The other day while sorting pitfall samples I came across a pair of chelicera. As they were separated from their former owner it gave me a chance to photograph and view some of the specific anatomy. (I should note that I checked all the spiders in my samples, and they all had their chelicerae, so I am not sure where these ones came from.)
Chelicerae are the first appendage on the prosoma of a spider, although in embryonic spiders they are behind the mouth (Foelix 2011). The chelicerae are composed of two primary parts: 1. The Paturon or Basal area 2. The articulated fang. Spiders are subdivided into two major infraorders: Mygalamorphae and Araneomorphae. One can tell the difference in the infraorders by the orientation of the fangs; in Mygalamorphae they are articulated such that they run parallel to each other and the body of the spider, in Araneomorphae they are angled or perpendicular to the body (Beccaloni 2009).
This pair of chelicera is clearly from an Araneomorphae. My first pair of photographs (Photos 1 and 2) shows the anterior and posterior view, and you can see that the fangs would be perpendicular to the main body. If you look closely at the area where the fangs fold up against the basal area you can see the cheliceral furrow (Photo 3). In some spiders the edges of the cheliceral furrow are lined with teeth. This pair has both retromargin (towards the rear of the spider) and promargin (towards the front of the spider) teeth (Photo 4). Looking at the chelicera from the ventral side you can see how the fangs fit nicely into the furrow (Photo 3). Spiders that have cheliceral teeth use them to hold and crush their prey (Foelix 2011). In many cases the number and placement of cheliceral teeth are used to aide in identifying spiders to species.
The next photo clearly shows the opening of the venom gland (Photo 5). This is how the spider delivers its venom to its prey. You can see that the opening is not at the very tip of the fang. This location provides a mechanically stable fang and helps prevent the opening from getting clogged (Foelix 2011). You can also see the hinge joint that allows for the movement of the fang. The chelicerae themselves can also be moved, allowing the spider mobility and flexibility in prey capture.
The last feature I wanted to point out on this pair of chelicera is the boss or condyle at the laterobasal angle (Photo 6). A condyle in general is a prominence of bone resembling knuckles, so you can see why it gets it names here, as they are very knuckle like protuberances.
Chelicerae are a good multiuse tool for spiders. They can be used for protection, digging burrows, carrying egg sacs, and creating sound. As you can see in this photograph of a different spider’s chelicera there is a stridulatory file on the lateral edge of the chelicera (Photo 7). You can imagine the spider strumming this with their pedipalps like a human strumming a washboard or frottoir. One other interesting feature is shown on this spider’s chelicera; there is a tooth like structure, that rather than lining the edge of the cheliceral furrow protrudes from the anterior surface of the basal area. This feature is called a mastidion (mastidia pl.)(Photo 7).
Finding this free pair of chelicera was a great opportunity for me to become more familiar with some of the specific anatomy that is used in identification of spiders. Now in case you were wondering I will make a guess on the owner of my free floating chelicera. Based on the overall appearance, the presence of the boss/condyle, and the number and placement of the cheliceral teeth I would guess that these belong to a Lycosidae, but without the rest of the spider I am not sure I can get any more specific than that.
Beccaloni, J. 2009. Arachnids. University of California Press, Berkeley.
Foelix, R. F. 2011. Biology of spiders. Oxford University Press, Oxford; Toronto.