Friday, July 13, 2012

Defensive and offensive strategies

When a powerful predator attacks a soft and defenseless prey, the prey animal must use some strategy to save itself. But also, when a soft and defenseless predator attacks a strong and dangerous prey, it too must use some strategy, if it wants to gain a meal. Here is an example of each I happened to witness in the last two days.

The Goatweed Butterfly is a bright orange butterfly you might encounter flying up before you as you are walking on a path through open woodland. The caterpillar of this butterfly feeds, as you might guess, on goatweed, a tough and gritty weed of rough places. During its development from egg to chrysalis, the caterpillar uses various ingenious strategies to preserve itself from being eaten. The one I am talking about here was a fully grown caterpillar, and its first strategy was to roll a thick and tough goatweed leaf into a tube and secrete itself inside that tube. As further protection, the caterpillar's skin exactly resembled the color and texture of the goatweed leaf, making the caterpillar virtually invisible. For the final stroke, if a predator looked down the tube to see what was inside, the caterpillar, with the aid of two fake eyes, resembled some very formidable sharp-toothed creature looking back out. Here are two illustrations of a goatweed caterpillar demonstrating these strategies. (The tiny pale bump at the bottom of the "face" is the actual head; all the rest is fakery.)

Yesterday I was walking by a patch of goatweed idly looking to see if any of the leaves were rolled. What I found was a rolled leaf under attack. A red paper wasp (Polistes carolinus) was after the caterpillar. Although adult wasps for the most part feed on nectar, their larvae are meat eaters, and this species of wasp feeds its young entirely on caterpillars. The adult is big and powerful and uses no strategy beyond brute strength to overpower caterpillars, then chew them up into a ball and carry them back to the nest. But perhaps this wasp had looked down the opening in the tube and been put off by the eyes and fangs it saw staring back. When I saw it, the wasp was doing its best to chew through the tough leaf to get at the caterpillar directly. It went round and round, opening up a hole big enough that the caterpillar was visible inside, and several times I thought the wasp would grab it by the head and yank it out, but evidently it didn't actually to see it, even though it was chewing at the leaf directly by the caterpillar's head. I was waiting for the caterpillar to be dismembered when to my surprise the wasp gave up and flew off to look for some easier-to-reach prey. I thought the wasp would return, but when I myself came back by the leaf fifteen or twenty minutes later, the caterpillar was still visible inside, evidently unhurt.

Now for the other side of the equation. The eaves of our house, and the outer window frames, are filled with House Spiders. These little gray spiders are about as soft and defenseless as you can imagine. Sometimes in the morning I see a mud dauber wasp flying along at the level of the eaves, almost like a milkman doing his rounds, stopping at each cobweb, pretending briefly to be caught in it, and if a House Spider foolishly runs out, matter-of-factly picking it up and carrying it off. But these spiders have diabolical webs. Not the cobwebby tangle, but the guy wires that go from the top of the window frame down to the window sill. I was eating lunch looking out the window, when I saw a wasp that was walking along the window sill, get its hind legs tangled in one of these guy wires. In its effort to pull free, it snapped the wire, which evidently had been pulled taut with great elastic stress. It immediately jerked the wasp's abdomen upward. The wasp dug its front claws into the window frame, and held on, literally, for dear life. I grabbed my camera and rushed outside and did my best to record what happened next.

The spider dropped down in an instant and immediately began wrapping lines around the wasp's wings to contain them, and attached further guy wires to its rear end and hind legs. The wasp dug in the claws of its front four feet.

As long as the wasp was attached to the ground, which gave it a base to push against, its greater weight, its armored body, its muscular legs, made it a hundred times stronger than the frail spider. The spider now carried the guy wires up to the top to attach them, each under great elastic tension.

And finally it was too much, and the wasp was jerked up into the air. In an instant the wasp's powerful legs were helplessly flailing in the air.

The wasp seemed to realize how desperate its situation was. Its jaws gaped wide open, waiting for any part of the spider that came within reach. Its sting was fully extended, probing and probing for a target. In a calm and businesslike manner, the spider worked in close, but just out of reach of the wasp's weapons, spinning the spider around as it completely hog-tied it.

Now it was time to deliberately seek out a soft place between the armored segments, and deliver the fatal bite.

It was all over.

These two stories may seem to come down rather hard on the poor wasps, but that was just what made them unusual and, for me, interesting to record. In the millions of years of wasp vs. caterpillar, or wasp vs. spider, I would guess the wasps are ahead several million to one.

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