On Spider Intelligence

A few days ago, Tim Eisele put up another marvellous post, this one about window flies. It discussed insect vision and had this tidbit.

for small arthropods like insects, everything they see is something that we would consider to be a “macro shot”. And on top of it all, they are highly size-limited, they can’t have the honking huge lenses that we use on our cameras and microscopes for looking at things close up. For the smaller insects, like these, their whole eye has to be on the order of the size of a pinhead. So, how can they do it?

Some other arthropods, like spiders, basically can’t do it. They do the best they can with simple eyes (a single lens, and an array of light receptors to make an image), but even the best spider eyes are evidently low-resolution and what we would consider fantastically near-sighted. Which is why a lot of spiders pretty much give up on the whole “vision thing” altogether, settling for enough eyesight to basically detect light and movement, and letting it go at that.

That got me to thinking about spiders' brains. For example, is a tarantula smarter than a wolf spider or is it just the Costco-sized version of its little cousin? Here's a little bit I've discovered so far.

First off, back to the eyes. It turns out that jumping spiders can see pretty well.

The jumping spiders, Salticidae, are well known for their excellent eyesight. Their two AME eyes are greatly elongated and push deep into the animal's head. They work like a telephoto lens and have a movable retina to increase the visual field. Strangely they also have four layers of rhabdomeres. They are also known to be sensitive to four different wavelengths of light, 360 (Ultraviolet), 480, 500 and 580 nanometres. A jumping spider can distinguish objects at thirty to forty centimetres distance with its secondary eyes, once it is within about twenty centimetres of its prey it can see it with its AME main eyes.

They're an oddity in the arachnid world - like Tim said, most spider eyes are good only for detecting light and shadows. Back to the brain.

From the Arachnoboards comes this (spelling errors in the original, some corrections made).

When it comes to animals, other factors differ, like what proportion of the brain mass is made of connected neurons, is there any grey matter or the whole brain is only made of nerves intersections and nodes? In this regard invertebrates are way less endowed.

Finally, another key factor is how this brain is stimulated from the outside. Take two humans identical twins, have one live in a cell with no book or interaction or stimulus and you'll see that he won't measure up in any aspect to the one who grew up normally. Because of this, I would have a tendency to say that burrower Ts might be less stimulated since there is little intellectual stimulation when 90% of the living time is spent at the bottom of a tunnel where nothing occurs. On the other hand, an arboreal T has way more to deal with by simply wandering in the branches of a tridimensionnal tree, trying to analyse air movement to figure out if it's only a breeze or an airborne predator comming...

Going back to the spider anatomy discussed in the first link, we have this.

The nervous centre of a spider is situated in the prosoma. The central nervous system is compacted with the brain to produce a single mass of nervous tissue. This single mass can be divided into the lower star-shaped subesophageal ganglion and the upper spherical supraesophageal ganglion. A number of nerves arise from these ganglia and spread out to the body, making up the peripheral nervous system.

The supraesophageal ganglion can again be divided into the cheliceral ganglion and the brain. The cheliceral ganglion controls the musculature of the chelicerae, the pharynx and the poison glands. The brain is mainly concerned with association activities. It only receives information from the eyes, via the optic nerve.

Motor nerves that control the the legs and the pedipalps originate laterally from the subesophageal ganglion. Posteriorly the subesophageal ganglion gives rise to a set of nerves called the 'cauda equina' which pass through the pedicel to control the opisthosoma.

Imagine a central nervous system where all nerves and the spine as well is smushed into a single blob in your head and then radiate out from there. I think that's where the remark "the whole brain is only made of nerves intersections and nodes? In this regard invertebrates are way less endowed" comes in. The spider brain is then just a big switchboard with a tiny bit of intelligence for adapting to repeated stimuli. A bigger spider might have a bigger brain, but that brain might have the same intelligence as a smaller spider because the "switchboard" part is larger to deal with all of the added sensors across a larger body.

This is worthy of some more research. Thanks, Tim, for bringing to light another of God's wonders to explore!

Image used without permission from the Spider Anatomy page.