THE SOIL FAUNA: ANTHROPODS - The Soil - page 82

Many groups of insects live on or in the soil for at least part of their active lives. Even
if the adults are free-flying creatures which exceed our earth-bound view, their larvae
may play an important role in the soil economy - and sometimes in man's economy
too. This section concentrates on those with a mainly vegetarian way of life: aphids,
crickets and their allies, and the larvae of some moths, flies and beetles. Passing men-
tion should be made in this context to some booklice and larval thrips (thunderbugs)
that live in the litter. These last two groups lead generally unobtrusive lives in com-
parison with most of the other groups mentioned in that they do not force themselves
on man's attention by attacking crops. Termites are a large and important group in the
tropics, both for their influence on soils and for their damage to wooden structures,
but they do not occur naturally outside the Mediterranean area in Europe.
Greenfly or blackfly are familiar to most people as plant lice which often form
dense colonies on the stems and leaves of plants. It is not so well known that several
species of aphids live underground on the roots of plants tapping the sap in the same
way as their above-ground relatives. Some species are quite specific in the host plants
they attack, such as Aphis cliftonensis on the roots of rock-rose. Others attack a range
of related or even unrelated plants. The tulip-bulb aphid, for instance, occurs on the
bulbs or corms of tulip, iris, gladiolus and crocus. Several species have a curious al-
ternation of generations, one of which is spent on a tree and the other on the roots of
one or more herbaceous plants. Thus the pear-hogweed aphid and the poplar-lettuce-
root aphid divide their attentions as their names indicate; the wingless generation of
the dogwood aphid lives on the roots of grasses and cereals, and the elder aphid Aphis
sambuci migrates in summer to the roots of docks and various pinks and campions in
the Caryophyllaceae.
Many of these root aphids are attended by ants. Thirteen species of subterranean
myrmecophilous aphids were identified in a study on Staines Moor in Surrey. They
were all associated with the common yellow ant whose ant-hills are described later.
The aphids were clustered in cavities made by the ants, and it was estimated that the
ants consumed over 3,000 of the youngest stages a day in the summer, as well as feed-
ing on the carbohydrate-rich honey dew excreted by the aphids. This phenomenal cull
was made possible by the ability of the adults to produce four or more young a day.
The aphids may, indeed, have provided enough food to sustain the ant populations
with little need for other prey. In return the ants protect the aphids from other predat-
ors that would be less concerned with sustainable yields.
Frog-hoppers and their allies are closely related to aphids, and some of these
have subterranean larvae which feed on plant roots. The boldly coloured black-and-
scarlet adult Cercopis vulnerata is commonly seen on nettles and other vegetation in
the early summer. A very much rarer insect is our one species of cicada Cicadetta