Chelicerae

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The Chelicerae are mouth parts of the Chelicerata, a subphylum that includes arachnids, Merostomata (horseshoe crabs), and Pycnogonida (sea spiders). The chelicerae can be divided in three kinds: jackknife chelicera (green in the image to the right), scissor chelicera (blue) and 3-segmented chelate chelicerae (red).

Jacknife chelicera

The jacknife chelicera is subchelate (with fixed finger much reduced or absent) and is composed of two segments, it occurs exclusively in the Tetrapulmonata.

Jacknife chelicera presents two different forms: orthognathous and labidognathous.

The orthognathous chelicera is articulate in a manner that enables movements of the appendages parallel to the body axis. This kind of chelicera occurs in the Liphistiomorphae and Mygalomorphae spiders and in the related orders Amblypygi, Schizomida, Thelyphonida.

The labidognathous chelicera moves at right angles to the body axis. This kind of chelicera is rotationed and occurs exclusively in the Araneomorphae spiders.

The Orthognathous chelicera, found in Mygalomorphae spiders.

The Labidognathous chelicera, found in Araneomorphae spiders.

Spider chelicerae

The chelicerae consist of a base segment that articulates with the cephalothorax and a fang portion that articulates with the base segment. Think of a two-segment finger, the outer segment of which consists of a hollow fang. Except for two very small families of spiders, they all have venom that is injected into their prey through openings in the tips of their fangs. The glands that produce this poison are located in the two segments of the chelicerae, and, in most spiders, extend beyond the chelicerae and into the cephalothorax. Chelicerae are also found in related creatures such as the scorpions.

Phidippus audax, jumping spider:The basal parts of the chelicerae are the two irridescent green mouthparts.

This overhead drawing shows the chelicerae in black, the surface of the cephalothorax in brown, the spiders legs in reddish brown, and the poison glands and surrounding muscle tissue (used to squeeze out the venom) in green.The fang portion of the right chelicerae can be seen projecting into the space between the two chelicerae.

The second drawing shows a side view of the cephalothorax of the spider in brown (with the legs removed) and the two portions of the chelicera.

Scissor chelicera

The scissor chelicera is chelate and composed of two segments and occurs in the orders Pseudoscorpiones and Solifugae.

3-segmented chelate chelicerae

This is the primitive condition and occurs in arachnids such as the Scorpiones and the Opiliones. It also occurs in non-arachnid Chelicerata such as the Xiphosura and Eurypterida. Arguably the chelifores of the Pycnogonida are homologous to the chelicerae and should be included in this count.

 

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