WHY A GARDEN SPIDER
DOES NOT GET STUCK IN ITS OWN WEB
by Ben Prins, Netherlands
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Of the many spiders living in our neighbourhood the garden spider (Araneus diadematus) is one of the best known. Especially in autumn the large and heavy females catch the eye, their abdomen swollen with eggs. Hanging head down in the centre of their magnificent orb webs they are waiting motionless for insects getting caught by the mass of sticky threads. As soon as that happens, the spider orients its front into the direction of the disturbance and moves without hesitation along one of the threads radiating away from the hub, to rapidly secure its prey .
This action of the spider is not risky in itself, since all the silk used for making the hub, the radiating spokes and the strong threads between which the web is hanging, is not sticky. Only the silk produced to construct the spiral thread connecting the radii, is covered with a strong glue. Yet, when we observe the spider walking at high speed along one of the spokes, it regularly touches the sticky spiral. And once having reached its prey, it does not hesitate to seize the threads of the sticky spiral with its legs, when approaching the victim as close as possible in order to immobilise it by wrapping it in masses of silk and giving it a fatal, venomous bite. Evidently the garden spider is not afraid of getting stuck in its own web and walks easily along the non-sticky as well as along the sticky threads. How does it manage to do so?
To answer this question, we are forced to catch a garden spider and to investigate the tip of one of its legs at high magnification. What we see at first glance are the two dark, serrated walking claws (see figure A). They are used for getting a firm grip on the relatively smooth surface of the branches and leaves, between which the spider constructs its web, and for moving over the soil. In front of them there is a somewhat smaller, strongly hooked third claw, that is surrounded by a number of bent hairs. The striking difference between these hairs and the other hairs elsewhere on the spiders legs is not only that they are conspicuously curved, but also that they are provided of quite a number of slender spines. These hairs and the third claw play together a crucial role in the ability of orb spiders (Family Argiopidae) to move around in their own webs.
TIP OF A LEG OF THE GARDEN SPIDER
We shall trace what happens with help of figures B, C and D. When the spider places the tip of one of its legs against a thread (see B) the hooked third claw is tilted backwards, its pointed end directing obliquely up. The thread, which always consists of two or more components of at most 0.005 mm across ( 5 micrometres), is pushed against the elastic, spiny hairs. Then the third claw rotates forward (see C), the hook seizes the thread, and forces the thread and the elastic hairs backwards. The leg is holding the thread now in a firm grip with only a minimal surface area of the thread in touch with the spines on the hairs and with the inner margin of the third claw. To loosen the thread the hook of the third claw is simply lifted and the rebounding elastic hairs, retaining their original upright position, propel the thread away from the tip of the leg (see D). In this way even a sticky thread becomes easily detached.
TIP OF A LEG OF THE GARDEN SPIDER
Although this mechanical device alone may be sufficient for a garden spider to move freely through its web without running the risk of becoming entangled, yet another provision may play a role. In a number of popular books it is stated that an oily secretion covers the legs, preventing the spider from being glued to the sticky threads. However plausible this possibility may look, not a single scientific publication could be traced as yet, which supported this statement. For the moment, therefore, the only answer to the question raised in the title, is the very special way, in which the tip of the legs of any web weaving spider is built. But an oily cover of the legs may represent an (unproven?) additional necessity for orb spiders, the only spiders producing a spiral thread covered with a glue, to avoid being glued to their own web.
Comments on the article can be sent to Ben Prins via Jan Parmentier.
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