A few weeks ago, I introduced you to a project I’m working on in my lab with social spiders in this post here. In that post, I talked about overarching differences between solitary, subsocial, and social spiders that will factor into my research question about spider brains – we’ll get to the question in a few posts.
I thought I’d move in this post to discussing the spider brain, which resides in the cephalothorax – or the first section (not the silk spinning abdomen) of the spider. When I started this project, I thought a spider looked like my drawing to the right – and many of our popular representations of the spider incorrectly show the legs coming off the abdomen (think Halloween decorations). It’s important to remember that the legs actually come out of the first section, the ‘head’ of the spider; it plays into the really cool layout of the brain/central nervous system.
To the left is a picture of the ‘ventral’ portion of the spider nervous system – called the subesophageal ganglion – the V shaped bit in the center of the picture, in lighter blue (the darker blue is muscle – wow, these spiders are strong!). It sits really close to the belly of the spider, because this portion of the CNS is responsible for movement in the spider and thus needs to be close to the legs. It takes up most of the head, with several discrete sections, radiating out from a central body. The two sections at the top of the photo innervate the pedipalps – sensory organs near the mouth in spiders. The other eight sections each innervate one of the spider’s legs, and the very bottom of the photo is where the nerves go to the abdomen.
These structures are made of motoneurons (neurons that control movement) that go out, into their respective organs/legs and sensory neurons that come in – giving chemical and mechanical information from hairs that cover the body and legs. In the smallest of spiders, these regions can extend pretty significantly into the legs as the spider has a limit to how small its brain can be and still function.
The subesophageal ganglion is really large, compared to the ‘brain’ portion of the central nervous system – called the supraesophageal ganglion (so named because the esophagus runs between the sub and supra sections of the spider CNS). The supraesophageal ganglion is pictured to the right and is about a third the size of the subesophageal ganglion; you can see the central body, the strip at the bottom, and the main mass of the brain in front of it. This is the part of the brain responsible for receiving input from the eyes, learning, memory, and other pre-programmed behavior (more classic ‘brain’ activities). It is dorsal to the subesophageal ganglion, meaning it sits (unsurprisingly) closer to the eyes while the sub is closer to the legs.
Below and to the left are some photos of my 3D reconstruction of an Anelosimus guacomayos brain – enjoy! You can really see the difference in size between the supra and sub, and the large space above the sub where the stomach of the spider sits. In my next post I’ll talk about some of the incredible behaviors this tiny CNS is capable of – more than you’d think! Does the spider brain look like you expected? Cool – or creepy? Share your thoughts with me in the comments below!
Sources: Check out the paper linked below for more great views of spider brains, and a good diagram showing the sub/supra divide at the esophagus.
Park Y, Moon M (2013). Microstructural Organization of the Central Nervous System in the Orb-Web Spider Araneus ventricosus Araneae: Araneidae). Applied Microscopy, 43, 65-74.
Many thanks to Leticia Aviles for the specimen.