Great buggy looking designs, as usual for you Paul.
I see a difference between the two that appears to be consistent in the images I've looked at, at least for larvae, and that is that abnormis' prothorax is more rectangular, while carolinensis is more oval -rounded, esp at the caudal (rearward) edges. Do you see this too?
Now that you point it out, I do!
Its funny, but when working on species you don't have a lot of personal experience with , you have to resort heavily on the keys. Once you've done it enough you start to recognize other differences. A good analogy would be handing a key to somebody who's never seen elephants before. After looking at toenails and hair tail counts referenced in the key, the individual is able to determine which one is Indian and which one is African. After that telling them apart is easy!:)
I guess I'm not surprised to see the variations in coloration, although in the central NY waters I frequented, A. abnormis was usually a very dark brown critter with lemony underparts, compared to the brighter A. carolinensis.
Yes, you describe the common color schemes. While generally useful, there are enough variations that they aren't dependable for determinations.
Gonzo had an interesting comment a while back where he referenced a conversation he had with Dr. Chandler. He pointed out that in some watersheds the variabilities are so obscure among the three major species of nymphs that telling them apart may require association with male adults to identify with absolute certainty.
Generally, I've found the best way to tell the nymphs apart are the presence of the following characters:
- thin roughly parallel sided pale occipital bar with pale unattached thin commas near the ocelli or absent; broken or indistinct 'W' mark; legs either solid colored or with long dark longitudinal bars or streaks; terga w/ dark central bands, or solid dark; anal gills usually absent.
- thicker tapered pale occipital bar with pale attached heavy triangular or rounded markings near ocelli; distinct 'W' mark; legs with pale bands or blotches; terga w/ dark apical bands or solid dark; anal gills usually absent.
- very similar to carolinensis
but usually with a large dark spot intruding into the center of the pale occipital band making it appear very thin there, or if not, a very thick W mark; terga w/ dark bands both apically and basally or solid dark; anal gills usually present.
As Dr. Chandler pointed out, these characters are far from absolute, but I believe they are much more dependable than going by shade of dorsal habitus alone.