Invertebrates
Beetle
Coleoptera · also called beetles
If you picked a named animal species at random, the odds are close to one in four that it would be a beetle. No other group of animals comes near. The reason is a single piece of engineering — the front wings turned into a hard case — combined with more than 300 million years to put it to use.
Overview
Beetles are the order Coleoptera, around 400,000 named species and probably over a million in total, on every continent except Antarctica. The one feature they all share is the elytra, forewings hardened into covers that meet in a straight line down the back over folded flight wings. Beetles have chewing jaws and go through complete metamorphosis, and some species eat almost every kind of organic matter there is.
Why the case matters
The elytra let a beetle push into soil, rotten wood, bark, and leaf litter — abrasive, tight places barred to soft insects — while keeping its flight wings safe and folded underneath. Combined with a larva that lives and feeds somewhere completely different from the adult, this opened an enormous range of ways to make a living, and beetle lineages spread to fill them, especially alongside the flowering plants.
Chemistry and size
Beetles are heavily armed. The bombardier beetle fires a boiling, caustic spray in pulses; blister beetles bleed the toxin cantharidin. Sizes span four orders of magnitude, from featherwing beetles under half a millimetre to the titan beetle at 17 cm and the goliath beetle among the heaviest insects alive.
Quiet declines
Most beetles are common, but many specialists are slipping. Dung beetles are poisoned by veterinary drugs passing through livestock; dead-wood beetles vanish with old forest; flightless island and cave species are highly vulnerable. Broad studies of insect biomass show sharp regional losses that include beetles.
Status
Beetles as a group are Not Evaluated. The order as a whole is in no danger, but a large number of individual species, tied to old trees, natural dung, or single islands, are threatened or already gone.
Beetle: common questions
Why are there so many kinds of beetle?
Several advantages have stacked up over a very long time. The hard wing cases let beetles exploit tight, abrasive spaces such as soil, bark, and leaf litter that are closed to soft-bodied insects, while still keeping the ability to fly. Complete metamorphosis splits the life cycle so larvae and adults do not compete. Beetles diversified alongside the flowering plants, with lineages specialising on particular plants, fungi, or dung, and each new plant group opened new beetle niches. They are also an old order, over 300 million years old, and have survived mass extinctions. The biologist J. B. S. Haldane is said to have remarked that a creator, if one exists, must have an inordinate fondness for beetles.
What is the difference between a beetle and a true bug?
They are different insect orders with different mouths and wings. Beetles, order Coleoptera, have chewing jaws and hard forewings that meet in a straight line down the back. True bugs, order Hemiptera, have mouthparts fused into a piercing-sucking beak and forewings that are usually half leathery and half membranous, overlapping to form an X or triangle pattern on the back. In everyday speech people call almost any small crawling insect a bug, but entomologists reserve the word for the Hemiptera, which includes aphids, shield bugs, cicadas, and bed bugs, and none of those is a beetle.
Are beetles helpful or harmful to people?
Both, depending on the species. On the harmful side, beetles such as the boll weevil, Colorado potato beetle, various grain and flour beetles, and wood-boring species cause large agricultural and structural losses, and bark beetles have killed vast areas of forest. On the helpful side, dung beetles bury and recycle livestock waste worth billions in free ecosystem services, ladybirds and ground beetles eat crop pests, carrion beetles clean up carcasses, and many beetles pollinate plants or serve as food for other wildlife. The great majority of beetle species have no direct effect on human affairs at all.
How do bombardier beetles spray boiling liquid?
They carry two separate stores of chemicals in the tip of the abdomen, one holding hydrogen peroxide and hydroquinone and the other holding enzymes. When threatened, the beetle lets a dose of the first mixture into a thick-walled reaction chamber containing the enzymes, which trigger a fast reaction that releases oxygen, generates heat, and drives the temperature to around 100 degrees Celsius. The pressure fires the caustic, near-boiling spray out through a nozzle the beetle can aim, in a rapid series of pulses that also protects the beetle's own tissues from overheating. It is enough to make birds, frogs, and ants drop the beetle at once.