A spider for Christmas
Published 26 January 2022
In many warmer parts of the Old World, the spiny orb-weavers of the subfamily Gasteracanthinae are among the most eye-catching of all spiders. As well as constructing complex, easily seen webs in the manner of other orb-weavers, these spiders draw attention by their bright colours and ornate structure, often with prominent arrangements of spines on the abdomen. Here in Australia, their dramatic appearance has lead to their often being referred to as “Christmas spiders”. The exact reason for this drama is uncertain. The spines are generally presumed to be for defence but the coloration has been subject to multiple proposals from an aposematic warning to functioning as a lure for flying insects.
The taxonomic history of the Christmas spiders is a complicated one, going back to the early years of arachnology. Not surprisingly for such distinctive animals, a large number of species were described by early authors. However, species of spiny orb-weavers are often very variable, leading to a significant number being described as new on more than one occasion. As with other orb-weavers, males are much smaller than females, and the spines on the abdomen tend to be more poorly developed. Coloration within a species can vary considerably in brightness, tone, and patterning. Structural features such as the arrangement of spines and the development of sigilla (impressions on the dorsal surface of the abdomen that mark the placement of internal muscles) can still provide reliable indicators of species identity, as (of course) can features of the genitalia. You have to learn to look past the superficial daubings and focus on the underlying form.
Christmas time again
Published 24 December 2022
With their distinctive appearance, the gasteracanthines have long been recognized as a coherent group. As well as the colourful, spinose abdomens, gasteracanthines are distinguished from most other orb-weavers by the presence of a sclerotized ring around the spinnerets. Other noteworthy features include the presence of a paramedian apophysis on the male pedipalp, and a carapace that is broader than long and rectangular in females (Scharff et al. 2020).
The spinose abdomen and sclerotised ring around the spinnerets are shared by the Old World gasteracanthines with an assemblage of New World orb-weavers dubbed the “micrathenines”. However, recent phylogenetic analyses of the Araneidae have agreed that the gasteracanthines and micrathenines represent independent lineages that have converged on a similar morphology from separate ancestors. One of the most dramatic examples of this convergence is between the southern Asian Macracantha arcuata and the South American Micrathena cyanospina. Both these species bear large curving horns towards the rear of the abdomen that are longer than the rest of the body.
As noted above, gasteracanthines exhibit strong sexual dimorphism with males being considerably smaller than females. To court a potential mate, the male attaches a line of silk to the female’s web and then vibrates it to lure her over. During copulation, the female returns to the centre of her web, carrying the male with her. Unlike other orb-weavers, female gasteracanthines rarely if ever eat their prospective mates (Elgar 1991). Males can feel much more confident in the kindness of strangers.
Elgar, M. A. 1991. Sexual cannibalism, size dimorphism, and courtship behavior in orb-weaving spiders (Araneidae). Evolution 45 (2): 444–448.
Scharff, N., J. A. Coddington, T. A. Blackledge, I. Agnarsson, V. W. Framenau, T. Szűts, C. Y. Hayashi & D. Dimitrov. 2020. Phylogeny of the orb-weaving spider family Araneidae (Araneae: Araneoidea). Cladistics 36: 1–21.