Geoscience Reference
In-Depth Information
F
IG. 3
Section through soil humus impregnated with agar jelly showing the 'primitive' insect
Campodea staphyl-
inus
(Diplura). This species is colourless and blind but has highly developed tactile senses. It does not
burrow but moves through the soil cavities using its antennae to locate a pathway when moving forward,
and its equally well-developed posterior feelers' (cerci) when moving backward under confined condi-
tions. (Photograph J.M. Anderson.)
This approach is clearly an advance over macroscopic analyses of litter habitats
but many important criteria are still left out. We could perhaps identify the kinds of
leaves present, up to a certain stage of disintegration, but their chemistry and relative
palatability - the presence of sugars, cellulose, waxes, tannins and lignin - would still
elude us. It is as if we tried to distinguish between caster sugar and salt by eye alone.
H
UMUS
We should think of this surface litter and decomposing organic matter, not just as an
inert physical habitat for mites and other organisms, but more like the house which
Hansel and Gretel found. This, you will remember, was made of ginger-bread, chocol-
ate and barley sugar. The various components of fresh litter - fruits, leaves, stems and
bark - differ greatly in their chemical make-up, and these differences are reflected
in their rates of breakdown. The soft parts of leaves, containing sugars, proteins and
starch within the cells, are quickly attacked and digested by earthworms, millipedes,
springtails and other soil animals. One can often find a perfectly skeletonized leaf in
