The 66th plate from Ernst Haeckel's Kunstformen der Natur (Art Forms of Nature), showing spiders.

Species

1. Tegeocranus hericius 0.6mm, Europe
2. Tegeocranus latus 0.9mm, Europe
3. Tegeocranus cepheiformis 0.62mm, Europe
4. Leiosoma palmicinctum 1.0mm, Europe
5. Phrynus reniformis (natural size), East India
6. Arkys cordiformis (2x), America
7. Gasteracantha cancriformis (3x), Brazil
8. Gasteracantha acrosomoides (5x), Madagascar (now Acrosomoides acrosomoides)
9. Gasteracantha geminata (3x), East India
10. Gasteracantha arcuata (2x), Java (now Macracantha arcuata)
11. Acrosoma hexacanthum (4x), Brazil (now also Gasterocantha cancriformis)
12. Acrosoma spinosum (2x), South America (now Micrathena schreibersi)
13. Acrosoma bifurcatum (4x), Brazil (now Micrathena furcata)
14. Oxyopes variegatus (3x), Germany (now Oxyopes heterophthalmus)
15. Epeira diadema (3x), Germany (now Araneus diadematus)

Spiders (order Araneae) are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. Spiders are found worldwide on every continent except Antarctica, and have become established in nearly every ecological niche. As of 2008, approximately 40,000 spider species and 109 families had been recorded by taxonomists.

Anatomically, spiders differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax and abdomen, and joined by a small, cylindrical pedicel. 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 that have been modified into spinnerets that extrude silk from up to six types of silk gland within their abdomens. Spiders' 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-web spiders. Spider-like arachnids with silk-producing spigots appeared in the Devonian period about 386 million years ago, but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from 318 to 299 million years ago, and are very similar to the most primitive surviving order, the Mesothelae. The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appeared in the Triassic period, before 200 million years ago.

Ernst Heinrich Philipp August Haeckel (February 16, 1834 – August 9, 1919), also written von Haeckel, was an eminent German biologist, naturalist, philosopher, physician, professor and artist who discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms, and coined many terms in biology, including phylum, phylogeny, ecology and the kingdom Protista.