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Identifying the Brown Recluse Spider

Monday, October 10, 2011 | Mid-Cities Pest Control

One can readily learn how to identify recluse spiders with less than a minute’s training. Whereas most U.S. spiders have 8 eyes, typically arranged in 2 rows of 4, the recluse spiders have 6 eyes arranged in pairs (dyads) with one anterior dyad and 2 lateral dyads (Fig. 1). All 13 species of U.S. recluses (11 native, 2 non-native) share the same eye pattern. In many publications, the violin pattern on the cephalothorax (the first body part to which the legs attach) is mentioned as a diagnostic characteristic (Fig 2). Although it is quite consistent in adult brown recluses (although it can fade in preserved specimens), many western U.S. recluse species and some young brown recluses have virtually no contrasting pigmentation in the violin region (Fig. 3, 4). In addition, recluse spiders have abdomens that are devoid of coloration pattern and their legs are covered with fine hairs but lack thickened spines.

Figure 1

Figure 2

Close-up of the cephalothorax of the brown recluse spider, Loxosceles reclusa, (from Missouri) showing the pattern of 6 eyes arranged in dyads A preserved brown recluse spider showing the strongly contrasting and well-defined violin pattern on the cephalothorax as well as the patternless abdomen covered with fine hairs.

Figure 3

Figure 4

A desert recluse, Loxosceles deserta, (from near Indio, Calif) showing the lack of a strongly defined violin pattern. The cephalothorax of a preserved desert recluse showing the characteristic recluse eye pattern and a uniformly colored cephalothorax

Misidentification of spiders as brown recluses is not uncommon both in the lay and medical communities. Many of these mistaken spiders are similar in only one trait with actual recluse spiders, with some only sharing the characteristics of brown color and eight legs.

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