Tuesday, June 17, 2014

A mystery solved

Last year when we began making a concerted effort to learn the spiders of Arkansas, I spent several nights in my backyard with my headlamp on looking for and photographing the Arboreal Orbweaver, Neoscona crucifera, the commonest of the big orbweb-spinning spiders, and a good representative to study the behavior of this group of spiders. The Latin species name, crucifera, means 'cross-bearing,' referring to the cross-like marking on the abdomen.

The general procedure for these spiders is for the big females to come out at dusk and build a large orb-shaped web stretched out between  two trees. (When they are fairly close to the ground it is quite easy to run into them if you are walking around without a light.) They spend the night catching whatever blunders into their web (they are formidable hunters and can catch prey much bigger and heavier than they are), then when it begins to get light in the morning, they take the web down strand by strand, generally eating it to get back all the protein they had put into it. During the day they get in under the eaves or in some other protected spot, to eat at their leisure whatever they caught the night before. Typically they have sturdy legs, a small carapace, and a fat round abdomen.

As is often the case with spiders, the males are quite different.

They are small compared to the female. Proportionally, their legs are longer and slenderer, their carapace is broad, and their abdomen is tiny. On the female the palps are like a pair of miniature legs, thin and threadlike, on either side of her mouth, and are used to manipulate the food she is eating. The male's palps are like big boxing gloves and are full of complicated apparatus for carrying his sperm and inserting it into the female's reproductive opening. If you see a male at all, you will find him living off to the side of the female's web, where possibly he scrounges some food from her leavings, or possibly he eats nothing at all.

Now this was part of my general knowledge of the orb-web spinning spiders before I began, so when I went out one evening and saw what I am going to show you now, I was flummoxed, I didn't know what to think (you have no idea how emotional the study of spiders can be can be).

Here was a large Neoscona crucifera with the cross pattern and fat abdomen of a female, which had a big professional-looking web spread between trees, which was catching and wrapping up and feeding on big game such as this big strong beetle, and yet which had very obvious swollen male palps.

I was putting together an album of photographs of Arkansas spiders which I hoped would be helpful to people who were trying to identify them in the field, and which might show little interesting bits of behavior, if I were lucky enough to discover any. But this was inexplicable. Somewhat guiltily I hid these photos away in a file, and decided not to mention them.

The one thing I noticed about this spider was, it was a subadult, it was in its last instar before molting into adulthood. I could tell that by the fact that the palps were swollen and beginning to mature, but did not yet have their complex sexual machinery, which would only come with full maturity.

I took those pictures July 25th last year, and that's where things stood until now, nearly a year later. What happened is, I received as one of my birthday presents from Cheryl Bernd Heinrich's latest book, The Homing Instinct. He's my favorite naturalist/writer, and here is part of the reason why: With his constant close and dogged and trained observation of every living thing around him, he sees things no one else sees.

In this case he had an orb-web spinning spider living in the rafters of his Maine woods cabin. When he sat at his desk working, he could look up and observe the spider. He began throwing insects to it and learning different things about it. He thought it might be a male, except that it was big, and everyone said males, relative to females, were small. It had a big web, and everyone said, the males don't make a web. Anyway, one day it stopped feeding; no matter what he offered it, it wouldn't take any more. Its fat belly began to shrink. Then, it shed its skin, and when it came out its shrunken belly was even smaller than before. On the other hand, its legs were a third longer. And its palps had turned complicated. It never made another web. One day it took off running, presumably searching for a female to move in with.