Wednesday, June 9, 2010

An anomalous male Eastern Forktail, and how to distinguish a disorderly disarray of damselflies

By far the most common local species, I've likely seen thousands of these small damselflies but to date this is the only Ischnura verticalis I've observed exhibiting a discontinuous antehumeral stripe.

Two days later I stumbled across another Eastern Forktail with a pinched or reduced shoulder stripe.

An imposter that might be mistaken for an Eastern Forktail: there were about a dozen of these small damselflies (about 30 mm long) in the tall grasses just under the dam shortly before sunset.

Note the vulvar spine on S8 ... definitely an Enallagma female.

The dorsal view is not the best due to the poor lighting. But the central blue oval spot on S9 is diagnostic, as are the small blue ocular spots, the narrow yellowish antehumeral stripe and heavy black humeral stripe, the pale yellow on the legs and the orange pterostigmata. This damselfly is a female Rainbow Bluet (Enallagma antennatum). I'm looking forward to photographing the males ... but where are they? Only the females were out and about ...

This damselfly is a teneral male Eastern Forktail ...

... and the next two images depict a teneral male Eastern Forktail with a broken (virtually nonexistent) shoulder stripe. Now have a look at the lateral view of a male Rainbow Bluet (Enallagma antennatum). The red tinted eyes of the teneral Eastern Forktail may be deceptive but (aside from the shoulder stripes) the ocular spots and the patterns on the terminal abdominal segments are quite different.

A male Fragile Forktail (Ischnura posita) has an "exclamtion mark" shaped shoulder stripe. This damselfly is also generally smaller than the Eastern Forktail and the last abdominal segments of this species are never blue.

Although they look similar at first glance mature female Fragile Forktails and Eastern Forktails can also be distinguished by their shoulder stripes. The shoulder stripe is broken in the mature female Fragile Forktail shown below, but solid in the mature female Eastern Forktail.

The following two immature female Ischnura posita haven't developed the pruinosity typical of the older females. Note the pale blue of the broken shoulder stripes and the sides of the thorax.

Thus far I have only encountered blue Fragile Forktail females in this area but according to the books they can be tan or grey. The females below were photographed at the marsh bordering the Eastern Ontario Trail, where this insect is abundant and not likely to be added to the endangered species list any time soon. Another good habitat to seek out these tiny damselflies are the placid pools of water hedged by low, dense vegetation along the Moira River. Now and again I've found the odd stray foraging in flower gardens a fair distance from the nearest water.