Friday, June 11, 2010

"Blue Type" Bluets: Marsh Bluet (Enallagma ebrium) and Hagen's Bluet (Enallagma hageni)

Color and pattern alone won't tell us whether this small bluet (length about 30 mm) is a Marsh Bluet (Enallagma ebrium), Familiar Bluet (Enallagma civile), or Hagen's Bluet (Enallagma hageni). But we have a bit more information to work with; zooming in on the image below provides a good oblique view of the claspers.

The lateral detail of the claspers below, viewed from a slightly different angle, is a bit blurry but good enough. The upper claspers or cerci are distinctly forked. This is a Marsh Bluet (Enallagma ebrium).

Another male Marsh Bluet and a closeup of its claspers. The light was better and the forked cerci are more in focus in this photo.

This damselfly also has the usual "blue type" attributes: small blue ocular spots, relatively wide humeral stripes and a large black spot on S2.

Again the appearance alone insufficient to tell us the species. We can tentatively eliminate the Familiar Bluet since the cerci would appear visibly longer, even in the field without magnification. Although the image below is a bit rough around the edges, it's clear that the cerci are narrow, roughly triangular in profile, and turn up slightly at the tips ... this damselfly is a Hagen's Bluet (Enallagma hageni).