Friday, January 14, 2011

Insect Alchemy: The Maturing Female Western Forktail (Ischnura perparva)

Female damselflies of the genus Ischnura, the Forktails, often go through a pretty dramatic color change during their time as an adult. In many species they start out wearing some pattern of black-and-orange, and as they mature the orange areas dull to some less interesting hue like muddy yellow, and they may simply darken altogether. During this transition in color, females of some species start developing a coat of pale gray pruinescence—a fine, waxy, powdery film, which eventually covers much of the exoskeleton.

The Western Forktail (Ischnura perparva), abundant and widespread in the Pacific Northwest and much of western North America, is one of those species that performs this insect alchemy, changing from gold to silver. The orange stage is typically referred to as “immature”, but orange and pruinescent females are both seen copulating with males so they aren’t necessarily reproductively immature. If you didn’t know any better you might think each was a different species. Here’s a series illustrating the transition...

A classic immature female with orange areas on the head, thorax, legs, and abdomen.

This is intermediate with the orange areas on the thorax turned to dull grayish-yellow, the black shoulder
stripes broader, and the abdomen almost all black now. A very thin coating of pruinescence is starting to
show. Note the change in eye color.



Further along with lots of pruinescence—especially on the abdomen, but the thoracic stripes
are still clearly visible.

Fully mature with the thoracic stripes almost completely obscured.


Note how the eye color changes to vivid green with a sharply defined black “cap” and that the lower sides of the thorax turn pale green. The transition to the fully mature pruinescent stage seems to occur relatively rapidly since intermediates like the second individual are not often seen.

So here’s an exception (as I delved into the world of odonates I learned pretty quickly that there are exceptions to almost everything, and even some exceptions have exceptions—it get’s complicated): a very small number of female Western Forktails are androchromatic (or male-colored) and the pale areas on the upper part of the thorax are green instead of orange and there are blue rings near the tip of the abdomen. To continue the alchemy analogy, I guess this would be turning copper and cobalt into silver. I've only seen one example of an androchromatic Western Forktail which is pictured below.

An androchromatic immature female with a light coating of pruinescence. The dark red spots behind
the head are water mites—I’ll talk about those in a future post.

This variant is pretty rare, although it may not be quite as scarce as we think since they end up covered with pruinescence just like the typical gynochromatic (or female-colored) females, and by then they all look alike. It’s a matter of timing. Keep your eyes open for them.

4 comments:

  1. Facinating information .... thank you

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  2. Thanks for the great explanation. Now I know that the pics I took last year are Pacific Forktail.

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    Replies
    1. Oops, I actually meant to post that on your Northwest Forktails post. Oh well - lots of good info.

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    2. No big deal, Matthew. Thanks for the comments!

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