Sunday, October 10, 2010

An Ichneumon Wasp

I don't know what surprised me more today - the unseasonable string of warm days, the wildflowers still in bloom along rural roads in west-central Wisconsin coulees or spotting this black insect on a late flowering Queen Anne's Lace.

I photographed it because I thought it would be easy to identify - the white on the back, tail and antennae.

I was wrong.  I looked through the insect field guide.  I couldn't find it. 

I had to resort to again.  And again, I got a quick response:  a male Vulgichneumon brevicinctor, one of the most common of the 5,000 or more species of ichneumon wasps in North America.  There may be as many as 100,000 species world-wide, making it the largest of animal families.

Once I got a name, I recalled that I actually had read something about them some time ago.

I'd read that they're not like other wasps.

They're not social.   The females have long ovipositors and use them to lay their eggs in caterpillars.  When the eggs hatch, the larvae of these wasps are parasites.  They convert their hosts into their own personal food factories.

Here's what I found out today:  Ichneumons are species-specific in their choice of hosts.  The female ichneumon uses her antennae to locate a host for her eggs.   (The word ichneumon comes from the Greek word for hunter.)

Because many of their hosts are agricultural pests (caterpillars and beetles) they are considered to be "beneficial" insects.   Some species of ichneumons however, parasitize harmless spiders and other wasps.  One specializes in the aquatic larvae of the caddisfly.  Another pierces wood to reach its host.

When the ichneumon eggs hatch, the larvae feed on the body of the host, keeping the host alive just long enough for them to pupate and for the adult wasp to eat its way out.  (Inspiration for the sci-fi horror film Alien).

The gruesome life cycle of the ichneumon wasp has troubled many naturalists and theologians, including Charles Darwin.   In 1860, Darwin wrote about it in a letter to Asa Gray:

There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice.

You can read more about this in an essay on Nonmoral Nature by Stephen Jay Gould.

By the way, the feeding behavior of adult ichneumon wasps is less disturbing.   They feed on water and flower nectar.

No comments:

Post a Comment