Sunday, September 2, 2012

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.

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