Here is another fly photographed during my recent visit to Argentina. I had little doubt when I saw it that it, too, was a member of the family Stratiomyidae; however, unlike the previous species this one was a far more cooperative subject for photographs. I have learned to carry a small utility tool in my pocket that contains an even smaller scissors—these have proven to be quite handy for gently snipping flowers or plant parts on which insects are perched without disturbing them. Holding these detached plant parts in front of the camera has several advantages, including the ability to photograph the subject while standing (or sitting) in a stable, comfortable position, providing more flexibility in choosing the background, and allowing me to “rest” some part of my hand or arm against some part of the camera or flash heads to “fix” the subject-to-lens distance. The fly remained quite calm through all these machinations, allowing me to focus on getting the composition, exposure, background, focus, and other technical aspects of the photos to my liking. Easier said than done of course, but a cooperative subject at least makes it more possible.
I thought this fly looked an awful lot like our North American species of Odontomyia, so I sent the photos to stratiomyid expert Norman Woodley (Systematic Entomology Laboratory, Smithsonian Institution) for his opinion. Norm wrote back:
The stratiomyid fly…is in the subfamily Stratiomyinae, tribe Stratiomyini, which includes Odontomyia. I think that your fly is in the genus Psellidotus. Psellidotus is similar to another genus, Hedriodiscus…these two are easily separable in the Nearctic Region but the distinction becomes fuzzy in the Neotropics, especially in South America. We don’t have the species you photographed in the USNM collection. The majority of species in Psellidotus and Hedriodiscus in the Neotropics are very poorly known outside of their original descriptions.
As before, this stratiomyid also exhibits stunningly patterned eyes, and like nearly all of the examples that I have seen the horizontal nature of the banding suggests an ability to see horizontally polarized light in similar fashion to many species of tabanids (Horváth et al. 2008). Again there seems to be a link between the ability to see horizontally polarized light and insects with aquatic lifestyles, as such visual capabilities have been demonstrated for a variety of other aquatic insects. While the biologies of most Neotropical stratiomyid species remain unknown, larvae of the subfamily Stratiomyinae are (like tabanid larvae) known to be aquatic (Brown 2009). Stratiomyine adults that exhibit these horizontal banding patterns may, like tabanids, also be able to see horizontally polarized light, which would be useful for finding mates and suitable sites for laying eggs.
Brown, V. B. 2009. Manual of Central American Diptera, Volume 1. NRC Research Press, 714 pp.
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.
Copyright © Ted C. MacRae 2012