Spider-like arachnids with silk-producing spigots appeared in the Devonian period about 386 million years ago, but these animals apparently lacked spinnerets.
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 webs vary widely in size, shape and the amount of sticky thread used. Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of glands. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure. 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 insects, spiders do not have antennae. Similarly, arguments can be formed against use of the term abdomen, as the opisthosoma of all spiders contains a heart and respiratory organs, organs atypical of an abdomen ). Īnatomically, spiders (as with all arachnids) differ from other arthropods in that the usual body segments are fused into two tagmata, the prosoma, or cephalothorax, and 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. However, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by the over 20 different classifications that have been proposed since 1900. As of August 2021, 49,623 spider species in 129 families have been recorded by taxonomists. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exceptions of air and sea colonization. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk.
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