August 30, 2012 8 Comments
In late July I began blacklighting on a weekly basis at different locations along the Mississippi River in an effort to gain more detailed information on the distribution of certain tiger beetles along that great watercourse. While attraction of tiger beetles to ultraviolet lights is well documented, it seems to me to be an underutilized method for collecting tiger beetles and recording distributions. Perhaps this is because only certain species are attracted to lights—principally members of the genera Ellipsoptera and Habroscelimorpha [and even one species, Habroscelimorpha striga (Elusive Tiger Beetle), that is seen almost exclusively at blacklights], while others, including the more commonly encountered and speciose genus Cicindela, are rarely attracted to lights. As a result, when I setup a blacklight on a sandy beach along the Mississippi River in La Grange in far northeastern Missouri, I expected to see Ellipsoptera cuprascens (Coppery Tiger Beetle), which I had seen there during the day many years ago, and hoped to see Ellipsoptera macra (Sandy Stream Tiger Beetle), which I have so far seen only in northwestern Missouri. I did not see either of these species, but what I did see was even more unexpected—Cicindela repanda (Bronzed Tiger Beetle).
It is ironic that I should be so excited to see this species—it is only the most common species of tiger beetle in Missouri (and probably across much of eastern North America), where chokingly thick populations develop each summer along every waterway in the state. As a species, it is remarkable infidel when it comes to habitat selection, proximity to water appearing to be its only real requirement. I have seen them on virtually every type of stream/river/pondbank regardless of soil type—sand, mud, or any mixture of the two—and note them to be common even on concrete boat ramps (although I have yet to find larval burrows in the latter habitat!). Yet, I have never seen them at a blacklight! Perhaps it was just a matter of time, as until this year I myself hadn’t done much blacklighting for tiger beetles. Populations of this species build as the summer progresses, and it could be that once numbers reach their peak in mid- to late August, a few will find their way to a light that happens to be placed in their midst while the majority of individuals bed down in their overnight burrows.
It’s easy to see what might attract them to the light other than the light itself—prey! At every location along the Mississippi River that I’ve blacklighted this summer, choking throngs of caddisflies inundate the sheet within the first half-hour after sunset. Piling up in layers beneath the stupefying light, the caddisflies are a limitless bounty of easy pickings for the tiger beetles, who greedily grab the hapless trichopterans in their toothy, sickle-shaped mandibles and then use their maxillae and digestive juices to macerate them to a juicy pulp that can be sucked dry. I have watched tiger beetle adults feeding on many occasions, but I never noticed until examining these photographs that the feeding beetles hold their antennae back and out of the way against their head and pronotum. Contrast the antennal position of the feeding beetles in the above photographs with the forward position of the antennae in the non-feeding beetles in the remaining photographs. Perhaps this is an adaptation to prevent the antennae from being grabbed and damaged by struggling prey.
Recall also my recent lamentations about lacking good photographs of this species, due initially to lack of effort and later to a rare failure when I did try to photograph them. A half dozen individuals made their way to the light this night, and I was able to coax a few of them off the sheet and onto the surrounding sand for a few photographs in more realistic and natural surroundings. I still don’t consider these to be the photographs that I want for this species, as they do not show any of the thermoregulatory behaviors exhibited during the day such as stilting, sun-facing, or shade-seeking that make for such marvelously iconic tiger beetle poses. For that, I will need to give them another shot on a hot day while summoning every ounce of tiger beetle stalking skill that I can possibly muster. Still, these last two photos (and a few others not shown) are several steps above the single, frustratingly distant lateral profile shot that I had for this species before this night.
Copyright © Ted C. MacRae 2012