3/7/2024 0 Comments Ground dwelling spiders in ks![]() It is estimated that the world's 25 million tons of spiders kill 400–800 million tons of prey per year. The species Bagheera kiplingi was described as herbivorous in 2008, but all other known species are predators, mostly preying on insects and on other spiders, although a few large species also take birds and lizards. The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appeared in the Triassic period, before 200 million years ago. True spiders have been found in Carboniferous rocks from 318 to 299 million years ago, and are very similar to the most primitive surviving suborder, the Mesothelae. Spider-like arachnids with silk-producing spigots ( Uraraneida) 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. 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. Īnatomically, 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, there has been debate among scientists about how families should be classified, with over 20 different classifications proposed since 1900. As of November 2023, 51,673 spider species in 136 families have been recorded by taxonomists. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every land habitat. 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 limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk.
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