Argiope cross-weaving spiders; stabilmenta and tiny males

I have come across these Argiope orb-web spiders, a number of times, with their characteristic woven crosses on their webs. These ones photographed in northern Thailand, are either the Multi-coloured Saint Andrew’s cross spiders (Argiope versicolor), or the very similar to Garden cross spiders (Argiope pulchella); the two were once thought to be the same species and are impossible to separate in the field (Levi, 1983). Andrew’s Cross spiders (Argiope keyserlingi), which are found in Australia are also very similar and there are at least nine Argiope species in southeastern Asia (Malayan Peninsula, Burma, Thailand). All of these Argiope species have a narrow head, and very wide thoracic region (see below).

All Argiope spiders build web decorations – called stabilimenta – which are made of white aciniform silk (the same type that is used to wrap up prey items!). Web spiders such as this can manufacture up to seven different types of silks, each with different degrees of strength and extensibility, which are used in different parts of the web and for different functions (Eisoldt, et al., 2011). There are various theories regarding the purpose of the stabilmentum, including to attract prey, or distract predators, or perhaps even to warn flying birds of the presence of a web (to avoid having to mate it again!). The fact that the female lines up her legs against the four parts of the cross (see above) suggests to me that it is also a way of ‘hiding’, or at least reducing her visibility on the web, although the brightly coloured thorax does make her body rather apparent! The males are much smaller than the female (see below), but also adopt a similar X-shaped  posture, with their legs held together in pairs, forming a cross! The one shown below has lost two of his legs (read on!).

Juvenile female St Andrew’s cross spiders (Argiope keyserlingi), and presumably juveniles of the other species as well, make a different sort of web: a complex  disc-like (discoid) stabilmentum (see below). Perhaps they are just learning how to do the knitting at this stage?!

Here is another image (below) of a complex, disc-like (discoid) stabilmentum, woven by a juvenile (?) Agriope spider from Tanzania, taken from the Wikipedia site on web decoration. Maybe they are more concerned with remaining hidden – behind the round stabilmentum – when small, than when mature? The stabilmentia get proportionately smaller, as both the spiders and their webs, get bigger (Seah, 2002).  Also, the X-shaped stabilmentia are where mating occurs. They are sometimes called writing spiders because they make these incredible patterns; more like scribbling than writing though!

The male of this particular pair appeared to have lost two of his legs (see below). An accident? Or had he lost them whilst trying to seduce the female? Female orb-weaving spiders often try and kill (and then eat!) the male before copulation can take place (Elgar, 1991)! The female may also attack and attempt to consume the male after copulation as well! The courtship behaviour of these Argiope spiders can be quite complex, but varies from species to species. In some cases, courtship takes place in the middle (at the hub) of the female’s web. The male cuts a hole in the web close to the hub (there is a hole in the photo, below!) and inserts a special mating thread which he vibrates to attract the female. If she likes his tune, she moves the short distance from the hub onto the mating thread ‘where she assumes an acceptance posture’ (Robinson & Robinson, 1980). Mating occurs on this thread. In other species, the male constructs a mating thread outside the female’s web, and plays his vibratory tunes away from her web. The female then moves across her web and out onto the mating thread, where mating occurs. Had mating just taken place here? The circumstantial evidence was a hole in the web and a male missing two legs! Perhaps, but we cannot know for sure.

Whilst I was taking these pictures, I noticed that the female captured a small weevil (see below, encircled in white).

It did not take the female long to truss up the little beetle! These photographs (above and below) were taken less than a minute apart.

Soon, the female was tucking into her little, wrapped-up snack (below).

Wasp spiders, Argiope bruennichi, also make stabilmenta; I came across this one with a rather unimpressive stabilmentum in northern Spain (below). At first I thought that the upper part of the stabilmentum (above the spider) had become damaged, but a little Googling showed that stabilmenta of this sort – where only the pattern below the spider looks like an intended zig-zag, appear to be typical of this species. E.g. see here. There is a genetic basis to the production of stabilmenta, which differ considerably in terms of shape, size, and frequency of occurrence, between Argiope species, but all of these features are also affected by environmental conditions, especially the type and abundance of prey.

Perhaps the Garden Cross spiders (Argiope sp.) could teach our European Wasp spiders how to improve their knitting! I wonder how stabilmenta evolved? Did they start with one zig-zag, then add another and eventually hit upon the idea of a cross (where they can hide with their legs held together in pairs!)?

References

Eisoldt, L., Smith, A., & Scheibel, T. (2011). Decoding the secrets of spider silk. Materials Today, 14(3), 80-86.

Elgar, M. A. (1991). Sexual cannibalism, size dimorphism, and courtship behavior in orb-weaving spiders (Araneidae). Evolution, 45(2), 444-448.

Kim KW, Kim K, Choe JC. 2012. Functional values of stabilimenta in a wasp spider, Argiope bruennichi: support for the prey-attraction hypothesis. Behavioral Ecology and Sociobiology 66: 1569–1576.

Robinson, M. H., & Robinson, B. (1980). Comparative studies of the courtship and mating behavior of tropical araneid spiders. Comparative studies of the courtship and mating behavior of tropical araneid spiders., (36).

Seah, W. K., & Li, D. (2002). Stabilimentum variations in Argiope versicolor (Araneae: Araneidae) from Singapore. Journal of Zoology, 258(4), 531-540.