Wild Side: Dung flies

Every creature has its place and its job to do.


In the natural world, everything is a resource. Even the most trivial or improbable niche has its occupier. Perhaps the most glaring example of this principle would be astonishing fecundity of life supported by animal dung.

This is no news to you if you’ve ever hung around livestock. Stables, paddocks, pastures, and feedlots are typically rife with flies that feed on or breed in manure. And there are entire families and subfamilies of insects that have evolved specifically to exploit this unappealing but fertile resource.

The key thing is that the digestive process of an herbivore breaks down the otherwise durable structure of plant cells, quite literally freeing the nutrient-rich cell contents from their shell. Many of those nutrients are absorbed through the intestines to support the life and growth of the herbivore; from a cow’s perspective, of course, that’s the whole point of digestion.

But the process of absorption is by no means complete, and what a cow or horse intestine tosses back into the environment is basically plant matter with the hardest part of digestion already taken care of. That’s why manure is a good fertilizer, and why so many arthropods associate with dung.

The point was driven home to me on a recent sunny morning as I naturalized at the Land Bank’s small but delightful Little Duarte’s Pond Preserve. Tracks on a public walking trail showed that a single horse had passed by a few hours before I did; the horse had thoughtfully left behind a pile of manure that, by the time of my arrival, had attracted a swarm of flies.

A quick scan with my binoculars suggested that multiple fly species were present, so I shortened the range. It was a perfect set-up: dung fresh enough to be irresistible to flies, good light for photography, and just a bit of breeze so I could hunch over the upwind side of the dung and photograph in complete comfort.

First on the agenda was photographing one of the most beautiful flies I know: Scathophaga stercoraria, also known as the golden dung fly. Males of this species are covered in yellow fuzz, and it’s not a subtle yellow, either. Females, alas, lack the fuzz and, for some reason, are much less numerous in most situations. In this case, I found only one female, but about a dozen of the garish males were chasing each other and exploring the pile of manure.

The females of this species lay their eggs on dung, and that is where the larvae develop. Adults of both sexes visit dung for reproductive purposes, and perhaps they feed on it some, as well. But primarily, adults are predators on other flies. Though Scathophaga is in a group of flies that generally has spongy mouthparts, adapted to soaking up liquid food, in this genus those mouthparts have evolved into a stiff spike capable of piercing the exoskeleton of other insects. I watched this process take place, as a blow fly wandered too close to a male dung fly. Wham! In seconds, the dung fly had pounced, enveloped its prey in its densely spined legs, and subdued it with a stab to thorax.

A second very attractive fly that caught my eye proved to be Physiphora alceae. In the family Ulidiidae, also known as picture-winged flies, this elegant little job sported an iridescent green body and distinctly striped eyes. Adults of this species are said to feed solely on flowers. But this is another fly that lays its eggs on dung (or sometimes, just decaying plant matter). I only found a single individual of this species, which may have been a female getting ready to lay eggs or a male looking for a female to mate with.
In addition to these two stunners, the dung pile hosted two very familiar representatives of the blow fly family (Calliphoridae). As a family, blow flies both feed on and lay eggs on a wide range of unpleasant material: Dung, but also carrion and rotting plant material. In this case, members of two genera were present, Calliphora (“blue-bottle flies”) and Lucilia (“green-bottle flies”). Determining which species I had found probably would have required taking specimens and examining them under magnification; the key details are very hard to see in the field, or even in a photograph.

Similarly, a small, streaked gray fly defined precise identification; I recognized it as a member of Muscoidea, the same broad group that contains the familiar house fly, but have learned that precise identification of such flies from photos is often impossible. I had better luck IDing — though not photographing — two other types of flies: a single miniscule, antlike number in the family Sepsidae, which marched about constantly flashing its wings like flags, and an abundant but tiny and constantly moving species, Sphaerocera curvipes, in a family known, appropriately, as lesser dung flies. Scores of these were present.

So the final tally was seven species representing six families (that’s just the flies; I haven’t even mentioned the beetles!). Two of those families (and one beetle family) were completely new to me. Such is the magical draw of fresh dung, and now you know why I can’t pass a pile of manure without zeroing in for a look.