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Saddleback bites the dust

Not long ago, I was delighted to come across a small redbud tree filled with caterpillars. There were at least five species noshing away, including three saddleback caterpillars, Acharia stimulea. Saddlebacks grow up to be rather ordinary little brown moths, but they're truly spectacular as larvae. It's as if a sea slug was saddled with a tiny lime-green horse blanket.

However, I noticed that the individual in these photos was not looking especially lively. Even though the caterpillar looked pretty good - slightly faded, perhaps - something was amiss. So in we go for a closer inspection.

Uh oh. While making macro photos - those little columns of spines will give you one heckuva sting by the way - I noticed a major problem for the poor little cat. That tiny white cylindrical object is an egg from a tachinid fly. These bristle-bodied parasitoid flies are among a caterpillar's worst nightmares. In this case, I think that's probably the old case of the egg - as we shall see, it looks as if the fly has already spun its gruesome magic.

Tachinid flies, which rather resemble house flies, are major caterpillar predators and attack the cats by sticking an egg to the the larva's exterior. In short order, the fly grub hatches from the egg and bores directly through the caterpillar's skin and into its interior.

Once within the caterpillar, the ever-growing fly grub begins consuming non-vital hemolymph fluids and tissue. Clever parasitoids that they are, it would not behoove the grub to kill its victim until the last possible moment, as a living host is better able to move about and better avoid other predators such as birds. In a final frenzy, the fly grub goes ape and eats all of the innards before bursting from the caterpillar and going off to pupate.

In this photo, we can clearly see what must be the grub's exit hole. What a show that must have been, and I'm sure you wish I had caught the grub bursting from the caterpillar's husk on video so you could enjoy that bit of cinematic loveliness as you ate your breakfast. But such a video was not to be - I suspect that these tachinid fly grubs tend to emerge under cover of darkness.

As I was making these photos, this tiny chalcid wasp alit on the caterpillar husk and began looking around. This little wasp is looking to parasitize the fly by laying eggs on either the fly grub or its puparium. The world of parasites and parasitoids (the latter generally kill their hosts) is truly strange and multidimensional.

I made this photo a few weeks ago. While photographing this caterpillar - a yellow bear, Spilosoma virginica, I believe - I noticed a tachinid fly perched nearby. The fly watched the bristly caterpillar's every move, occasionally shifting position to get other viewing angles. Yellow bears are heavily beset with stiff hairs, and the fly was undoubtedly waiting for an opportune moment to get at some unprotected part of the caterpillar. I watched for a while, but had to leave before the fly moved in. I wish I had had the time to stick around and film the actual attack.


Russell Reynolds said…
LOve your blog Jim ,, it always gets me looking for something new in the great world of nature. This was a great story .
Anonymous said…
Thank you for your wonderful observations. The wasp appears to be a chalcidoid, possibly from the family Eulophidae.

Caterpillars can wind up being veritable school bus' of parasitoids, harboring many different kinds, both parasitoids (including braconids) and hyper-parasitoides.

Thanks again and keep up the great imaging!
Jim McCormac said…
Thank you for the excellent info, Hans, especially regarding the wasps ID!

The miniature world of parasitoids is infinitely fascinating if not somewhat horrifying. The ever-improving macro camera capabilities allow us better looks into this world, and I will keep photo-documenting this stuff.

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