Archive for June, 2015

The Golden Orb-Weaving spider – Nephila clavipes (Linnaeus, 1767) of the family Nephilidae – is known from USA to Argentina. In Costa Rica, it occurs in lowland and premontane tropical rain forests. Females make large aerial webs in which they usually occupy the centre. Orb-web spiders are effective predators and can easily subdue prey that is significantly larger and heavier than the spider (see on photo).

Two females of Nephila clavipes with prey; Costa Rica.

Two females of Nephila clavipes with prey; Costa Rica.

One of the most peculiar characteristic of this species, as well as of other Nephila species, is an extreme sexual size dimorphism, where dwarf males can be many times smaller and lighter than the females (see on photo). Numerous hypotheses have been proposed to explain the factors that may give rise to such size dimorphism in spiders. Some of them are briefly discussed here.

In the case of Nephila, it is argued that that large size in females could be driven by selection on female fecundity (= the potential reproductive capacity), acting to increase the number of offspring produced. With the high level of juvenile mortality, the production of larger numbers of offspring is crucial for survival of the species. Thus such size dimorphism is almost always due to female gigantism rather than male dwarfism.

As was demonstrated for some African species [e.g., Nephila pilipes (Fabricius 1793)], females continue to grow after reaching maturity. The females mature at varying body sizes and instars and then continue to grow by molting the entire exoskeleton except their copulatory organs (=genitalia). Apparently, this is why in Costa Rica Nephila clavipes is represented by mature females of markedly variable body sizes (although, to date, a post-maturity molting has not been described for this species).

In a short video presented below (courtesy of Alex Villegas, Costa Rica) it is shown how a dwarf male of Nephila clavipes is approaching a giant female in its attempts to mate, alas unsuccessfully this tiem. Indeed, the male is to be careful in order not to be mixed up by the female with a potential prey.

Further reading:

Kuntner, M. & Coddington J.A. 2009. Discovery of the largest orbweaving spider species: the evolution of gigantism in Nephila. – Plos; DOI: 10.1371/journal.pone.0007516

Kuntner M., S. Zhang, M. Gregorič, and D. Li. 2012. Nephila female gigantism attained through post-maturity molting. – Journal of Arachnology 40(3):345-347. DOI: http://dx.doi.org/10.1636/B12-03.1

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