Tuesday, August 27, 2019

Spider Origami

Spider folds blade of grass,

Enclosing her eggs and herself

Never to see the sun again.

(Click to enlarge)
A few years ago I spotted some neatly folded grass blades in a clump of Switchgrass in the front yard. A closer look showed that the grass blade had been folded twice, the first fold about six inches from the tip and the second fold brought the tip back up. The edges of the grass were stuck together by silk, in such a way that they made a small, three-sided purse, with tapered ends. I found two more grass blades folded in the same manner, so I decided to open one up to see what was inside. 

Nest found opened after violent thunderstorm.
The white material is the silk that fastened the edges of the nest "box."

I immediately regretted doing this. A small spider with a clump of eggs was inside the little grassy box and it stayed with the eggs, never even attempting to run. I refolded the grass blade, hoping that the spider would repair the damage I’d done.

In subsequent years I always found at least three of these spider nests in the same patch of Switchgrass. Most of the time the spider folded the leaf blade twice to make the nest, but,
A nest with three folds
occasionally I found one that was made with three folds.

This year I decided it was time to figure out what kind of spider builds these neat little hideaways. I googled “spider folded leaf nest” just to see what would turn up. One of the hits on the first page led me to a book with an illustration of a nest that was identical to the ones I had been finding. The spider responsible was identified as Clubiona riparia (Riparian Sac Spider, Bank Sac Spider, Leaf-curling Sac Spider). The biggest problem with applying that identification is that the Riparian Sac spider is known from riparian habitats in Canada, Alaska and the northern USA. (Riparian is a term applied to the wetlands bordering rivers and streams.) Our front yard is definitely not riparian – we’re at least a mile from the closest river. But the nest illustrated was identical to the ones in our yard, so perhaps it’s a related form.

The book indicated that the female deposits her eggs within the nest and remains with them until they hatch or she dies. In the words of famous entomologist John Henry Comstock, the nest serves, “as a nursery for the spiderlings and a coffin for the parent.” Other sources repeat this information with the embellishment that her body provides the hatchlings their first meal. I couldn’t find any citation to support this information.

Maternal care is known in several groups of spiders. Some jumping spiders remain with their eggs in a silken cocoon until they hatch. Wolf spiders carry their egg sac attached to their spinnerets until they hatch; then the young spiderlings are carried about on the back of their mother. A few spiders feed secretions from their body to their newly hatched offspring. In return, the young spiders eat their mother’s body.

The construction of the nest is a bit of spider origami. The two-fold version is done in such a way that the vascular system of the grass blade is not damaged. If it were, the blade that forms the nest would dry out and turn brown, making it conspicuous to predators. Since the blade usually retains its green color, maybe it’s better to think of the blade being bent, rather than folded.

I tried to reverse-engineer the nest construction. First, I cut a long, narrow strip of paper. Then I folded it in half, making the fold so that the upper end lay at a small angle to the lower end. Then I used a 1 inch long piece of tape to attach the two edges. Because the fold was not at 90 degrees the untaped side stays gapped open. This leaves room for the spider. The next part is tricky: I bent the original upper end backward and slightly twisted, so that it would cover the space between the free edges of the “box” formed by the first fold. Then I taped the edges together to make a kind of twisted, three-sided box. I don’t know if the spider folds the grass blade in this order, but, if you try to do it, you’ll see how the chamber is formed.

You have to remember that the spider is doing this all from the inside. It’s like sewing together a sleeping bag from a single strip of cloth while you’re on the inside, standing on it.

I leave constructing the three-fold version as an exercise for the reader.