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The predator from within...

If you have a young caterpillar that repeatedly falls off its food-plant despite being awake and able to move, and seems to lack coordination and shows no interest in eating, it may been infected by a parasitoid wasp.

The young caterpillar shown below played host to the larva of a type of parasitoid wasp (perhaps a species of Apanteles), which devours a substantial portion of the caterpillar's innards. The larva then emerges from the caterpillar's body and immediately uses the caterpillar's silk to spin itself a cocoon, thus adding insult to injury. Within a week or two, a tiny wasp (pic below) will hatch from the cocoon. These wasps had been incorrectly identified on this page as Trichogramma wasps, but the latter are far smaller and have shorter antennae and stubbier wings, and do their damage to butterfly eggs rather than to caterpillars. [thanks to Jean in Portland].

The caterpillar shown above is dead. Although the wasp larva, while growing inside the caterpillar, appears to spare the muscles and nervous system of the caterpillar so that the caterpillar will be able to hold onto the twig or leaf and also retain its reflex to move its head away from contact with another caterpillar, the caterpillar usually becomes entirely unresponsive within a few hours after the wasp larva emerges.

Below is a picture taken through the low-power lens (described in tools) of 2 wasps and the cocoons from which they hatched. That's a standard-sized paperclip next to them, so you can see that they are very small. Not the sort of wasp that we worry about encountering.

Notice the wonderfully long antennae!

Point-of-view is important in deciding our alliance with one creature or another.

Do a Google "Images" search on 'caterpillar wasp' to find out lots more about parasitoid wasps. For example, this is an excellent page about these wasps (and much else about butterflies): http://www.learnaboutbutterflies.com/Enemies%20of%20Butterflies.htm