Wednesday, July 24, 2013

Behavior watching

You get your digital camera and your macro lens and you quickly find out what wonderful subjects arthropods make for photography. Then you realize what a learning tool you have, and you blow up the picture you have taken on your computer screen and hold it side by side with the illustration in your field guide, and you match the details, and then you learn the name of your subject, and you read up on it a bit, and then you go back into the field and watch your creature with new, informed eyes. It's a funny thing, but most of us don't really see something until we put a name on it. But once we do, then we are set up for the next stage, which is to begin observing it, really seeing it for the first time. That's when you can start behavior watching, and the very most fun of all, trying to record that behavior on your camera. Maybe in the end I will get a video camera (Cheryl is taking more and more movies on her camera, and perhaps I will try to put them on this blog if I can work out how to do it), but for now I am very pleased if I can catch a glimpse of some interesting behavior just with a series of still photos.

Insects and other arthropods have been fine-tuning their behavior for millions of years, and they are sometimes bizarre beyond belief. Here are a couple I have seen in the last few days.

We have a black walnut tree and a pecan tree in the yard. We don't get nuts from them. Our chief source of pleasure from them is that from time to time they have an outbreak of Walnut Caterpillars. These are wonderful large shaggy black caterpillars, and during an outbreak the tree can look like it has giant tent caterpillars without tents. (They don't by the way do any permanent damage to the tree at all.) Here is the caterpillar.

Here, multiplied by a hundred times, is what they look like during an outbreak.

Now some caterpillars with long hairs have stinging spines and it can be quite painful to touch them, but these do not. These are merely long hairs. They are, however, a first line of defense. It's very hard for birds, for instance, to swallow many of these ready-made hairballs without quickly clogging up their throats and stomachs. For the most part, it is only cuckoos who have the specialized techniques for eating them. And the long hairs also cause some problems for parasitic flesh flies and tachinid flies and the ichneumon and braconid wasps who try to lay eggs or larvae on the skin of caterpillars so they can bore their way inside and devour them from within. It's hard to get through the hair and down to the skin. But the caterpillars have a further line of defense which is quite amazing and amusing to see.

If an ichneumon wasp or a hungry bird turns up in among the caterpillars they all begin sharply jerking their heads and tails perhaps once a second. The wonderful thing is, the hundred or so caterpillars somehow synchronize the jerking, so that they make all their jerks at exactly the same moment. The effect is not of several small caterpillars jerking, but of one very large and perhaps formidable creature doing it. No one quite knows how these insects, with their poor eyesight, manage this precise synchronization with one another.

But that's only the beginning of their talents. Now, as they bolt down leaves and grow fast, from time to time they must pause to shed their skins. While they are shedding, and hardening up after their molt, they are soft, inactive, and very vulnerable to predation. They don't use protective colors to hide themselves, so a large aggregation of them can be seen from a distance. I described them earlier as looking something like tent caterpillars without a tent. Well, tent caterpillars have their tent to take shelter in when they are molting. What do the walnut caterpillars do?

I learned what they do when I came out the other morning and looked at our walnut tree and saw a big ball of some living thing down near the base of the trunk. Here's what it looked like:

And here is what had happened: Every caterpillar on the tree (if you look carefully you can see at least three different age classes) left the leaves at the top of the tree and climbed down to the bottom of the bare trunk and gathered in this ball. Again with magical synchronization, they had all come down to shed their skins at the same moment. As a friend suggested to me, instead of a single vulnerable caterpillar lying exposed, this resembled a big hairy animal.

I kept checking on this ball throughout the day, and as caterpillars hardened up and were ready, one by one at intervals five or so feet apart they marched back up to the top of the tree.

The next morning nothing was left but a bag of head capsules and empty skins.

A couple of days later we put on our headlamps and did a night op looking for spiders. I have reported before in this blog that on an earlier operation I found a small colony of Scytodes spitting spiders living in the siding of one corner of our house, and coming out at night to hunt. This is a frail, slow-moving, and, I imagine, rather timid spider that has perfected a hands-off method of hunting. They walk up close (but not too close) to a suitable small insect, point their fangs forward and they spit a mixture of glue and venom at it. They shiver their fangs, making a criss-crossing stream that envelopes their prey. By the time the prey begins to think it is in danger, it is already pinned to the ground with the venom sinking in.

Well, this night I caught one in the act. Mosquitoes around the house roost in large numbers on flat sheetwebs of spiders. They must land so softly and keep so still the spiders don't notice them. And being on the web must give them protection from most non-spider predators. But not this lone mosquito, which made the mistake of roosting on a scrap of Scytodes web.

In this first picture the spider is walking slowly towards the mosquito, spitting as it goes. It looks like a wing and a leg are beginning to get matted up.

In the second picture, slightly more from the side, you can see the chelicerae on the side towards us held out towards the mosquito, with the fang swiveled out (from this angle we can't see the fang on the far side). One wing and one leg are definitely tied up.

In the third picture the legs are completely tied up, the fangs still spitting. You can't actually see the stream of spit. For one thing, this is taken at night with a flash. But many years ago we watched in broad daylight as a spitting spider caught a fly, but we still couldn't see the stream, only little dots appearing where it struck the ground in a ring around the fly.

Let's look at the last two pictures from closer up to see more detail.

We didn't actually see the shivering of the fangs. We only read about that afterward. But the next chance we get, we will watch for that carefully. The more you observe, the more you learn. The more you learn, the more you actually see. For instance, the day after we saw the ball of walnut caterpillars here, we were out at Big Lake, and spotted a ball of walnut caterpillars at the base of a pecan tree there. Without our new knowledge, we would never have noticed it.

No comments:

Post a Comment