Geoscience Reference
In-Depth Information
metre in September and October from an area of limestone grassland with a well de-
veloped litter layer at Wytham Wood, Oxford. Spread out evenly on a grid, this last
density would be equivalent to about one spider every three centimetres but in the
three-dimensional turf environment it would not appear so great.
Spiders may be divided into four groups, or 'guilds' to use the ecological term,
according to the way in which they catch their prey: web spinners, jumping spiders,
ambush spiders, and wandering or running spiders. In the first group we have the
money spiders, Linyphiidae, which feature predominantly in many samples from ar-
able soil, turf and litter. This large group of small spiders abounds from the sea's edge
to the tops of our highest mountains. It includes some of nature's most bizarre cre-
ations with stalked or lobed 'heads' bearing a ring of eight eyes for all-round vision,
as befits hunters which are also prey to many larger predators in the dense micro-un-
dergrowth of grass turf. Linyphiids are responsible for the hammock-like webs and
carpet of gossamer that sometimes adorns the grass on dewey mornings. Ground-liv-
ing species may only spin rudimentary webs or mere tripwires across soil crevices or
in leaf litter as described in
chapter 1
. The young of several abundant species have
perfected the art of parascending on silk threads as a means of dispersal. Such aero-
nautic activity takes place all through the year but is most noticeable in autumn when
the population densities are highest.
The significance of this behaviour is illuminated by some interesting observa-
tions on the spider fauna of sewage filter beds by E. Duffey. The beds provide an ar-
tificial subterranean environment which supports high densities of some linyphiids.
Leptorhoptrum robustum
was recorded at the Birmingham Sewage Works at a mean
density of 29,000 per cubic metre over a 28 month period. The deep layer of clinker
affords numerous small air spaces where temperature and humidity remain fairly con-
stant, and where sewage input provides a rich nutrient resource. These conditions sup-
port a high population of enchytraeid worms and fly larvae on which the spiders feed.
When the flow of sewage is shut off for maintenance work, the bed begins to dry out.
This results in massive mortality of the prey species, and triggers a wholesale emig-
ration of the spiders. The surface of the filter beds becomes covered with silk at this
time, and workmen have reported bites from the spiders which cause irritation and
swelling.
L. robustum
occurs widely in marshy areas in Britain where periodic drying
out would create the need for dispersal to more favourable sites.
There are several ground-living spiders that use webs to line burrows under
stones or down large cracks. Here they wait to pounce out upon passing insects that
make contact with the silk threads on the surface. The purse spider
Atypus
constructs
a closed finger-like tube which lies flaccid over the soil surface, and may extend for
20 centimetres from the burrow. When a beetle, earwig or other insect walks over the
