Widow spiders (genus Latrodectus) are widely feared but poorly known. They are a medically important group with a worldwide distribution. Most species favor xeric conditions. Some species adapt well to synanthropic (human-altered) conditions and are readily dispersed by humans (Forster, 1984; 1985; Forster and Forster, 1999; Garb et al., 2004; Ono, 1995; Ori et al., 1996).

They have remarkably diverse sexual biology involving cannibalism, kleptoparasitism, mate guarding, mating plugs, post-copulatory sterility, and varying degrees of sexual size dimorphism (e.g., Anava and Lubin, 1993; Andrade, 1996; 1998; Andrade and Banta, 2002; Breene and Sweet, 1985; Forster, 1992; 1995; Ross and Smith, 1979; Segev et al., 2003; Snow et al., 2005; Snow and Andrade, 2005a; b).

There are currently 31 valid species of Latrodectus. Widow spiders are most diverse in Argentina (seven species), the Middle East (seven species), South Africa (six species), and North America (five species). There are undescribed species and nomenclatural problems, so the taxonomy of the group is unstable.


The male palp (intromittent organ) is distinctive with a long spiral embolus. The epigynum (female genitalia) features an atrium (large unified copulatory opening leading to a pair of independent copulatory ducts) and dumbbell-shaped spermathecae (female sperm storage organs). The famous red hourglass marking on the ventral part of the abdomen is not present in all species.

Medical Importance

Black widow venom is a neurotoxin that can cause latrodectism, a mild to severe medical syndrome. The bite itself is usually not perceived but local pain may occur several minutes after the bite. Typical symptoms of latrodectism include swelling of the lymphatic nodes, profuse sweating, rigidity of the abdominal muscles, facial contortions, and hypertension (Maretic, 1971; 1975; 1983). Antivenin is available to counteract the effects of Latrodectus. Cases of latrodectism are known from all geographic regions where widow spiders occur. Across the globe envenomations appear to be patchy. Actual or alleged envenomations in the various geographic regions are attributed to the various local widow species, which differ significantly in toxicity, aggressiveness, and ecology (Finlayson, 1956; Maretic, 1971; 1978; 1983; 1987; McCrone, 1964; McCrone and Netzloff, 1965; Müller et al., 1989; Müller et al., 1992; Shulov, 1940; Shulov and Weissman, 1959). Even without antivenin treatment, the mortality rate is low (on the order of 5%, mostly in small children; Schmidt, 1993).

One of the key factors influencing the epidemiology of latrodectism is the ecology of local Latrodectus species. Latrodectus tends to be urbanized in North America and Australia, while in Argentina and the Mediterranean region, Latrodectus is more likely to be found in rural or natural areas (Maretic, 1971; 1978). In regions where Latrodectus is not urbanized, latrodectism is often associated with farm workers and summer months, when farming is most intensive. In regions with urbanized Latrodectus species, latrodectism affects a broader cross section of the population, and bites may occur at almost any time of the year (Maretic, 1978).

Discussion of Phylogenetic Relationships

Garb et al. (2004) have produced the first phylogeny of Latrodectus species using the mitochondrial gene cytochrome oxidase I. This analysis produced a generally well supported tree. —