Hacklegill Mayflies: A Closer Look

It seems every time I sample, I find quite a few of the same large tusked mayflies. I enjoy collecting them because of their size and somewhat intimidating looks, but when I bring them home, they invariably bury themselves in the gravel. After doing some research on them, I discovered the reasons behind their behavior.
These mayflies are in the Potamanthidae family, which are a group of Old World burrowing mayflies. There are only four species of this family in the U.S., and two are found in my state, Anthopotamus myops and A. verticis. The genus name has changed somewhat recently, which made my search slightly difficult, but according to Aquatic Insects of North America by Merrit, Cummins, and Berg, Anthopotamus is the currently accepted genus name. Now, this particular key only took me to genus, and I had to look at distribution maps to figure out which of the four species they could be, so there is some slight uncertainty as to what I have exactly.

I recovered this carcass after what I assume was a damselfly attach, it was missing most of its innards.

So, what are those needle-sharp looking tusks used for? Cleaning detritus from the setae on their forelegs. For such a nasty looking body part, they play such a subtle role in this animal's ecology. I speculated for a time, that this group of Ephemeroptera (mayflies), were in fact carnivores, which isn't common, but also isn't rare. In my research I found that they in fact, are detritivores. So they basically eat FPOM (fine particulate organic matter) that is either swept off of surfaces by the waving of their forelegs, or suspended in the water. One paper I read found that close to 90% of their diet was detritus with a few diatoms thrown in, with the diatoms simply being consumed becasue of their being on the wrong rock at the wrong time. This got me thinking, how in the world did these researchers look at the gut contents of such tiny creatures? I know the obvious answer, microscopes, but how were they certain of what they were looking at? It makes me appreciate the field of aquatic entomology even more.

After finding out what their tusks were used for, I researched their affinity for hiding in my gravel. I found that their prefered microhabitat were interstitial spaces in gravel and pebble-sized substrates (basically the "between" places). This perplexed me since my collecting methods are flipping over rocks and collecting what's underneath. I then found that they prefer shallow waters (<1m) in areas with low current velocities. This described the areas I found them in quite well, except that there were few gravelly areas, only silt deposits and larger rocks. Another paper noted that they commonly had negative phototactic responses, in other words, they avoided light. So in my mind, they inhabit the bottoms of these rocks due to their locations in the streams and their effectiveness at blocking light.

As for the name Hacklegill mayflies, this has to do with the long feathery gills found spreading laterally on their abdomen.

Papers on Anthopotamus:

Bae, Y. J., and McCafferty, W. P. 1994. Microhabitat of Anthopotamus verticis (Ephemeroptera: Potamanthidae). Hydrobiologia 288: 65-78.

Bartholomae, P. G. and Meier, P. G. 1977. Notes on the life history of Potamanthus myops in southeastern Michigan (Ephemeroptera: Potamanthidae). The Great Lakes Entomologist 10: 227-232.

McCafferty, W. P. and Bae, Y. J. 1992. Filter-feeding of the larvae of Anthopotamus (Ephemeroptera: Potamanthidae). Annals of Limnology 28: 27-34.