Spider Spider Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs able to inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other orders of organisms. 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. , at least 45,700 spider species, and 113 families have been recorded by taxonomists. However, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by
Parasitic flies of domestic animals transmitted organisms to develop, multiply and be exposed to further vertebrate hosts where they can complete their life cycle. Only the adult flies are involved in this biological transmission, in contrast to biological transmission by other arthropods such as lice or ticks in which all active stages of the life-cycle feed on blood. Typical genera are "Culex", "Aedes", and "Anopheles". The basic structure of dipteran flies is illustrated in the diagram. Veterinary parasitology also covers arthropods in the Class Acari, the ticks of domestic animals and mites of livestock which have distinctly different structure from arthropods in the Class Insecta.