Loxoscelidae

The thoracic furrow of these spiders is longitudinal and the carapace is flat. The sternum is pointed posteriorly, and the tarsi have two claws. Loxosceles is a large genus in this family; there are over 50 species in the western hemisphere.

Loxosceles laeta Males and females are 6-9 mm long and the body is uniformly brown to brownish black. The characteristic violin-shaped mark on the cephalothorax is not distinct. The palpal tarsus is longer than wide (in L. reclusa it is wider than long). Female leg 4 is longer than the other legs (in L. reclusa leg 2 is longest in both sexes). Indoors, their webs may be 30 cm in diameter and usually placed in corners; corner spider is a common name for this spider in South America. The egg-sac is loosely constructed and contains about 50 eggs. This species is distributed in South America, but isolated populations also occur in southern California. In South America this species is known as arana de rincones (spider of corners) and arana de detrás de los cuadros (spider ofbehind the pictures).

Brown recluse spider, violin or fiddle-back spider, Loxosce-les reclusa (Fig. 18.7c) Males and females are 7-12 mm long and, except for marking on the cephalothorax, the body is uniformly brown. The cephalothorax has a dark brown violin-shaped mark in the middle, with the neck of the violin directed backwards. Eyes are arranged in three groups of two. The palpal tarsus is wider than long (in L. leata it is longer than wide). The abdomen is ovoid, and in the female it is dark brown; the abdomen of the male is light brown. The venter of both sexes is brown and the spinnerets are short. Legs are long, up to twice the body length, and they are not obviously setose. Mating occurs from February to October, but typically in June and July; a single mating is sufficient to fertilize several batches of eggs. Fecundity for one season is about five egg-sacs, with about 51 eggs each; total egg production varies from 31 to 300. Hatching occurs in about 33 days, and development to adult takes about 336 days. Adult males live 301-796 days and females 356-894 days. The preferred temperature range for this species is 23-27 °C. The web is an irregular maze of random threads, and there is often a silken tube or retreat where the spider rests. Adults feed on prey caught in their web and they hunt away from the web at night. Natural habitats include around rocks or other small cavities on the ground, sometimes on cliff faces. In the urban environment they occur around houses, sheds, outbuildings, and indoors. This species occurs widely in southeastern and central continental USA.

Loxosceles rufescens Males are about 7 mm long and females about 7.5 mm long; the body markings are similar to L. reclusa. This species is known from Europe, primarily from theMediter-ranean region, but it also occurs in urban areas in the USA. It has been reported in several cities, includingBoston, New York, Philadelphia, Washington, DC, Los Angeles, and Spokane, Washington. Outside its normal range it usually remains in isolated populations in artificial habitats.

Loxosceles unicolor Males and females are 6-9 mm long; the body is uniformly light brown to yellowish brown. The violin-shaped mark on the cephalothorax is indistinct. Spiderlings hatch 15-19 days after formation of the egg-sac. Development ofmales takes 290-680 days and 276-562 days for females. The bite causes pain and skin necrosis that develop into a localized, skin ulceration. This species often occurs around houses in the urban environment. It is distributed in California, Arizona, New Mexico, Nevada, Utah, and Texas.

Other Loxosceles Males and females of Loxosceles arizonica are about 8 mm long. This species occurs in Arizona, New Mexico, western Texas, and southward into Mexico from Coahuila to Baja California. L. devia males and females are about 7.5 mm long. The violin mark on this species is not distinct or is sometimes lacking. It occurs in southern Texas and the adjacent states of eastern Mexico. In Brazil, the common medically important species are: L. gaucho, L. similis, L. adelaida, L. intermedia, L. hirsuta, and L. amazonica.

Loxoscelism is caused by the bite of L. reclusa, L. laeta, L. rufescens, and other Loxosceles species. The venom of these spiders produces mild to severe pain within 2-8 h, and a thick weal around the bite. At the site of the bite, the cells die and the necrotic area usually becomes dark and dry. An open ulcer develops in 7-14 days and persists for 2-3 weeks. As well as cutaneous lesions, the bite can result in systemic reactions, particularly in children. Bites by L. laeta produce similar reactions, but sometimes a deep-skin form of necrosis results, and healing is very slow.

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