Side Spikes

Finding a number of dragonfly and damselfly nymphs yesterday only planted the idea of looking for more. So today, I went down to the Cannon River and looked for more nymphs. Getting from parking lot to river bank took a while as the rampant spring ephemerals along the trail distracted me. Slowly, flower by flower I made my way.

The first swoop through the river silt yielded an Armored Mayfly nymph. These are wonderfully bizarre insects. They resemble tiny aquatic armadillos. Or, perhaps a better comparison, they look like tiny aquatic versions of the armored dinosaur Ankylosaurus, both protected by conspicuous side spikes.

A few scoops further down the shore, a large dragonfly nymph appeared. Recently molted, the nymph's long, pointed abdomen jutted out of a clump of muddy-hued river silt, an eye-catching pale green. Had it not recently molted, its color would have been equal to the silt, and it would have been much more difficult to spot. Compared to the mayfly nymph (around 7mm in length), the dragonfly nymph was gigantic (around 40 mm). I recognized it immediately as a clubtail dragonfly nymph, exactly what I'd hoped to find.

Clubtails or gomphids (a colloquial shortening of the family name Gomphidae) were a great discovery for me when I began studying dragonflies. To begin with I was shocked that I'd never known of them before. And surely I had been where they had been but had simply never noticed them. Once I made their acquaintance they quickly they quickly took a place among my favorites. The appearance of the adults in late May and early June is much anticipated.

What species of clubtail was it? I didn't know. Seven species are known from this stretch of the river. The most pragmatic method of identifying it to species is to rear the nymph to adulthood and I planned to do that. But I'd also attempt to use available keys to try to identify the nymph. Currently there are no published guides to the living nymphs and very few online photographs for comparison (a word of caution when working with online photos---many online photos are misidentified). There are several treatises that contain information about the clubtail nymphs. The two that I have on hand are The Odonata of Canada and Alaska, a three volume masterpiece by Edmund M. Walker (with an assist by Philip S. Corbet on volume three), and Dragonflies of North America by James G. Needham, Minter Westfall, and Michael May. The keys in both of these books lead to the genus Stylurus.

Dragonflies of North America has the further key to the species of this genus. Keying out an insect can be satisfying when it works, or simply frustrating when it doesn't. There are difficulties in terminology. And technical difficulties, such as the need for high power microscopes or dissection. It takes practice, patience, and perseverance to master and given key. In this instance, the key is simplified because all but two of the species are not known from this watershed. It is only necessary to differentiate between the two species Stylurus amnicola and Stylurus notatus. This requires a bit of measurement and the calculation of some simple ratios. According to the key the ratio of the dorsal length of abdominal segment 9 to the dorsal length of segment 8 should be greater than 1.5 for Stylurus notatus. For Stylurus amnicola, the ration of the length of abdominal segment 9 to the width of abdominal segment 9 should be equal or less than 0.75. The first ratio (9L/8L) for this nymph equals 1.56---good for Stylurus notatus. The second ratio (9L/9W) equals 1.11---too large for Stylurus amnicola. So, if the keys are correct and my measurements are correct, this is Stylurus notatus, the Elusive Clubtail.

Interestingly, this species has not been officially recorded, either by voucher or by photograph, on the Cannon River. It has been observed flying over the river on numerous occasions but never captured. It's called "elusive" for a reason. If I can successfully rear the nymph, this will be the first record.

Publicado el 26 de abril de 2017 a las 03:13 AM por scottking