A Trip to South Carolina in late April, 2001 produced two species of the subgenus Hylogomphus: Gomphus parvidens (Piedmont Clubtail) and Gomphus apomyius (Banner Clubtail). The keys for these two species in Dragonflies of North America (Needham, Westfall, and May, 2000) work, but at least in the populations in three streams/rivers where I sampled exuviae of Gomphus parvidens (n=30) had additional diagnostic features.
In structure and size, the South CarolinaGomphus apomyius exuviae (n=30) match my New Jersey specimens very closely. G. apomyius was on the coastal plain, typical for that species. As the vernacular name implies, I found G. parvidens above or very near the fall line.Map Here I have never collected G. parvidens exuviae, but I have spent a lot of time with G. apomyius in New Jersey. I was a little surprised that the larvae are so dissimilar, since the adults look so much alike.
Some consistent differences are:
The prementum on G. parvidens is relatively straight sided, narrowing
slightly toward the distal end. The prementum on G. apomyius narrows
rather abruptly about 2/3 - 3/4 toward the distal end. The median lobe is
straight or very slightly convex on G. parvidens, and much more strongly
convex on G. apomyius.
On most G. parvidens, there is often no lateral hook on abdominal segment 6 or
at best a very small hook. On some, what appeared to be a hook, was just
tufts of hair. Dragonflies of North America does show a small lateral hook
on segment 6 for G. parvidens (Fig. 217 d) pg. 338). This might be variable throughout the species range, but I doubt if it would be as large as on G. apomyius. The lateral hook on segment 6 of G. apomyius is approximately 60-70% as long as the large hook on segment 7.
On G. parvidens, the mid-dorsal length of abdominal segment 9 is less
than 4 times its width at the widest point. On G. apomyius, the mid-dorsal
length of abdominal segment 9 is more than 4 1/2 times its width at the widest
Although doubtfully diagnostic, the abdomen is very hairy on the South Carolina G. parvidens specimens. Many have long hairs scattered on top of the abdomen, and very heavy along the dorso-lateral line. The streams where I found G. parvidens had a lot of very fine, soft silt. The rivers where I found G. apomyius are very sandy, and these nymphs could rub off the hairiness in the course sand, and had a polished look.