Spiders order Araneae are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom , [ 2 ] and spinnerets that extrude silk. As of September [update] , 52, spider species in families have been recorded by taxonomists. Anatomically , spiders as with all arachnids differ from other arthropods in that the usual body segments are fused into two tagmata , the cephalothorax or prosoma, and the opisthosoma , or abdomen, and joined by a small, cylindrical pedicel.
However, as there is currently neither paleontological nor embryological evidence that spiders ever had a separate thorax-like division, there exists an argument against the validity of the term cephalothorax, which means fused cephalon head and the thorax.
Similarly, arguments can be formed against the use of the term "abdomen", as the opisthosoma of all spiders contains a heart and respiratory organs, organs atypical of an abdomen.
Unlike insects , spiders do not have antennae. In all except the most primitive group, the Mesothelae , spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure.
Their abdomens bear appendages, modified into spinnerets that extrude silk from up to six types of glands. Spider webs vary widely in size, shape and the amount of sticky thread used. It now appears that the spiral orb web may be one of the earliest forms, and spiders that produce tangled cobwebs are more abundant and diverse than orb-weaver spiders. Spider-like arachnids with silk-producing spigots Uraraneida appeared in the Devonian period , about million years ago , but these animals apparently lacked spinnerets.
