What we can discover on a spider's leg
by Wim van Egmond
the Netherlands

During 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.)

But 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.

(Image of tarsal organ taken with 40X obj.
Cropped to show more detail)

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.

 

Further reading:

Rainer F. Foelix, Biology of Spiders (Harvard University Press)

Michael J. Roberts, Spiders of Britain and Western Europe.

Spider websites:
Venomous spiders

 

The slide was a Northern Biological Supplies slide (N.B.S.) prepared by Eric Marson

Comments to the author Comments to the author sent via our contacts page quoting page url plus : ('wegmond','')">Wim van Egmond are welcomed.

or visit Wim's HOME PAGE 

Copyright all material: Wim van Egmond 1999-2000

 

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