Spider. Coloured scanning electron micrograph (SEM) of the underside of a spider (order Araneae).Spiders are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Magnification: x 13 when printed at 10 centimetres wide.