THE SOIL FAUNA: ANTHROPODS - The Soil - page 67

a small stump or nothing at all. This trend is clearly associated with an increasingly
subterranean way of life, and is accompanied by other adaptations such as reduction
in lengths of limbs and antennae, reduction in eyes, absence of hairs or protective
scales, and lack of pigment ( Fig. 23 ) .
Other families show similar adaptations to more or less subterranean life styles,
and this has resulted in an ecological classification of Collembola into surface-living
and true soil forms. The Entomobryiidae contain many of the large, surface-living
species belonging to the genera Tomoceros, Orchesella ( Fig. 23 ) and Entomobrya it-
self. Most species of the family Onychiuridae are typical soil forms. They have spe-
cial glandular cells dotted about the body called pseudocelli (see Fig. 23 ) as they were
originally thought to be sensitive to light like simple eyes (ocelli). In fact, these ap-
pear to be used in defence against predators by producing an unpleasant secretion;
many millipedes similarly produce repellant secretions from paired stink glands on
certain body segments.
R.A. Brown gives a graphic description of an encounter between a predatory
gamasid mite Pergamasus lapponicus and the springtail Onychiurus armatus. “When
P. lapponicus meets an adult O. armatus in a void there is little escape for the collem-
bolan. The mite can easily outrun it, and O. armatus cannot spring away as it lacks an
effective furca. However, when the mite starts to feel O. armatus with its front legs,
the collembolan stands still and within 2-3 seconds a drop of milky liquid appears
on abdominal segment five in the vicinity of a collection of organs known as pseudo-
celli. The collembolan then bends its abdomen forwards over its head and attempts
to smear this liquid around the mouthparts of the attacking mite. If it succeeds, the
mite quickly drops the collembolan and backs away, frantically rubbing its mouth-
parts with its front legs”.
Despite such protective and escape mechanisms, predatory mites may normally
exert a considerable restraining effect on Collembola populations. A differential sus-
ceptibility to DDT is often correlated with an increase in Collembola in sprayed areas
( chapter 9 ) .
Collembola show marked seasonal variations in numbers associated with an abil-
ity for rapid multiplication when conditions are favourable. Many species feed on
fungal mycelia and so thrive in moist conditions in mild temperatures when fungi pro-
liferate. Others feed directly on decomposing plant and animal material. Like many
other soil animals, populations often show very patchy distributions in what, to us,
may appear quite uniform areas. Collembola aggregations arise partly from clusters
of eggs laid in areas with good food resources but scent signals, pheromones, are also
secreted which attract passing Collembola into the area.
W OODLICE AND SOIL HABITATS