Damselflies — On Bluets (and ‘Yellow-Greents’)

"Boreal Bluet Damselfly"

Boreal Bluet

I came across this damselfly along the shores of the North Saskatchewan River in N.E. Edmonton (12 September, 2011). It seems simple enough to identify it as a ‘bluet’ (Genus Coenagrion or Enallagma) ’cause its pink blue. But in the world of entomology, nothing is as easy as that. Alberta is home to at least seven species of bluet, many of which, at first glance, are entirely indistinguishable from each other. So how does one narrow down bluet identification? It’s almost all in the claspers…

"damselfly claspers"

The claspers in question… (cropped from above image)

So…what’sa ‘clasper‘?

In the days of old these were called “abdominal appendages”, however, the upper (superior) abdominal appendages are now referred to as “cerci“, and the lower (inferior) are called the “paraprocts”. Together these appendages allow male damselflies to clasp the female’s neck (actually the prothorax) as part of the mating process. The cerci shape is key to determining the differences between the various similar bluet species. Using Acorn’s guide†(pg 155) to claspers  I narrowed the ID down to either the Northern Bluet (E. cyathigerum) or the Boreal Bluet (E. boreale). The detail on the cerci (above), which are blunt,  seem to indicate that it is most likely the Boreal Bluet‡.

However, didn’t I say nothing in entomology is simple? Female bluets not only don’t have claspers, they also ain’t necessarily blue! Many bluet females are yellowish-green and need to be identified using other features.

As I have mentioned before, one of the pleasures of bug photography is in finding the unexpected when you first view the image enlarged on a monitor. In many cases the surprise comes in the discovery of phoretic or parasitic attachments. This damselfly is carrying a cluster of mites behind the mid legs. These are most likely water mites from the family Arrenuridae. As larvae they attach themselves to the aquatic larval damselfly, and they begin feeding on the adult after emerging.

"Bluet Damselfly with mites"

Note the mite hitchhikers under the thorax

†Information from Damselflies of Alberta by John Acorn, with additional info. found at http://deepblue.lib.umich.edu/bitstream/2027.42/55115/1/3560.pdf

‡ID consensus: John Acorn and Terry Thormin.

"Northern and Boreal Bluet Claspers Compared"

Crude representation of cerci difference (drawn from illustration in Damselflies of Alberta)

This entry was posted in Acari, Alberta, Coenagrionidae, Insect, macro, Odonata, Parasitism, Summer, Zygoptera and tagged , , , , , , , , , , .

2 Comments

  1. Susannah Anderson 3 October, 2011 at 4:38 PM #

    Hi, Adrian,
    I have included the link to this post in this month’s Circus of the Spineless #66.

    • Adrian D. Thysse 3 October, 2011 at 4:40 PM #

      Thanks, Susannah!
      I always forget about submitting to carnivals.

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