Not quite adult

For the past few years I’ve had research plots in northwestern Tennessee. Each summer, once a month or so, I make the 5-hour drive to the site and spend the afternoon taking data. Any normal person would then check in to their hotel room in town, watch television, and make the drive back to St. Louis the next morning. Of course—I’m not normal, I’m an entomologist! The southeastern lowlands of Missouri, where over the years I’ve found (and continue to find) a number of good spots for collecting insects, are tantalizing close. Instead of retiring immediately to my hotel room, I’d rather head back to the lowlands and find a good spot for setting up a blacklight. It might be midnight before I finally get to a hotel room, but it’s all worth it. Some of the most interesting insects that I’ve featured here during the past few years have come to blacklights on one of these trips, including the primitive longhorned beetles Parandra glabra and Mallodon dasystomus and adult male bagworms, Thyridopteryx ephemeraeformis.

Subadult female, poss. Maccaffertium sp. | Mississippi Co., Missouri

Subadult female mayfly (prob. Maccaffertium sp.) | Mississippi Co., Missouri

Most of the spots I’ve found are located along the Mississippi River, a downside of which is the overwhelming abundance of aquatic insects that are often attracted to the lights. Caddisflies (order Trichoptera) are the worst, sometimes swarming the lights with such frenetic abundance that to check the sheets one must button the collar, hold the breath, dash in quickly to look at the sheet, and retreat just as quickly lest the fluttering hordes find their way up the nostrils, down the ear canals, and into the eyes. Mayflies (order Ephemeroptera) also can be attracted in great numbers, although they tend to be, fortunately, much calmer and better behaved on the sheet than their trichopteran counterparts. Normally, I pay little attention to these insects other than what is required to avoid breathing them—their abundance almost makes them unnoticeable. On one particular night in early August, however, my eyes caught the soft glow of a ghostly-white insect sitting on the underside of a leaf some yards away from the light. I looked closer to see it was a mayfly, and so pale was its coloration that I knew it would make for a striking photograph against the black night sky.

The reason for its milky-white coloration is due to a unique aspect of mayfly developmental biology—they are the only insects to develop fully functional wings before their final molt to adulthood. This stage, called the sub-adult or subimago, emerges from the water where it spent the past year as a nymph (also called a naiad) and flies to nearby vegetation, but it is still not mature. One additional molt is required, wings and all, before the insect finally reaches adulthood and can spend the few remaining hours of its life in single-minded pursuit of a mate. Sub-adult mayflies are distinguished from their adult counterparts by their paler coloration and opaque rather than clear wings. We can also tell that this individual is a female because no claspers are visible at the tip of the abdomen (which males possess for mating) and its relatively small eyes (the eyes of males almost completely cover their head).

My thanks to Dr. Robert Sites, who initially suggested this might be a species in the family Heptageniidae, and to Roger Rohrbeck for confirming my subsequent identification as probably belonging to the genus Maccaffertium.

© Ted C. MacRae 2014