What we can discover on a spider's
the last summer I have been making many 3D images of
spiders and insects. Since I was curious how they would
look under the microscope (and I am rather clumsy with
chemicals as well as a bit lazy) I ordered a series of
prepared slides. Maurice Smith sent me some very
interesting slides he sells in his on-line shop. (For
more information, see the bottom of the page)
One of the slides contained a couple of legs of a garden spider (Areneus diadematus). I knew that just one leg contains many interesting features. In this article I like to show you some of the things you can find on a single spider leg.
Garden spider eating a fly
(Click image to view large version)
|The garden spider belongs to the orb-web spider family (Araneidae). It catches its prey with a large web. It does not rely that much on its eyesight (which is rather poor) but more on organs that can sense the slightest trembling of the web. It is not strange that the legs contain many sensors that can detect such vibrations in the web. It is easy to demonstrate this by holding a tuning fork against the web. The spider will leave its retreat thinking its the beating of the wings of an insect.|
Tactile hair, showing its texture The wide strand on the background is a tendon (photographed with 25X obj.)
what do these receptors look like? The spider's leg is
obviously very hairy. Almost all these hairs are sensory
hairs (in fact they are more like bristles), and act as
receptors that can detect touch and vibration.
There are several types of sensory hairs. You can identify them by the way they stand in their socket. The large tactile hair above emerges obliquely. There are also much smaller so called trichobothria (they project vertically from their socket). They are extremely sensitive to air currents or low frequency air vibrations. Spiders can sense a flying insect from several centimetres. Some spiders are able to detect and catch a flying butterfly in mid-air.
There are also chemosensitive hairs, the tip of the leg contains a series of these hairs the spider uses to taste.
(Image taken with 40X phase-contrast obj.)
There is also a whole variety of other receptors. So-called slit sensilla and lyriform sensilla can be found all over the leg (most lyriform organs are situated close to the leg joints.) With these sensilla the spider senses it's own movements but they also enable it to detect sound.
Each slit has a thin membrane in which we can see a small dot. This is a dendrite. This nerve cell detects deformation of the slit.
(Image taken with 40X phase contrast obj.)
Rather difficult to find is the tarsal organ. It is a small spherical pit situated near the end of the leg (the last segment called the tarsus). They are supposed to detect odor as well as humidity.
of tarsal organ taken with 40X obj.
Within the legs I could also see the long tendons for the movement of the limbs. The legs are stretched because they are under pressure of body fluids. The spider only has to pull the cord to bend the leg.
|But I saved the most spectacular
part of the spiders leg for last. The foot of a spider is
a very interesting object to study under the microscope.
You can see how the garden-spider is able to cling to the
threads of its web. The foot possesses two claws, one
downward pointing hook and several serated hairs. The
hooked claw grabs the thread and the serated hairs hold
the thread in the hook.
There is a difference between web building spiders and spiders that stalk their prey like wolf-spiders and jumping-spiders. These don't possess the hooked claw. But they often have tufts of hairs on their foot that enable firm adhesion to a surface by means of capillary force.
(Image taken with 25X obj.)
|Detailed illustrations of how the
spider walks in it's web can be found in the Micscape
article Why a garden spider does not get
stuck in it's own web
Since spiders are easy to find and abundant it is not difficult to study these interesting creatures. If you like to study a prepared slide like I did, go to the OnView Shop to order spider legs or whole mounts of spiders and insects.
Rainer F. Foelix, Biology of Spiders (Harvard University Press)
Michael J. Roberts, Spiders of Britain and Western Europe.
The slide was a Northern Biological Supplies slide (N.B.S.) prepared by Eric Marson
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Copyright all material: Wim van Egmond 1999-2000
Published in the January 2000 edition of Micscape Magazine.
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