While walking the grounds of my company’s experiment station in Fontezuela, Argentina, I encountered a massive European elm (Ulmus laevis)—its trunk enveloped by an unidentified woody vine with large, ball-clusters of small, green flowers. Despite their inconspicuous appearance, the flowers were highly attractive to insects, primarily honey bees and smallish, black and yellow vespid wasps. One of the wasps caught my attention—it was not quite as narrow as the others, and it flew a little differently. Closer inspection, of course, revealed that it was not a wasp after all, but rather a fly. I wasn’t sure what it was, but it was extremely flighty and wouldn’t allow me to get close enough even to attempt looking at it through the viewfinder—much less going for a composed shot. Now, if it were a tiger beetle I’d probably spend the next 2 hours “working” it to get that in situ shot. But, hey, it’s just a fly (with apologies to my dipterist friends)! I trapped it in a vial and collected some flowers and foliage from the vine with hopes of giving it a day or so to settle down enough to allow a few photographs in a more controlled environment.
That proved more difficult than expected, and a few photographs was all I could get. Even after a day in the vial it was extremely flighty, and every time I released it onto the flowers in the white box I had setup it immediately tried to take flight. I decided it must at least be hungry, so I whipped up some sugar solution and painted a small amount onto one of the leaves, then placed the open end of the vial over the leaf to let the fly find the sugar. That worked—briefly! The fly paused just long enough to allow me to fire off a half-dozen shots or so while it drank and preened before once again attempting to take flight. No amount of coaxing back to the sugar could interest the fly—she’d had enough, and so had I. I thought the fly had a soldier fly-ish look to it (family Stratiomyidae), and this was confirmed by dipterist Martin Hauser who wrote:
It is a female Hoplitimyia, maybe mutabilis…but the species are a mess. There are at least two species in the US, and more in South America. They have aquatic larvae…
Among flies, tabanids and syrphids seem to get all the attention from insect photographers because of their contrastingly colored eyes, but this fly had every bit as much eye as those better known families! Considering how broadly across the order Diptera that one finds these stunningly patterned eyes—72 species out of 23 families according to Lunau & Knüttel (1995), an obvious question is what is their purpose. Considering that the patterns and coloration are often sexually dimorphic, it’s tempting to think it has something to do with mate selection, especially with their large size and resulting prominence. However, Horváth et al. (2008) presented evidence that the ventral eye surface of many tabanids are stimulated by horizontally polarized light. Such capabilities are common in aquatic insects, suggesting some function in locating water for finding hosts, mates and suitable sites for laying eggs. This still doesn’t explain why the patterns are often sexual dimorphic, although one can imagine that males and females experience different selective pressures for specific visual cues that could have an effect on the resulting color pattern. Comments from any dipterists that happen by this blog and have greater insight into this question would be greatly appreciated.
Horváth, G., J. Majer, L. Horváth, I. Szivák & G. Kriska. 2008. Ventral polarization vision in tabanids: horseflies and deerflies (Diptera: Tabanidae) are attracted to horizontally polarized light. Naturwissenschaften 95:1093–1100, DOI 10.1007/s00114-008-0425-5.
Lunau, K. & H. Knüttel. 1995. Vision through colored eyes. Naturwissenschaften 82(9):432-434, DOI: 10.1007/BF01133678.
Copyright © Ted C. MacRae 2012