“Why the zig-zag lines on the web?” asked a Love MacRitchie walk participant.
He was pointing to a rather unique web of the Saint Andrew’s cross spider, as we were strolling along Venus Loop on our January 2019 walk.
This question spurred our topic for this month’s blog post as we searched through the online repository for some answers.
The Saint Andrew’s cross spiders (Argiope spp.) belong to the group of orb-web spiders that weaves silk webs to capture prey. Unlike the usual webs that are almost invisible to our naked eye, the Saint Andrew’s cross spiders add patterns to make their webs more apparent. Such patterns vary from species to species, and can even differ on a daily basis within the same individual. The specific pattern seen in the photo above is known as the ‘cruciate’ pattern (Bruce, 2006). Silk web decorations seen in the photo above are also known as stabilimenta. The term came about with the initial suspicion that such decorations help to improve the stability of the web. There are also other spiders who use vegetation and prey leftovers, instead of silk, to decorate their web.
Why the Showy Web?
This interesting and seemingly counter-intuitive occurrence has sparked the curious minds of many researchers. There are three popular hypotheses to explain the occurrence of web decorations (Bruce, 2006; Théry & Casas, 2008).
- Attracting Prey
- Keeping Predators at Bay
- Preventing Web Damage
1. Attracting Prey: the silk decorations are found to reflect ultraviolet light and these might be visual signals that help to attract prey.
2. Keeping Predators at Bay: the decorations are suggested to help the spider appear larger in size to deter gape limited predators or to serve as warning signals. It might also serve to confuse predators on the exact location of the spider on the web.
3. Preventing Web Damage: the decorations serve to make the spider web more obvious to larger animals, such as birds. This lowers the chance of accidental damage caused when these animals zoom by.
It seems like there isn’t one sole explanation for the function of silk decorations and it has been suggested that silk decorations might serve different purposes in different contexts. Researches have been carried out both in laboratory and field environments to test out these hypotheses. In fact, a team of researchers at the National University of Singapore have also been examining the functions of web decorations through experiments (see Li et al., 2003; Li et al., 2004).
There are definitely many mysteries in the forest waiting to be unravelled and many lessons that we and our future generations can learn from the forest. But first, the forests will need to be conserved in order for the less known tales to be told.
Bruce, M. J. (2006). Silk decorations: controversy and consensus. Journal of Zoology, 269(1), 89-97.
Li, D., Kok, L. M., Seah, W. K., & Lim, M. L. (2003). Age-dependent stabilimentum-associated predator avoidance behaviours in orb-weaving spiders. Behaviour, 140(8), 1135-1152.
Li, D., Lim, M. L., Seah, W. K., & Tay, S. L. (2004). Prey attraction as a possible function of discoid stabilimenta of juvenile orb-spinning spiders. Animal Behaviour, 68(3), 629-635.
Théry, M., & Casas, J. (2008). The multiple disguises of spiders: web colour and decorations, body colour and movement. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1516), 471-480.