118: Arthropods

TAXONOMY

Arthropoda means “joint-footed” in Latin and describes arthropods’ jointed bodies and legs connected to a chitinous exoskelelton.⁵ The majority of arthropods are benign to humans and environmentally beneficial. Some clinicians regard bites and stings as inconsequential and more of a nuisance than a threat to life. However, some spiders have toxic venoms that can produce dangerous, painful lesions or significant systemic effects. Important clinical syndromes are produced by bites or stings from animals in the phylum Arthropoda, specifically the classes Arachnida (spiders, scorpions, and ticks) and Insecta (bees, wasps, hornets, and ants) (Table 118–1). Infectious diseases transmitted by arthropods, such as the various encephalitides, Rocky Mountain spotted fever, human anaplasmosis, babesiosis, and Lyme disease, are not discussed in this chapter.

Arthropoda comprises the largest phylum in the animal kingdom. It includes more species than all other phyla combined (Fig. 118–1).⁵ At least 1.5 million species are identified, and half a million or more are yet to be classified. Araneism (pertaining to spiders) or arachnidism (spiders including other arachnids) results from the envenomation caused by a ­spider bite. “Bites” are different from “stings.” Bites are defined as creating a wound using the oral pole with the intention for either catching or envenomating prey or blood feeding,^96,195 or for the purpose of feeding such as in arthropods that have mouthparts for chewing or sucking (plant sucking). “Stings” occur from a modified ovipositor at the aboral pole that is also able to function in egg laying as in bees and wasps. In scorpions the sting is not a modified ovipositor and the “tail” is not a tail but the metasoma section of the abdomen. Stinging behavior typically is used for defense. Most spiders are venomous, and the venom weakens the prey, enabling the spider to secure and digest their prey. However, there is one family of spiders, Uloboridae, which do not have a venom ...