Geoscience Reference
In-Depth Information
tennae. An ability to telescope the segments into one another also enables an animal
to elongate and contract quite markedly with corresponding constriction or expansion
of the body. This helps it to burrow through loose soil in much the same way as an
earthworm.
The development of lithobiids is similar to that of millipedes in that the number
of legs increases with every larval moult. In the other two groups, the adult number
of legs is present from the first stage onwards. Truly, Nature has tried most alternat-
ives. After hatching, the tiny geophilids are unable to fend for themselves, as young
lithobiids can, and they depend on their yolk-mass for the first six to eight weeks.
During this period, they remain in a compact group in the brood chamber excavated
by the female who wraps herself round them thus maintaining a high humidity and
affording protection. The total life span is five or six years, a surprisingly long time
for such apparently delicate creatures.
S
PIDERS
There are some 632 species of spiders in Britain, divided among 31 families, but few
of them penetrate the soil to any extent and so they have generally received perfunc-
tory treatment in soil biology texts. It is true that most spiders live above the ground
altogether, weaving their webs in low herbage and bushes, or climbing on trees or
walls. Nevertheless, many are to be found living among leaf litter, in cracks in the
ground and in the larger spaces around grass roots, under stones and even on bare
ground. Here they serve as an invisible brake upon the populations of springtails, flies
and other small arthropods that live in these superficial habitats. We are largely ignor-
ant of their importance in the soil economy, as indeed we are about most predators,
but a start has been made in recent years in determining their contribution to the en-
ergy dynamics of ground-living communities.
There have been several attempts to estimate the size of spider populations of
various habitats based on straightforward searching within a square frame placed on
the ground, by extracting from soil cores using washing and sieving techniques or
heat, and by pitfall trapping. None is entirely satisfactory because the more active
kinds of spiders tend to be overestimated in pitfall traps and underestimated in soil
and litter samples. More important, though, is the structure of the litter layer and the
time of year at which the samples are taken. A grass ley grazed and trampled by cattle
has a very poor spider fauna. On the other hand, an undisturbed grassland or fen with
a good layer of decomposing litter can support a very high population. Gordon Salt
and colleagues at Rothamsted obtained an estimate of up to 130 a square metre from
an area of pasture land after first cutting off the vegetation close to the surface. By
comparison, Eric Duffey obtained estimates which ranged from 800 to 1000+ a square
