Thechelicerae (/kəˈlɪsəriː/) are themouthparts of the subphylumChelicerata, anarthropod group that includesarachnids,horseshoe crabs, andsea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulatedfangs, or as a type ofpincers. Some chelicerae, such as those found on nearly allspiders, are hollow and contain (or are connected to) venom glands, used to injectvenom into prey or a perceived threat. Bothpseudoscorpions and harvestmen have additional structures on their chelicerae that are used for grooming (papillae in pseudoscorpions,cheliceral teeth in Opiliones).[1] InParatrechalea, males and females have shown to have a chelicerae dimorphism, because the chelicerae is used as a mating signal for females.
Chelicerae can be divided into three kinds: jackknife chelicerae, scissor chelicerae, and three-segmentedchelate chelicerae.[2]
The jackknife chelicera is subchelate (with fixed finger much reduced or absent) and is composed of two segments. This type of chelicera occurs exclusively in theTetrapulmonata.
Jackknife chelicera are described in two different forms: orthognathous and labidognathous. Orthognathous chelicerae are articulated in a manner that enables movements of the appendages parallel to the body axis. This kind of chelicera occurs in theLiphistiomorphae andMygalomorphae spiders and in the related ordersAmblypygi,Schizomida andUropygi. Labidognathous chelicerae move at right angles to the body axis. This kind of chelicera is rotated and occurs exclusively in theAraneomorphae spiders.[3]
The chelicerae consist of a base segment, sometimes called the "paturon", that articulates with thecephalothorax (orprosoma) and afang portion that articulates with the base segment.[2] Almost all spiders have venom glands and can inject the venom through openings near the tips of their fangs whenbiting prey. The glands that produce this venom are located in the two segments of the chelicerae, and, in most spiders, extend beyond the chelicerae and into the cephalothorax.[2] The fang, the organic functional equivalent to ahypodermic needle is what penetrates the skin, fur, or exoskeleton of the spider's target—spider mouthparts are primarily intended for envenoming a spider's prey in most species, typicallyinsects and other smallarthropods.[2] The basal portion includes all or part of the spider's venom glands, which can be squeezed to control the amount of venom forced out of the glands.[2] Such control permits a spider to administer either adry bite, a dose appropriate to the nature of the prey or enemy, or a maximal dose.[2] The control is also necessary for actions such as the spitting of venomous silk by members of the familyScytodidae; they depend on that mechanism both in hunting and defence.
When a spider bites, the two parts of the chelicerae come together like a folding knife, and when making a threat display or actually preparing to bite, the spider will open the angle of the fangs with the basal portion of thechelicerae and also open the angle of the basal portion with the cephalothorax.[2] In the tarantulas and otherMygalomorphae, the horizontal separation of the tips of the fangs does not change much, but in the other spiders the tips of the fangs move apart from each other as well as elevating.[2] Even the tips of the fangs of the rather large spider shown above are quite sharp, and the spider's body is well adapted to driving the fangs into flesh. Some spider bites, such as those of theSydney funnel-web spider, are reported to have penetrated toenails and soft leather shoes.
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The uncate chelicera is chelate and composed of two segments and occurs in the ordersPseudoscorpiones,Solifugae,Ricinulei, andAraneae[4] (e.g.,brown recluse,cellar spider, andcrevice weaving spider).
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Having three-segmented chelate chelicerae is the primitive condition and occurs inarachnids such as theScorpiones and theOpiliones, as well as in non-arachnid Chelicerata such as theXiphosura andEurypterida.[5] The chelifores of thePycnogonida may be homologous to chelicerae.[6]
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