One of the most extraordinary structures in nature (or out of it) is a spider's web. Spider silk is inconceivably fantastic; to say that it's a protein produced by a gland (spinneret) at the rear of the animal's abdomen really falls a long way short of conveying it. It is said that a strand the thickness of a pencil could stop an aeroplane in flight - and it's flexible! What chance does a moth have? Moreover there are seven types of silk, each produced by a separate gland in the spinneret; while no spider can produce all seven, some, including the orb weavers such as Argiope and Eriophora species, can produce five. Further these different types can be combined to form a huge array of threads of different appearance and function. Much more to be said on this in future postings!
However today I'm just going to comment on one type, prompted by a story about Korean researchers who appear to have resolved the function of the wonderful ribbons of oddly conspicuous silk in the webs of some Argiope species, the most familiar of which in eastern Australia is known as the St Andrew's Cross Spider A. keyserlingi. I say 'oddly conspicuous' because it would hardly seem to be of benefit to the spider to make its food trap too visible.
|St Andrew's Cross Spider in web, Nowra.|
Some on-line sources report that this species occurs over much of Australia, but I trust the
Atlas of Living Australia which says that it is restricted to the east coast.
The ribbons are known as stabilimenta; at least eight other spider groups appear to have independently 'invented' stabilimenta. Their purpose has been debated, with the original suggestion involving strengthening and stablising - hence the name. Others include making the web visible to avoid accidental damage by large animals, breaking up the outline of the spider in the web for camouflage and reflecting ultraviolet to attract prey to the nest. This last suggestion is the one the researchers pursued, and indeed it appears to explain at least part of the purpose of the structure. This is based on their observation that the twenty families of large pollinating insects - including butterflies, moths, wasps, flies and beetles - caught in the decorated webs of a related Argiope species were twice as likely to be caught by these as by undecorated webs.
The key points here are that 1) the ribbons in the web reflect much more ultraviolet light than the rest of it does, and 2) we know that such insects are attracted to ultraviolet patterns reflected by their preferred flowers. The strong suggestion is that the spider is using this behaviour to lure the insects to their doom, and the spider's dinner table. Quite properly the researchers point out that this isn't necessarily the only purpose of the decoration, and may not even have been the original one, but this is how evolution works, adapting structures and behaviours for purposes as the need arises.
|A closely related and similar Argiope species from Litchfield National Park, Northern Territory.|
Note the characteristic posture, with the legs held in four pairs.
(The original article is here, but unless you have 35 euros to spend you can't have access to it! However it has been reported on elsewhere - just search for the title.)