There are many more species of fish (about twenty-eight thousand species) than of any other kind of vertebrate (amphibians, reptiles, birds, and mammals). And there are about thirty thousand species of orchid, in a single taxonomic family. But those seemingly impressive numbers fade into the background in comparison to beetles.
Numbering well over three hundred fifty thousand (and counting), there’s a species of beetle for every ecological job—predators and parasites, herbivores and detritivores, scavengers and pollinators. They range in size from a giant six or eight inches long down to a wee thing only a fraction of a millimeter in length. Beetles have been around for a long time. Their fossil history begins before that of bees and ants and long before that of butterflies. Beetles appeared at least two hundred thirty million years ago, already diversified in their ecologies. Their diversity got a boost from the appearance of conifers, and then again from the arrival of the flowering plants, as they began to exploit these new resources. In fact, they were probably the first insects to pollinate the early flowering plants, since there were no bees or butterflies yet.
The phenomenal diversity of beetles is impossible to capture in a short essay. So let’s reduce the problem (slightly) by considering selected taxonomic families: the weevils or snout beetles (Curculionidae) and the rove beetles (Staphylinidae). These two families are probably the largest, in terms of the numbers of species. And I can’t altogether leave out three other interesting, large families…
Consider first the weevils, with over eighty thousand species, according to some taxonomists. Most weevils feed on the flowers and leaves of flowering plants (which now number hundreds of thousands of species). With that long snout, they bite and chew the plant tissues. The most notorious species is perhaps the boll weevil, which feeds on flower buds and fruit of cotton and ravaged U.S. cotton crops in the 1900s. A few weevils are aquatic, including some that feed on native and introduced water milfoil. One acts like a dung beetle, collecting the dung of Australian wallabies for raising their larvae.
One branch of the weevil family includes the bark and ambrosia beetles, which can wreak havoc in conifer forests. There’s a variety of ambrosia beetles, whose adults and larvae feed on ambrosia fungi that grow on wood. Some of these beetles even carry bits of the fungus in special pockets, and so they inoculate new tunnels under the tree bark.
The rove beetles number at least sixty thousand species, with vast numbers still uncatalogued by taxonomists. Most of them are small and inconspicuous, often living in leaf litter, under loose bark, in caves, and other places that are usually beneath our notice, scavenging whatever they can find. Some feed on carrion, a few are external parasites on fly larvae, some feed on fungal spores, and some burrow in shoreline sand to feed on algae and diatoms. Here in Southeast, one species of rove beetle is the chief pollinator of western skunk cabbage.
Many rove beetles are predaceous, often on mites, round worms, and fly larvae. For example, some species hang out in the dung of ungulates (deer, cattle etc.) and eat fly larvae while others maim and eat adult scarab beetles. Some species are specialized to live in the nests of birds, rodents, and even gopher tortoises, where they prey on the larvae of fleas and flies. One type of rove beetle has a very specialized and obligate relationship with neotropical figs, which are pollinated by specialized wasps, whose larvae grow up inside the fig. Inside the fig, the beetle adults and larvae feed on the pollinating wasps.
Many species of rove beetle live in the colonies of termites, ants, bees; one even inhabits the nests of a communally nesting butterfly in Central America. They are ‘inquilines’ or tenants in the host nests. Although some of them just scavenge in the middens of the ants and termites, others are predators, parasites, or kleptoparasites (stealing the food of the hosts); these species are well disguised, so that the hosts do not eject or kill them. They actually smell like their hosts, and in some cases they also look like them. Some rove beetle tenants secrete substances that calm the worker ants or even entice the workers to retrieve tenants that wander too far away. The rove beetles that live in certain termite colonies have huge glands that secrete drops that the termites love to eat. Some tenants are even treated as members of the colony, fed and protected as if they really belonged; in fact, these tenant beetles sometimes tap on the mouthparts of the host ants to elicit a feeding.
Two other large taxa are somewhat related to the weevils: The leaf beetles (Chrysomelidae), with at least thirty-eight thousand and perhaps as many as fifty thousand species, feed on plant material in a variety of ways, including leaf mining. The longhorn beetles (Cerambycidae), with maybe thirty-five thousand species, are plant-feeders too; the larvae dig into wood or mine the nutritious under-bark.
I can’t omit the so-called ground beetles (Carabidae), with over forty thousand species. Many of them are ground predators, but some eat seeds, some are inquilines in ant nests, some feed on slime molds, and some have parasitic larvae. Some eat snails by sticking the long snout into the snail shell and pulling out the resident snail. The well-known tiger beetles are thought to be the fastest running insects, chasing down their prey out in the open. One species is so quick that it can catch springtails in mid-jump. This family includes the famous bombardier beetles, which spray hot and caustic secretions at any attackers.
That’s just a sketchy introduction to some of the diversity of beetles. Beetle taxonomists and other researchers spend lifetimes immersed in the relationships and ecological stories about beetles. There is always more to learn and some of that will be surprising.