organisms on earth, making up one-fifth of all plant and animal species. No other animals exhibit such a dazzling range of size, form, and color. Mostly small, sturdy, and compact, beetles are incredibly well-equipped to find food, reproduce, and avoid predators. Additionally, their collective roles as herbivores, hunters, and recyclers are critical to the sustainability of terrestrial ecosystems. In “The Lives of Beetles: A Natural History of Coleoptera,” beetle expert Arthur Evans presents an inviting and comprehensive introduction to the fascinating lives of the world’s beetles. In the below excerpt, learn about the varied and surprising defense strategies of beetles, from click beetles’ acrobatic maneuvers to bombardier beetles’ boiling bursts of caustic gas. 

Arthur V. Evans is an entomologist, educator, photographer, radio broadcaster, and video producer. His many books include “Beetles of Western North America” and “Beetles of Eastern North America” (both Princeton). He lives in Richmond, Virginia.

This excerpt has been reprinted with permission from “The Lives of Beetles: A Natural History of Coleoptera” by Arthur V. Evans, published by Princeton University Press, 2023.

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Defense Strategies 

Spiders, ants, robber flies, and other beetles rank high among their invertebrate predators. To avoid becoming prey, most beetles rely on morphological and behavioral adaptations.

Predation by birds and other diurnal predators is likely to have played a dominant role in the evolution of cryptic and aposematic (see page 54) coloration in beetles, while the stridulatory, chemical, and non-aposematic defenses of nocturnal species are especially effective deterrents against mammalian, amphibian, and invertebrate predators.

[photo caption] The powerful and oversized mandibles of male European stag beetles (Lucanus cervus) are used in battles with rival males, as well as for defense.

[photo caption] The prosternal spine of the eyed click beetle Alaus oculatus (Elateridae) forms part of a mousetrap-like spring and latch system. This system is used to right themselves when they fall on their backs. The sudden release of energy from the latch launches smaller species into the air at accelerations 100x greater than that experienced by astronauts during a rocket launch.

STRUCTURAL AND BEHAVIORAL DEFENSES

A beetle’s first line of defense is its thick and tough exoskeleton. Hister beetles (Histeridae) possess smooth, hard exoskeletons and can hold their appendages tightly against their bodies—all of which make it difficult for predators to attack and capture them. For larger species such as longhorn beetles (Cerambycidae), stag beetles (Lucanidae), and scarabs (Scarabaeidae), size alone — backed up by strong mandibles, horns, powerful legs, and sharp claws — may be enough to discourage all but the hungriest of predators.

In addition to biting mouthparts, many tiger (Cicindelidae) and ground (Carabidae) beetles have long, slender cursorial legs that are adapted for outrunning predators. Adult flea beetles (Chrysomelidae) use their muscular, jumping hind legs to catapult themselves out of harm’s way. 

When on their backs, click beetles (Elateridae) can right themselves by flipping into the air with an audible click, by contracting their ventral muscles to latch a peg on their prosternal spine to an opposing lip on the mesosternum. Pressure builds and the surrounding softer cuticle is loaded with elastic energy, just like compressing a spring. The peg is soon released when it snaps past the lip, releasing a huge amount of energy that abruptly launches the beetle into the air at accelerations about 300 times that of gravity.

Modified body shapes are considered defensive adaptations, too. The carapaces of tortoise beetles (Chrysomelidae) have broadly flanged margins that shield their appendages from ants and other predators. When attacked, they simply hunker down and remain steadfast on their food plant, aided by the sticky pads under their tarsi. Some Hybosoridae and Leiodidae can quickly roll up into a ball, with their appendages carefully tucked away, and remain motionless for extended periods of time.

[photo caption] When attacked by ants and other predators, tortoise beetles (Chrysomelidae) tightly press their broadly flanged pronotal and elytral margins against the surface of a leaf, thus protecting their legs and underside.

Thanatosis, or death feigning, is employed by hide beetles (Trogidae), some darkling beetles (Tenebrionidae), zopherid beetles (Zopheridae), weevils (Curculionidae), and others. When disturbed, these beetles “play possum” by pulling their legs and antennae up tightly against their bodies and remaining still. Most small predators quickly lose interest in these beetles and move on to more suitable prey.

CHEMICAL DEFENSES 

The defensive chemical compounds of beetles are produced by glands or extracted from ingested foodstuffs and stored in special chambers or in the blood (hemolymph). Ground beetles and predaceous diving beetles (Dytiscidae) possess specialized thoracic and abdominal organs that produce aldehydes, esters, hydrocarbons, phenols, and quinones, as well as various acids. For example, bombardier beetles in the genus Brachinus (Carabidae) can produce small, boiling clouds of caustic hydrogen peroxide gas mixed with hydroquinones and various enzymes that literally explode from their bodies with an audible pop. This potent cocktail is delivered with considerable accuracy through an anal turret. Burying and carrion beetles (Silphidae) emit oily defensive anal secretions that reek of ammonia. Many rove beetles (Staphylinidae) and darkling beetles have eversible anal glands that disperse a wide range of defensive substances. Stink beetles in the genus Eleodes (Tenebrionidae) characteristically lower their heads while raising the tip of their abdomens high just prior to releasing their noxious loads.

[photo caption] When attacked, bombardier beetles in the genus Brachinus (Carabidae) can eject boiling hot clouds of noxious compounds from an anal turret.

[photo caption] Red milkweed beetles (Tetraopes tetrophthalmus) limit their exposure to the plant’s toxins by first chewing through the leaf’s midrib; this bleeds off the latex sap before they begin to feed.

[photo caption] Paederus beetles (Staphylinidae) do not utilize reflex bleeding as a defense mechanism. The beetle’s toxin, pederin, is only released if the beetles are rubbed or crushed on human skin.

While many species protect themselves via camouflage and other cryptic behaviors, others are decidedly conspicuous and stand out among their surroundings. Soldier beetles (Cantharidae), lady beetles (Coccinellidae), fireflies (Lampyridae), net-winged beetles (Lycidae), and blister beetles (Meloidae) are typically sluggish and chemically defended insects that warn potential predators of their foul smell and taste by sporting bright, contrasting colors. Such bold and conspicuous markings that repel experienced predators are called aposematic colors. Bioluminescence in fireflies is also a form of aposematism.

Some beetles co-opt the chemical defenses of their food plants. Boldly marked species of Tetraopes (Cerambycidae) feed only on milkweeds, the leaves of which contain paralytic toxins known as cardenolides. Undeterred, Tetraopes larvae consume milkweed roots and sequester the plant’s toxin in tissues that eventually develop into elytra. Adults eat leaves of milkweed, but they limit the amount of the plant’s toxin they ingest by first chewing through the midrib at the base of the leaf to bleed off its toxic and milky latex.

When crushed or rubbed against the skin, some female Paederus rove beetles (Staphylinidae) release pederin, an incredibly toxic compound. Paederus relies on endosymbiotic Pseudomonas bacteria for the production of pederin. Exposure to pederin, variously known as Paederus or linear dermatitis, spider lick, and Nairobi fly dermatitis, results in a slight rash to severe blistering.

Lady beetles and blister beetles discharge bright orange or yellow hemolymph containing noxious chemicals from their leg joints, a behavior known as reflex bleeding. Lady beetles produce bitter alkaloids that function as a feeding deterrent. Blister beetles exude cantharidin, an incredibly caustic compound that also functions as a powerful feeding deterrent. Lacking their own chemical defenses, male antlike beetles (Anthicidae) obtain cantharidin from dead or dying blister beetles for their own protection and to attract mates. While copulating, the males transfer cantharidin to the female, who then confers this defensive chemical compound to her offspring.

[photo caption] Fungus weevils (Anthribidae), such as Platystomos albinus from Europe eastward through Caucasus and Asia Minor to western Siberia and Mongolia, are masters of camouflage. Their colorings allow them to almost disappear amongst the lichens and fungi living on the tree bark.

CAMOUFLAGE, MIMICRY, AND MIMESIS

Somber-colored longhorn and fungus (Anthribidae) beetles, mottled in various shades of browns, grays, and greens, are effectively camouflaged against the lichen-encrusted bark of trees. Relatively large and seemingly conspicuous species possess disruptive color patterns and/or highly reflective surfaces that, while resting on their food plants, make them look decidedly less beetle-like to predators.

Thought previously to be involved primarily with sexual selection, researchers now postulate that iridescence functions in some beetles as a form of aposematism, while in others it is actually a form of camouflage. For example, the elytra of many beetles are packed with minute dimple-like punctures that reflect light differently from the surrounding surface. From a distance, these tiny bright points of iridescence, combined with a different quality of light reflected off the rest of the elytral surface, produce dull greens and browns that help them to blend in with their surroundings.

BEE AND WASP MIMICS

[photo caption] Lacking defenses of their own, some beetles mimic the appearance or behavior of harmful or otherwise noxious species of insects, an adaptation known as Batesian mimicry. The boldly marked and fuzzy body of Trichius gallicus zonatus, a chafer (Scarabaeidae) from Sardinia and North Africa, as well as other similarly marked jewel (Buprestidae), bumble bee scarab (Glaphyridae), and longhorn (Cerambycidae) beetles, all strongly resemble those of stinging insects, a ruse that is further reinforced by their bee- or wasp-like movements.

Boldly marked checkered beetles (Cleridae) jerkily running along the tree limbs strongly resemble stinging ants or wingless wasps known as velvet ants. But stinging insects are not the only models for beetles seeking protection. Some species of click beetles (Elateridae) and longhorn beetles (Cerambycidae), along with various moths and cockroaches, mimic distasteful fireflies (Lampyridae), soldier (Cantharidae), and net-winged beetles (Lycidae).

Müllerian mimicry involves two or more defended species inhabiting the same region that share similar aposematic markings. Predators quickly learn to avoid them, thus protecting all similar-looking species. Although best known in butterflies, Müllerian mimicry complexes do occur in beetles, especially among boldly marked and chemically defended net-winged beetles that include similarly colored and unpalatable moths.

Eyespots and sudden flashes of color may startle or confuse would-be predators. The outsized eyespots of eyed click beetles (Elateridae) are thought to momentarily confuse or startle predators, but this hypothesis has yet to be rigorously tested. When buried head first in a flower, the bold eyespots on the backsides of Trichiotinus beetles (Scarabaeidae) may suggest the face of a stinging wasp. Dull-colored jewel and tiger beetles often reveal possibly startling flashes of bright iridescent colors when they lift their elytra to take flight.

Mimesis is a form of mimicry where an organism resembles an inanimate or neutral object from the point of view of a predator. Small and chunky warty leaf beetles (Chrysomelidae) are presumably overlooked by predators and insect collectors alike because of their strong resemblance to caterpillar feces. Hide beetles (Trogidae) and minute, mud-loving beetles (Georissidae) are frequently encrusted with dirt or mud and resemble pebbles or small earthen clods, while some Gymnopholus weevils have plants and fungi growing on their backs.

[photo caption] Trichodes alvearius, a checkered beetle (Cleridae) widely distributed across Europe and north Africa, relies on its bold markings, or aposematic coloration, to discourage attacks by predators. They are thought to be part of a mimicry ring that includes toxic blister beetles (Meloidae) and burnet moths (Zygaenidae).

[photo caption] It has long been suggested that the oversized eyespots of Alaus oculatus, an eyed click beetle that is widespread in eastern North America, might startle or confuse predators, but this hypothesis has yet to be carefully tested.