Beetle-mania

expression of an inordinate fondness

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.

beetles-on-skunk-cabbage-by-bob-armstrong
Rove beetles on skunk cabbage. Photo by Bob Armstrong
beetles-mating-on-skunk-cabbage-by-bob-armstrong
Rove beetles mating. Photo by Bob Armstrong

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.

Seasonal body modifications

surprising changes in size and shape

A relatively simple –and familiar to most of us –kind of seasonal and reversible body-modeling occurs in bears preparing for hibernation, and beavers getting ready for snoozing in their lodges, or deer anticipating food shortages in winter. These species all put on fat in fall, becoming distinctly more portly. Come spring, they are more svelte, having burned up that winter fat. Humpback whale females migrating from Hawaii to the food-rich northern waters are much slimmer than in fall when on their way south. Similarly, migrating birds typically put on fat before they migrate and arrive at their seasonal destinations in trimmer condition.

Sometimes the reversible remodeling occurs in internal organs. Bar-tailed godwits and other birds that migrate very long distances without feeding on the way markedly reduce their digestive organs and regenerate them and resume feeding upon arrival. Chickadees, jays, nuthatches, and nutcrackers get bigger brains in fall; the part of the brain associated with processing spatial information is the hippocampus, which actually acquires more neurons. The temporarily larger hippocampus allows them to remember the widely scattered locations where they have stored seeds and retrieve them during the winter.

Brood-parasitic birds, which lay eggs in other birds’ nests, also have improved spatial memories—just in the nesting season. Two species of cowbirds in which the females monitor the locations of potential host nests have enlarged brains (hippocampus) in the nesting season. But in a third species of cowbird, in which both male and female monitor host nests, both sexes have seasonal changes in brain size.

Males of other birds (for example, starlings), get temporarily enlarged brains, especially the parts associated with singing, in spring, when males sing to advertise territory and attract females. European titmice that engage in strongly seasonal singing have corresponding seasonal changes in brain size, while those that vocalize year-round do not. Captive house sparrow males kept near females sang more often and had bigger brains than those isolated from females. These studies have focused on male birds and the production of song, but what about the females that listen to those songs?

It seems that the more we look, the more instances of seasonal remodeling we find. Nevertheless, I was not prepared to learn of recent work showing that skulls, specifically the brain case (and the enclosed brain) get seasonal changes too. Even though these bones are well-ossified and hard, with good, firm sutures between the various cranial bones, they too can be reversibly remodeled on a seasonal basis. These studies were done with the common red-toothed shrew of Europe and with two species of weasel (one called stoat in Europe and ermine or short-tailed weasel in North America, and one called weasel in Europe but least weasel here). Although all three species show the seasonal changes, there are interesting differences among them and between male and female in one species.

All three species showed a change in skull shape as the animals matured, from a more rounded and relatively large juvenile skull in their first summer–when they disperse and establish their individual territories–to a more flattened, relatively smaller adult form in their first winter. Then, each species showed a temporary, reversible increase in braincase size in the second spring and summer, with the acquisition of an increased behavioral repertoire involving mating and territoriality. There is then a subsequent winter decrease, accompanied by thinner skull bones—with hints that there were greater seasonal changes in geographic areas with more severe winters. At least in the shrews, there is no evidence of increased numbers of neurons, but in all three species different parts of the brain are involved in the increase and in the decrease. Precisely how this is achieved is not clear.

The researchers suggest that this skull remodeling is a way to conserve energy for these small mammals that have extremely high metabolic rates and high levels of activity all year long. Their slim body design, with short legs, is not conducive to putting on much fat and hibernation is not feasible.

However, while both male and female shrews exhibited approximately the same pattern, the weasels differed from each other. In the least weasel, after the winter decrease in braincase size, only males showed the spring-summer regrowth. There was no regrowth in the skulls of females. By contrast, in the ermine/stoat, both sexes exhibited a summer increase after a winter decrease.

The researchers related the variation in gender differences to the life histories of the respective species and genders. Both male and females shrews defend territories vigorously requiring much activity and perhaps recognition of neighboring individuals. Least weasels are very short-lived, seldom living more than a year or two, and females often invest resources in reproduction even in their first summer. They may not have enough resources to invest also in skull regrowth and little chance to gain by it, given the short life expectancy. Males, on the other hand, are busy with territorial defense, for which a larger appearance is often effective. Stoats/ermine are longer-lived and females don’t reproduce so quickly in the life history. Resource expenditure in reproduction is more spread out in time, and investment in skull regrowth may be more possible and more likely to lead to a possibility of future reproduction.

Winter wanderings

In early January, the ice on the ponds in the Dredge Lake area was good and solid, although there were isolated spots of open water where upwellings slowed the formation of ice. I traipsed around some of the trails and ponds, finding tracks of shrews, hares, and a mouse. Otters had slid over a beaver dam and then up a frozen slough, no doubt hoping to find a fish or two.

One day in mid-January, a friend and I explored a frozen pond, walking on snowshoes to spread out our weight, in case of a spot of weak ice. A little snow was falling, so it was a beautiful walk.

Beavers had made a small food cache near their lodge, including some hemlock branches. There were lots of spider webs and long, trailing silk threads used by airborne spiders. We wondered if any critters, in addition to some spiders, eat that silk to recycle the protein.

Around the bases of several trees at the edge of the pond, we noted the tracks of a small bird, probably a junco. It had apparently inspected each tree base quite closely, possibly picking insects from the spider webs that curtaining the gaps between the upper roots or searching for stray seeds.

A vole had crept out of one bank of a frozen rivulet, crossed he ice, and scuttled back to where it came from. My companion had observed such behavior in other places when the animal was seen to be a red-backed vole, so we assigned that perpetrator to those tracks. Deer tracks crisscrossed the pond ice, and deer had been feeding on the witches’ hair lichens that grew on small trees at the pond edge. My sharp-eared companion heard a brown creeper, which we soon saw as it hitched its way up a spruce trunk.

Many of the alders in this area had neither cones from last summer nor any male catkins for next spring. This was unlike other alder stands we’d seen, so we wondered why this stand was evidently reproducing very poorly. Perhaps the high level of water in the pond was too much for them.

We also noticed that here and in some other places the alders had retained many of their dried and shriveled leaves, instead of letting them drop to the ground. Blueberry shrubs sometimes do this too. In other regions, oaks, beeches, and other trees also retain many of their dead leaves throughout the winter. The term for retention of withered old flowers or leaves is ‘marcescence’. Marcescent leaves have attracted a good deal of speculation about why these plants do this, such as deterring deer and moose browsing, trapping snow for release of moisture in spring, or delaying decomposition until spring, when nutrients are most needed for growth. However, apparently very little investigation has explored those ideas. In some cases, particularly when marcescence is occasional and not regular, the retention of dead leaves may just happen incidentally because the weather suddenly changed in a way that prevented the usual mechanism of leaf-drop (formation of the cut-off or abscission layer at the base of the leaf).

A few days later, along the Auke Lake trail, (and later in other places) we noticed that many of the blueberry bushes had small galls on the twigs, often at the bases of marcescent leaves. The galls are really quite small, and I have to wonder how many times I have walked past them without noticing. They do seem to be more conspicuous against a snowy background…Some blueberry galls are made by midges or wasps, but these did not fit the descriptions of such galls, so the makers of these galls remain to be determined.

Toward the end of January, I went with a friend on the Pt. Bridget trail. Near the trailhead, we found a place where a weasel (I think) had fossicked about in the mud at the bottom of a hole in the snow, and come up to leave a string of its small, muddy footprints on the snow, before diving back down under the deep snow in a new spot. Somewhat to my surprise, the lower branch of the trail, along the edge of the big beaver meadow, was quite passable, provided one didn’t mind a couple of inches of water here and there. A moose had used the trail too, taking advantage of a deeply trenched part of the path—and a small wooden bridge—to avoid some of the post-holing that was required in the rest of the meadow. The bible-camp horses had left ample evidence of time spent on this side of Cowee Creek, on the beach fringe as well as in sheltered places under the conifers. Pawing away the snow and stirring the long, dead grasses, they also had clearly been looking for precocious green shoots under the snow…and had found a few.

As we left the area near the cabin, my companion spotted an owl, probably a short-eared owl, as it swooped down to some bare ground next to a tidal slough (the tide was out). It was probably trying to catch an unwary rodent, but we could not be sure it was successful. It soon flew up into the nearby trees, changed perches, and eventually took off across the wide meadows, screened from clear view by tall spruces.

A day or two later, when Plan A for a beach-walk was foiled by ferocious north winds on Lynn Canal, another friend and I eventually found a sheltered beach near Amalga Harbor. Moving slowly and quietly, we managed to share the beach with a trio of common mergansers that paddled slowly along the tide line. Then they all hauled out and snuggled up in a close-packed row to sun themselves.

I have learned a new word for verbal bric a brac like that found in this essay: bricolage. ‘Tis a very useful word for assortments of diverse things brought together in some more or less unifying way. There may be more bricolages here in the future.