Cheliceral fang

The Cheliceral fang of a spider is so called because the chelicerae of spiders consist of two parts, one containing all or part of the glands that produce the spider's venom and the other part a kind of organic hypodermic needle through which the venom is injected into prey animals or other animals that the spider bites, e.g., in self defense. The term "fang" ordinarily describes the two long prominent teeth of canines that are used to produce deep wounds, but the fangs of spiders are structurally more like the fangs of venomous snakes since their teeth are hollow and are used to inject poison. Spider fangs, however structurally similar, evolved some 400 million years ago (while snakes are some 150 million years old) and they are not made of calcium. Instead, they are composed of chitin, which is a kind of protein. The fangs grow again whenever the spider molts which is a great advantage in case the spider had lost or broken one of them; however, as chitin needs a few days to become hard, the spider is vulnerable before the fangs are fully useful again.

The biting apparatus of spiders is quite well adapted to penetrating much larger objects. Unlike human teeth which can exhibit problems with even large apples, the fangs of spiders can be directed more-or-less straight into a flat surface such as the palm of a human being. The reason is that unlike the human jaw which is hinged at each side and can only open a few degrees, the chelicerae of spiders are hinged where each chelicera meets the cephalothorax and again where the fang joins the basal segment. By opening both of these joints fully, the spider has great freedom to vary its angle of attack. The fangs themselves are curved to match the arc through which the chelicerae move, making the biting action streamlined and elegant.

Not only is the range of motion extremely advantageous to the spider who intends to bite larger animals, but the fangs themselves are of a very fine diameter and therefore very sharp as well. Some spiders that hunt their prey by secreting themselves in silken tubes through which they bite to impale insects that attempt to cross over these tubes have chelicerae that are very large and powerful in comparison to the moderate size of the spiders (fangs of the wandering hunting spiders such as "Lasiodora parahybana" can be as long as 20 mm which means that if the spider bites a human, the sheer mechanical damage itself has to be considered serious too). Other spiders, such as the widow spiders (e.g., "Latrodectus mactans", the Black widow) have tiny fangs whose length approximates the diameter of the lead of a mechanical pencil, i.e., less than one mm. However, their fangs are quite sufficient to penetrate to a depth of the skin that supports capillary circulation. Even much smaller spiders, whose venom will not produce such extreme effects, may succeed in injecting venom into humans. There is one record of a human being bitten by a male of the Latrodectus genus, and the males are typically about 1/10 the size of the females, with correspondingly shorter fangs (see Redback spider).