Here is something to keep arachnophobes up at night.

The inside of a spider is under pressure, like the air in a balloon, because spiders move by pushing fluid through valves. They are hydraulic.

This works well for the spiders, but less so for those who want to study what goes on in the brain of a jumping spider, an aristocrat of arachnids that, according to Ronald R. Hoy, a professor of neurobiology and behavior at Cornell University, is one of the smartest of all invertebrates.

If you insert an electrode into the spider’s brain, what’s inside might squirt out, and while that is not the kind of thing that most people want to think about, it is something that the researchers at Cornell had to consider.

Dr. Hoy and his colleagues wanted to study jumping spiders because they are very different from most of their kind. They do not wait in a sticky web for lunch to fall into a trap.