Thursday, November 22, 2012
Whoops! Make that Nigronia serricornis
The hellgrammite/fishfly that I found yesterday at the Lynch was Nigronia serricornis, not Nigronia fasciatus. I just discovered my mistake as I was looking for verification of the fasciatus ID. The two species -- serricornis and fasciatus -- are similar, but they are distinguished from one another by the position and size of the respiratory tubes on segment 8. These:
The tubes on N. fasciatus are relatively close together and long, extending beyond the end of segment 9; those on N. serricornis are "lateral" -- at the edges of the segment, and they are short, less than 1 mm in length. This distinction is made in an article published in Entomological News in 1965 (R. Duncan Cuyler, "The larva of Nigronia fasciatus (Megaloptera: Corydalidae), pp. 192-195.) On p. 194, Cuyler has this to say: "Larvae of Nigronia fasciatus closely resemble the larvae of the more widespread Nigronia serricornis Say, and a third undescribed species. It differs from these mainly in the greater length and closer proximity of the caudal respiratory tubules. In the other species the respiratory tubules are mid-lateral in position, conical, and less than 1 mm in length." The tubules on the nymph I found yesterday are less than .5 mm in length though that nymph was certainly not yet mature.
For more evidence on the ID of N. serricornis, I urge you to look at the illustrations of the respiratory tubes of N. serricornis and N. fasciatus in Merritt, Cummins and Berg, An Introduction to the Aquatic Insects of North America, on pp. 429 and 431. Those of N. fasciatus clearly extend beyond the end of abdominal segment 9; those of N. serricornis do not. Here is a microscope view of those of the nymph I found yesterday.
One final bit of evidence for us to consider. Take a look at the Nigronia serricornis photos of Jason Neuswanger in Troutnut.com (http://www.troutnut.com/hatch/3357/Hellgrammite-Nigronia-serricornis-Fishfly), and note the length of the respiratory tubes.
My apologies for the mistake -- amateur at work. The tolerance value of Nigronia serricornis is 4.6.