Red-spotted Argyrodes

Introduction

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 and complex webs of host spiders, such as the golden orb weavers (Nephila species) and tent orb weavers (Cyrtophora species).

Identification

Many common species of Argyrodes are black with silvery markings and often not noticed because they are small. Argyrodes incursus is a small dark brown to black spider with four small orange spots around the spinnerets and, in females, a large orange-red spot on the back of the abdomen. In males, a hard plate, or scute, covers most of the upper abdomen.

Habitat

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

Distribution

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

Other behaviours and adaptations

The host spider, Achaearanea mundula, is common in the shrubby woodlands and forest margins of coastal regions. It builds a large, complex web - a tangle of lines above with a leaf detritus retreat, supporting a horizontal sheet below upon which prey falls and is captured. In summer and autumn Argyrodes incursus females invade the webs of host females, often when she is guarding egg sacs or young inside her retreat. Their occurrence seems sporadic; host webs in one area may have none but elsewhere up to eight Argyrodes incursus may be present in a single web. The invader initially sits in the upper tangle threads or on the sheet, but it gradually advances toward the host spider. By some deception the smaller aggressor succeeds in biting and killing the larger host spider within her retreat. The successful Argyrodes incursusthen remains within the leaf retreat, making a mammoth meal of the host female, her eggs and young. In the process, its once ovoid abdomen swells impressively into a rounded sphere, giving the spider the appearance of an engorged tick. Mating may also occur in the leaf retreat, male and female Argyrodes incursus having been observed there together. Having fed and mated, the female performs the ultimate act of occupancy; she constructs her egg sac and rears her young within her victim's retreat.

Life history cycle

The intimate life cycle relationship described for Argyrodes incursus with that of its 'host' Achaearanea mundula suggests that an exclusive predator/host-prey association exists between these two species. Presumably, it evolved from an initial phase of kleptoparasitism. The success of such a one-sided association may depend on the 'host' species being considerably more abundant overall than its predator - as seems to be the case here.