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CARRION BEETLES &
PHORETIC MITES:
MASTERS OF
ANIMAL DECOMPOSITION

In the out-of-doors, not everything is as pleasing to the eye as our photos last week of a Scarlet Tanager, nor does everything smell as sweet as spring wildflowers. Indeed, there ARE aspects of natural history--although unpleasant to our human senses--that are critically important in the balance of nature. We were reminded of the concept this week when we chanced upon a bustle of activity along a trail beside Hilton Pond. What we found was the remains of a mid-sized Green Frog that had perished from unknown causes. The frog had dried in the sun and was inactive, of course, but its corpse had become a locus of frenetic movement as a variety of scavengers sought nutrients or places to lay their eggs. Although this tale may not appeal to everyone--especially over breakfast--we'll try not to be too graphic and encourage you to continue on to learn about creatures that helped decompose the unfortunate frog.

The dead frog itself was almost two-dimensional, flat against the grass with legs splayed in unnatural directions (above). Since its dark color blended in with the substrate we might have missed it entirely had there not been something--actually, someTHINGS--crawling on it. Atop the carcass was a veritable herd of half-inch-long organisms with black bodies that were yellow-white on the anterior end. We knew from experience these were American Carrion Beetles, Necrophila americana (formerly Silpha americana), and that they were redecorating the frog as a nursery--and larder--for their unborn progeny.

When an animal dies in the woods, it immediately begins to decompose internally as microorganisms in its gut are no longer held in check by natural defense mechanisms. Such initial decay yields an odor--albeit faint--that very, very quickly attracts the attention of flies. These incredibly diverse insects (below)--some with metallic green bodies and others with bright red eyes--arrive within minutes after a dead animal stops breathing. Flies sometimes find sustenance in the corpse, but primarily they look for places to lay their eggs--which in some species hatch within hours to produce tiny white maggots that burrow into a deceased host and help speed decomposition.

Not long after flies descend on the carcass, the first carrion beetles begin to arrive. These are highly specialized insects, with flattened bodies and flexible, sculpted wing covers that allow them to squeeze under dead animals and turn around in tight places most folks don't even want to think about. Typically a carrion beetle's abdomen sticks out past its wing covers, and its pronotum may be brightly colored, as is the case with the American Carrion Beetles shown below and in the topmost illustration. Just above the center of the top photo, there's also a smaller, dull black species of carrion beetle--probably Oiceoptoma inequale (formerly Silpha inaequalis)--the only one we saw like it over two days of observing the frog carcass.

In the ecological sense, carrion beetles don't like flies, mostly because fly maggots compete directly with beetle larvae. As a way to diminish competition, adult carrion beetles begin consuming maggots as soon as the fly larvae hatch. This allows a window of opportunity in which carrion beetles can mate and lay their own eggs before fly maggots have consumed the carcass completely. Unfortunately for the beetles, flies just keep on coming and laying eggs, so it's almost impossible to eat all the maggots that result. It would be great for carrion beetles if they had assistance in keeping flies at bay and, in fact, they do!

Although they're hard to see in the top photo, many carrion beetles on the dead frog were marked by one or more tan-colored dots. Closer examination revealed these dots were moving, and a view through our camera's macro lens showed they were immature eight-legged mites, with some beetles carrying a dozen or more (below).

Although we weren't able to specifically identify the mites--they could be Poecilochirus spp. or perhaps a related genus--we DO know what roles they play in our decomposition drama. First, the mites demonstrate "phoresis," the symbiotic relationship in which a non-parasitic organism hitches a ride on another. Second, after being transported to a corpse by carrion beetles the mites quickly get down to the business of eating fly eggs and even small maggots, thus reducing the fly population and creating more space and food for beetle larvae. Carrion beetles return the favor by hauling the non-flying mites to the next banquet of fly eggs when yet another animal dies. It's even been suggested that some mites perform maintenance on carrion beetles themselves, cleaning off bacteria the host insects are bound to pick up as they explore a carcass.

After a female carrion beetle's eggs hatch, her larvae dine on the corpse as long as food lasts. The beetle larvae eventually fall to the ground and form subterranean pupae, at which point the female beetle flies off to find another corpse--taking some of her mites with her. Meanwhile, back at the old carcass, immature mites left behind may have linked up with beetle pupae underground; thus, when a pupa opens after a few months and releases an adult carrion beetle, the new beetle already has its own complement of mites to transport on its maiden visit to a freshly dead animal.

Of course, the decomposition story doesn't stop with carrion beetles, mites, and flies. Other inch-long insects known as Rove Beetles often arrive and use their strong biting mouthparts to tear at the flesh of a dead animal. Ants and wasps may get in on the feast, and much larger Black Vultures (above right) and mammalian scavengers will pick at the carcass, either consuming the tasty parts or breaking everything else into smaller pieces more accessible to insects. In warm weather when there are plenty of invertebrate decomposers, a small animal the size of our Hilton Pond frog may disintegrate almost completely in a week or so, leaving only bone and nearly indigestible dried skin to mark its passing.

As unappetizing as this tale might seem, we should always be thankful for flies and carrion beetles and hitchhiking mites. Without this assemblage of decomposers our cars would soon be fender deep in dead opossums along the roadside, and frogs that expired through the years at Hilton Pond Center might be piled so high we couldn't search our trails for eye-pleasing Scarlet Tanagers and sweet spring wildflowers.

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