Soggy Sunday Soldier Flies: Stratiomyidae

Yet another hornet mimic - Soldier Fly Stratiomys sp.

One can generally tell a fly mimic from a social wasp or hornet model most easily by looking at the head: the antennae are usually a dead giveaway. Wasps and hornets have geniculate antennae, that is a few long basal segments joining a multi-segmented flagellum at about a right angle. Although 'geniculate' comes from the Latin for knee (genu, as in genuflect), the less technical term is 'elbowed antenna'. In the social wasps and hornets (and their solitary relatives), their elbowed antennae are typically much longer than the head is wide.

Long, elbowed (geniculate) antennae of Bald-faced Hornet

Although thread-horned nematoceran midges, mozzies, crane flies, and the like have long antennae with more or less distinct segments, almost all fly mimics are in the suborder Brachycera (Greek for short + horn). The brachyceran antenna is composed of a few distinct basal segments sporting a finger-like style or bristle-like arista composed of the remaining segments shrunken into obscurity. 

Non-mimic Soldier Fly with antenna ending in a style Odontomyia cincta Olivier, 1811

Most flies, including most mimics, have antennae shorter than the head is deep and rarely as long as the head is wide. That seems enough for their purposes, which may include everything from smell, to sensing gravity, to hearing.

Blending into the vegetation seems the goal of this Soldier Fly's colour pattern

Yet, as usual, there are exceptions. Some soldier flies (Family Stratiomyidae) in the genus Stratiomys (Greek for army or soldier + fly) do their best to look like hornets including sporting unusually long antennae. Each antennal segment seems to have been stretched by evolution to its maximum misleading length and elbowed to boot. 

Male Stratiomys sporting rather waspish antennae

Our previous very good wasp mimic Spilomyia sayi (Syrphidae) also has unusually long aristate antennae - much longer than in most Flower Flies except those like the exceptionally good yellowjacket mimics in the genus Chrysotoxum. It seems a bit strange to me that antennae seem so integral to a good wasp constume, but then social wasps are very particular about how they look. If fooling wasps is a primary goal of these mimics, then putting on a waspy face may be important.

Mystery male Soldier Fly apparently sending chemical messages

In general, insects lead far more complicated and interesting lives than we generally give them credit for. Of course, we know relatively little about even the best studied insects, so we make our assumptions in ignorance. And then most are so small and seemingly inconsequential.

Actina viridis (Say, 1824) - the most common Soldier Fly in the Home Bug Garden

The larvae of Soldier Flies are often aquatic or semiaquatic or at least inhabit wet soil, rotting vegetation, dung, or compost. They seem to mind their own business and not cause us problems, so they are likely to remain mostly obscure. Sometimes they are very common, like the little metallic green Actina viridis (Say, 1824), which perhaps breeds in our compost bin (or at least the adults hangout around it).

Microchrysa polita (Linnaeus, 1758) - a tiny Soldier Fly with a common  name - Black-horned Gem

Few of the 250 or so species in North American seem to have common names other than solider fly, but some with wider distributions have attracted some whimsy. At least one species, the Black Soldier Fly Hermetia illucens (Linnaeus, 1758), is actually useful in composting, as chicken feed, and frog and lizard pet food. This is largest soldier fly that I've ever seen - and a good wasp mimic with very long antennae like the rest of its genus Hermetia. So, here we can end this soggy Sunday morning on a positive note and a good insect.

I just want to bee ... Mistaken for a wasp by wasps

Spilomyia sayi (Goot, 1964), not a wasp, just a good mimic

Every summer from late July through August an unusual flower fly (Syrphidae) shows up in the Home Bug Garden. Many, if not most, flower flies are brightly and contrastingly coloured more or less like wasps or clothed in furry hairs like bees. But some look more like the stingers than others. That is true of this fly, Spilomyia sayi (Goot, 1964) (known in earlier works by the preoccupied name Spilomyia or Paragus quadrifasciatus Say,1824). 

Two syrphids of varying waspishness - both in looks and behaviour

As well as being coloured like many a vespine wasp, its antennae are longer and more wasp-like than most syrphids, its wings waspishly dark, and its demeanour as it struts across the heads of yarrow and goldenrod as recklessly fearless as any hornet. Most flower flies dodge quickly away when approached, but not Spilomyia sayi. In fact, this fly is easy to catch by hand - in which case it emits a loud and very bee-like buzz.

Flies have died from time to time, and birds have eaten them, but how about wasps?

But who is Spilomyia trying to fool? Well, it is pretty good at fooling people, but the usual argument for this kind of Batesian Mimicry is bird brains. I suppose birds do eat flower flies, from time to time, but how often does a bird swoop down on a goldenrod or yarrow in a field? I can't say that I have ever seen this happen, but I have seen many a fly scarfed up from such flowers by patrolling hornets. Could it be that these flies just want to be left alone by their fellow insects as they gather some nourishment on flowers that are often covered in wasps (many of which are stingless males)? I suspect that predatory wasps and hornets have exerted more evolutionary pressure on these flies than any bird. Alas, behavioural ecologists seem obsessed with birds, so I am not sanguine about seeing any studies that test this idea.

Blackjacket (Vespula consobrina) - a likely model for and predator on our local Spilomyia