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Argyrodes incursus (Family Theridiidae)

Red-spotted Argyrodes


Argyodes incursus is found along the central coast of New South Wales and on Lord Howe Island. (NSW, Lord Howe Island)


Spiders of the genus Argyrodes are distributed world-wide and are best known as kleptoparasites - spiders that live in, and steal prey from the large, complex webs of "host" spiders, such as the golden orb weavers (Nephila species) and tent orb weavers (Cyrtophora species). Many common species of Argyrodes are black with silvery markings and often not noticed because they are small. Argyrodes incursus is also small, but its body is dark brown to black and has four small orange spots around the spinnerets. Females bear a large orange-red spot on the back of the abdomen. In males, a hard plate, or scute, covers most of the upper abdomen. Body length is 3 to 4.5 mm in females and 2 to 2.5 mm in males.

Ecology/Way of Life:

The Red-spotted Argyrodes has been found only in the web of its larger host, the spider Achaearanea mundula (family Theridiidae; body length 5 to 7 mm). Unlike more typical Argyrodes species, which may also be present in these webs, Argyrodes incursus is not a food thief, but a killer that eats its host and her babies and moves into her home. The significance of its orange-red spots is unknown; they may a warning colour to deter predators.

The host spider, Achaearanea mundula, is common in shrubby woodlands and forest margins along the New South Wales coast. It builds a large, complex web, with tangle threads above, a retreat formed of leaf fragments, and a horizontal sheet of silk below, onto which prey falls and is then captured. In summer and autumn, females of the Red-spotted Argyrodes invade the web of a host female, often when she is guarding her egg sacs or young inside her retreat. Numbers of these invaders is variable, ranging from none to eight on the web of a single host.

At first, the invader sits in the upper tangle threads or on the sheet, and then gradually advances toward the host spider. By some deception, as yet unclear, the successful invader manages to bite and kill the considerably larger host within her retreat. The successful Red-spotted Argyrodes then remains within the leaf retreat, making a mammoth meal of the host female, her eggs and young. In the process, her once ovoid abdomen swells impressively into a rounded sphere, giving her appearance of an engorged tick. Observations of male and female Red-spotted Argyrodes together in the leaf retreat suggest that mating may also occur there. After feeding and mating, the female then performs the ultimate act of occupancy; she forms her egg sac and rears her young within her victim's retreat.

The intimate life-cycle relationship between the Red-spotted Argyrodes and its "host", Achaearanea mundula, suggests an exclusive association between the predator and its host/prey. Presumably, it evolved from an initial phase of kleptoparasitism; the success of such a one-sided association may depend on there being many more of the "host" species overall than its predator - as seems to be the case here.

Interaction with Humans/Threats:

No threats have been identified.

Other Comments:

Described in 1988 by Mike Gray and G.J. Anderson; the genus name is from the Greek: argyros = silver and -odes = like; the species name is from the Latin incursus = a hostile invader).

Further Reading:

Gray, M.R. and Anderson, G.J. (1988). A new Australian species of Argyrodes (Araneoidea: Theridiidae) which preys upon its host. Proceedings of the Linnean Society of New South Wales 111(1): 25-30.

Topics: Spider webs Silk Prey capture Kleptoparasitism


Text & map by Mike Gray; photograph by Mike Gray, courtesy of the Australian Museum.

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