Sunday, December 16, 2007

Orb web Spider Tetragnatha extensa

I am an amateur U.K. naturalist trying to discover all the things living in my garden.

Further to the wealth of excitement my first spider posting provoked amongst my legions of readers (hem,hem), photo 1 (click to enlarge) shows another spider I found hanging in the centre of his web in an overgrown corner of my back garden (at (0,2) - see here).

You'll note I refer to my spider as a 'he': Using a hand lens I was able to confirm the presence of palps ending in swollen bulbs.

Back in late-April, when the photo was taken, working with my copy of Spiders (Michael Roberts, Collins Field Guide) I felt I came to a reasonably confident identification of my spider as Tetragnatha extensa. Looking now at my modest-resolution photo, I don't claim to unreservedly stand by this (T. montata might be a alternative (?) for example), but in the absence of better evidence it'll have to stand.

Eight species of the Tetragnatha spider genus are found in Northern Europe. All spin orb webs. Shortly after completeing the web, they take a few seconds to bite a hole out at the centre, there to take up residence waiting for lunch to arrive.

Gnath in biology refers to the jaws, hence Tetra'gnatha "four jaws", a reference to the fieresome mouthparts of these spiders. Photo 2 is my (untrained amateur's) attempt to sketch what would very likely be your terminal sight were you unfortunate enough to find yourself a small fly trapped in an extensa's orb web! The long 'mouthparts' on which the fangs are hinged are known as chelicerae. The chelicerae are used in mating, with the male and female locking theirs together. Atop the chelicerae sits the spider's turret-shaped head with its eight eyes.

T. extensa is one of the commonest Tetragnatha species in Britain and apparently tends to favour a residence close to open water (a slight puzzle in this case since, aside from an open water-butt, that corner of my garden doesn't have any). My copy of The Biology of Spiders (Foelix Rainer, Oxford Uni. Press) refers to a Tetragnatha spider being able to walk on water at 15-20cm/sec (the book doesn't state the species, but from the description I take it to be extensa).

Finally, in case you're wondering about the curious black cylindrical object on the left of photo 1: This is a handy design I got from Dr. Robert's book above for an insect (/spider) viewer. Very simply it comprises two concentric cylinders (in this case the plastic lids from two antiperspirant sprays). The larger has a hole cut in the top. The idea is to pop the insect between the two, and cover the hole with cling-film ('plastic wrap' to those of you reading in the US). By sliding the smaller tube up like a plunger, the insect becomes trapped, immobile against the film and hence easy to examine with a hand lens. Provided you don't keep it there too long, the critter need suffer no ill effects and can be set free afterwards. Neat heh!

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