"The Brown Recluse Spiders" (Loxosceles Spp.)

The brown recluse spider, Loxosceles reclusa, is considered to be the most dangerous of 11 Loxosceles species (Gertsch and Ennik, 1983) now known to occur in the United States. The brown spiders of the genus Loxosceles (also known as violin or fiddle back spiders because of the dusky maculation of the carapace) are known from two principal world areas: temperate South Africa northward through the tropics into the Mediterranean region and southern Europe; and from temperate and tropical zones of North and South America. They further note that the North American fauna is dominated by species of the reclusa group which now numbers 50 species in North America, Central America and the West Indies.

In the United States, L reclusa is considered to be the most widespread and dangerous (venomus) of the reclusa group. Of lesser importance to public health are L. arizonica, L. devia and L. deserta which are native to the Southwest. Recent introduction in the United States of the laeta and rufescens groups include L. laeta and L. rufescens. Their propensity for inhabiting household goods, furnishings, etc. enabled both these species to be imported in overseas shipments. For example, specimens of L. laeta were found in the basement of the Museum of Comparative Zoology at Harvard University, Cambridge, Mass., but were subsequently eradicated. Naturally occuring populations of L. laeta now occur in Southern California and L. rufescens occurs along the southeastern and Gulf states. Both of these species should be considered dangerous to public health, however, L. laeta envenomation may produce cutaneous and systemic reactions similar to those produced by L. reclusa in man.

Distribution. The brown recluse, L. reclusa, is the most widely distributed of the reclusa group in the United States. The natural range of this spiders occurs from Texas northward to Nebraska, eastward through Iowa, Illinois, Ohio and Indiana; and southward through the Gulf states. Although there is some evidence for range dispersion into the Mid-Atlantic states, reports of this spider beyond its range extension are more likely due to concerted collection efforts and identification and transport through commerce or domestic household movement. This spider naturally occurs in outdoor situations within its range which includes debrids, bark, leaves, stones, utility boxes, wood piles, vehicles, etc., but has adapted quite well to indoor habitats. In residences, the brown recluse is commonly found in storage areas such as closets, attics, bedrooms and other dark recesses, cracks and crevices. They frequently inhabit clothing, boxes, toys, papers, furniture and other household items. These spiders are very adaptable and can be found active in temperatures ranging from 20 to 150 degrees F. However, they are more active in temperatures of 40 to 110 degrees F. The other loxoscelids, which have similar appearance and habitat preference, include L. deserta, L. arizonica and L. devia in the southwestern United States, L. laeta in Southern California and L. rufescens in the southeastern and Gulf states. Because of difficulty in distinguishing between species and potential confusion of the loxoscelids with other arachnid families, e.g. Filistatidae, positive identification is vital.

Preying Habits. The brown recluse and other loxoscelids are nocturnal and search for food such as firebrats, cockroaches, crickets or other soft-bodied species. At first light, wandering spiders will normally return to their retreat of an irregularly spun off-white web with their prey. However, spiders will also crawl into shoes, trousers or other clothing removed at night and to be worn the next day. People are most commonly bitten in bed, dressing or cleaning storage areas. Webs are commonly found in clothing, sacks, corners, walls, ceilings or other undisturbed areas. In the outdoors or cooler weather the brown recluse will spin a cocoon-like web for a retreat (shelter). The brown recluse can survive long periods without food.

Life History. After mating, which may occur from February to October within its natural range, 40 to 50 eggs are deposited in off-white, round, 1/4 inch diameter silken cases. These are usually found in dark sheltered places on the spider's habitat. The summer months of May through August are optimal times for egg laying. From one to five egg sacs will be produced in a lifetime which normally averages from one to two years; however four to five years is not uncommon. After the eggs hatch, the young spiderlings undergo at least one molt (shedding of skin) within the egg sac. Subsequently, second instar spiderlings will emerge from the egg sac and undergo six to seven molts during the next seven to 12 months before reaching the adult stage. The presence of shed skins (exuviae) and subsequent attachment in and around residences may be indicative of infestations and enable accurate identification. Careful inspection of attics (including exposed rafters and ceiling joists), basements, closets, boxes and other household goods should reveal old and active infestation sites. The immature spiderlings resemble adults in structure but have somewhat lighter coloration. Adult males and females will vary from light tan to dark brown.