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The Australian redback spider (Latrodectus hasselti) is one of the widow spiders (Latrodectus), a cosmopolitan group of small, round-bodied spiders. Although its native range is presumed to be Australia, L. hasselti has been inadvertently introduced to New Zealand, Japan, Southeast Asia, and possibly elsewhere (Garb et al. 2004 and references therein). The New Zealand endemic Latrodectus katipo appears to be the closest extant relative to L. hasselti. The phylogeny and biogeography of the genus Latrodectus have been reviewed by Garb et al. (2004).

Female redback spiders are black (occasionally brownish) with an obvious orange to red longitudinal stripe on the upper abdomen, with the red stripe sometimes being broken, and an hourglass-shaped red/orange spot on the underside of the abdomen. Females are about a centimeter long, but males are just a few millimeters. The males' red markings are often less distinct. The male's body is light brown with white markings on the upper side of the abdomen and a pale hourglass marking on the underside. Juveniles have additional white markings on the abdomen (http://australianmuseum.net.au/Redback-Spider, accessed 24 December 2009)

Latrodectus hasselti is well known for its sexual cannibalism. Females often consume males during copulation following the stereotyped self-sacrifice ‘‘somersault’’ behavior performed by the male (Forster 1992; Andrade and Banta 2002). After insertion of  one of his copulatory organs (palps), the male turns a somersault through 180 degrees so that his abdomen comes to rest against the female's mouthparts, whereupon she may begin to devour him (Forster 1992). The cannibalistic process is slow and males may use their two palps to copulate sequentially with a single female--inseminating one of her two independent sperm storage organs with each palp--then sacrifice themselves to their cannibalistic mates. This extreme strategy increases paternity for the mating in question, but also results in the male's death, eliminating the possibility of copulating with additional females (Andrade 1996; Andrade and Banta 2002). Andrade et al. (2005) documented a novel male trait—an abdominal constriction that appears during courtship—that allows a male to survive partial cannibalism from the first copulation and go on to mate with a female a second time. Inseminating both of the female's sperm storage organs by copulating with her twice dramatically increases the male's paternity share in the event the female proceeds to mate with additional males. This constriction allows males to overcome the potential fitness limit imposed by their own suicidal strategy by prolonging survival across two cannibalistic copulations.

A female may mate sequentially with different males and under some circumstances may choose to reduce the paternity share of one of the males by consuming him sooner rather than later (Stoltz et al. 2009).

As is true for other Latrodectus, during Latrodectus hasselti copulation, a discrete portion of the male's copulatory organ (the apical sclerite) breaks off and remains in the female's reproductive tract (Andrade 1996; Snow et al. 2006). This broken off apical sclerite reduces sperm competition from other males subsequently mating with the same female by acting as a "sperm plug", physically blocking access to the female's sperm storage organ (Snow et al. 2006).

Latrodectus hasselti is among the most medically significant spiders in the world. Humans are bitten far more often by females than by the smaller males. The bite often causes severe pain and other symptoms lasting for days (Isbister and Gray 2003). A variety of Latrodectus widow spider species are found around the world and their bites tend to produce similar symptoms. Bites are often followed by  the gradual onset of severe, unremitting local, regional, and systemic symptoms. These may include pain at the bite site or in the whole of the bitten limb, intense sweating, piloerection ("goosebumps"), and muscle fasciculations (twitching). Symptoms may become severe and generalized and include hypertension (elevated blood pressure), tachycardia (abnormally rapid heartbeat), nausea, vomiting, and abdominal pain. Although death is extremely uncommon, patients may be left with serious symptoms for days or weeks if left untreated (Graudins et al. 2001). There is an effective antivenom treatment for L. hasselti bites and several studies have found this antivenom to be effective in treating bites from other Latrodectus species as well (Gaudins et al. 2001). According to Wiener (2003), there have been no deaths from L. hasselti bites since antivenom became available in 1956.

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