Beetles are classified in the order Coleoptera (sheath-wing). The front pair of wings is hardened into cases (called elytra) that can fold down to sheathe the other pair of wings and much of the rest of the body. Few other insects have these.
There are thought to be over 400,000 species of beetle. That’s about 40% of all known insect species and about 25% of all known animal species. However, some researchers suggest that Diptera (flies) and Hymenoptera (wasps and bees) may actually outnumber the beetles. Nevertheless, many other beetle species remain to be discovered and described, and the total number of existing beetle species has been estimated at over a million.
Titan beetles have the biggest bodies, up to almost 17 centimeters long (almost seven inches). But certain Hercules beetles are a bit longer, with a total length of up to 19 centimeters, thanks to the prodigious horn on the head and thorax of the males. Both live in the tropical rainforests of America. In contrast, the smallest free-living beetle has a body length of less than half a millimeter. It’s found in Central and South America, and it feeds on fungi. By weight, the Actaeon beetle of South America is the heaviest: the larva is bigger than the adult and can weigh over 200 grams (That’s considerably bigger than a typical mouse).
The order is divided by taxonomists into over 200 families, reflecting the great diversity of body styles and lifestyles. One of the largest families is the Staphylinidae, known as rove beetles, with over 60,000 species occupying many habitats and living many lifestyles. Many are predators (one genus specializes on springtails!), but some scavenge, some are mutualistic with mammals, living in the fur and eating fleas, others are parasitoids, laying their eggs on the hosts, which become food for the larvae. We find them under intertidal rocks and roaming over skunk cabbage inflorescences.
One of the most interesting lifestyles of rove beetles involves the exploitation of army ants and termites. Those insects live in immense colonies and bring in prey for the larvae. They are therefore a rich potential source of essential resources, and rove beetles (along with lots of other critters) exploit this. Some are lodgers that don’t harm the larvae directly but feed on the prey that the adults bring in or just hang out on the edges of the colony, eating debris.
Still others integrate into the colony, being fed and tended by the hosts and even tending the host larvae and migrating with the colony when it moves. They are socially acceptable to the ant hosts, because they are mimics of the ants, morphologically and chemically. They have no elytra, a narrow, ant-like waist, long legs, and smell like a colony member. The mimics are often parasitoids, feeding on the host larvae. One sub-group of rove beetles that exploits army ants has evolved the lifestyle at least 12 times, independently, from a free-living ancestor (as most of the sub-group is). Each of those 12 symbiotic types is specialized to one particular type of army ant.
Those that mimic termites have enlarged abdomens, like their hosts. Some are parasitoids, feeding on the hosts’ larvae. But one highly unusual species has even evolved a termite-like “puppet” on its back that entices the termite hosts to feed it. The puppet is an outgrowth of the enlarged abdomen; its “head” sits just above the real, but much smaller, termite head. And it has appendages that mimic legs and antennae. It may eat food delivered by the termites and does not seem to be parasitic. This amazing critter lives in Australia. Don’t you have to wonder just how that puppet evolved — it could not have just sprung up, fully-formed, at the beginning. What might have been the developmental steps along the way?!
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
