OTHER SOIL ANIMALS - The Soil - page 105

drainage, could absorb the equivalent of 1mm of rain a minute falling on the soil sur-
face by allowing it to drain deeply into the profile.
Heavy rain and flooding can sometimes drive worms from their burrows. One
of us (B.N.K.D.) counted 38 mature Octolasion cyaneum along a half mile stretch of
country road one dull afternoon in early May after persistent rain. Dozens more could
probably have been found on the grass verges from where they came. Similarly, large
numbers of the allied Octolasion tyrtaeum were expelled from their burrows at Wood-
walton Fen, Cambridgeshire, in July 1968 after exceptionally heavy rain when parts
of the fen were flooded for several days.
In dry weather, burrowing species such as L. terrestris retreat to more congenial
conditions deep in the ground. Species without permanent burrows curl up in a cell
and go into a period of quiescence or diapause until conditions improve again. Earth-
worms have a remarkable ability to withstand desiccation; 70-75 percent of their wa-
ter content can be lost during prolonged dry periods. Nevertheless, a long drought is
as bad as a period of severe frosty weather in causing high mortality.
The extent of wet and dry conditions is thought to control the balance between
green and pink forms of Allobophora chlorotica. Pink is genetically dominant over
green since all the offspring from a green x pink cross are pink, but these offspring
are wholly or partially sterile. The pink form appears to live in drier soils such as
woodland, gardens and well drained pastures whereas the green form seems to favour
damper habitats.
Earthworms are hermaphrodite. The smaller, surface-dwelling species produce
many egg-capsules a year which hatch and grow into adults in 20-40 weeks. Their
adult life may also be only a matter of a few weeks. These are the so-called 'r-se-
lected' worms characteristic of variable or unpredictable environments, and having a
potential for rapid increase when conditions are favourable. The larger and deeper-
living species live in relatively predictable environments and are called 'K-selected'
species. They reproduce and grow more slowly. Lumbricus terrestris may take 3-4
years to become adult, and individuals have been known to live up to 30 years 1 .
Most species of earthworms favour neutral or slightly alkaline soils and need cal-
cium for their digestive calciferous glands. About a dozen species commonly occur
in grassland, while only a few, such as Lumbricus eiseni mentioned above, can tol-
erate the more acid conditions found in moorland, heathland and conifer woodland.
The brightly banded Eisenia fetida , the brandling worm of anglers, likes well rotted
manure heaps and can produce dense populations there. Several other species are also
found in compost and leaf litter or, like the red worm Lumbricus rubellus and the
bank worm Dendro-drilus rubidus , often congregate under dung pats in pasture. River
banks and marshy meadows appear to be the haunts of some less common species.