2
A. M A C F A D Y E N
and size and in all these respects are likely to overlap inanimate
materials, especially in the case of those mites, opilionids and insect
pupae which habitually coat themselves with mineral and vegetable
particles. In this case it seems most logical to classify the types of
methods according to the physical principles used despite the fact
that two or more of these are frequently combined in a single extraction
programme.
1. Sieving
Sieving is hardly likely, by itself to provide a means of separating a
wide range of animals from soil, but it is often a most useful adjunct to
other methods either for removing inanimate material outside the size
range which is of interest or for separating animals - perhaps a single
species - which is of economic interest. An example of the first technique is the use of a dry power-driven sieve such as that described by
Lange et al. (1955) and also used by Lloyd for removing leaves in the
litter fauna study mentioned above; in this case all the smaller fraction
is then treated by the special behaviour-type method - which could
not have been applied to material containing large leaves. It is more
usual, however for sieving to be done under water, often with the aid
of powerful jets. This was the basis of Morris’s (1922) wireworm technique in which a battery of sieves was used, stones and roots being retained by the coarser meshes and fine silt and clay passing through the
finer ones. It was shown by Ladell (1936) that this method by itself, is
both laborious and inefficient and he supplemented the use of sieves
with flotation on magnesium sulphate (see below). Both Ladell’s and
later flotation methods have incorporated sieves for the purpose of
primary separation of larger and smaller material.
It is also profitable to sieve the material in alcohol obtained from
Tullgren-type extractors under certain circumstances, eepecially when
it contains appreciable quantities of large plant material or large numbers of animals. In this case by using mesh sizes of about 1 mm and
200 p the counting can be done in two or three stages and removal of
debris of very different sizes leads to greater accuracy.
2. ,Sedimentation
As an alternative to separation on the basis of size there is a number
of possible methods of separation by density. In most cases these can
work on either static or dynamic principles, for example fluids of different densities can be used so that bodies of corresponding densities
also come to rest at definite boundaries, or differences in rates of sinking
can be exploited to achieve separation. Remarkably little effort has
been made to apply these principles to the sepration of arthropods
A. M A C F A D Y E N
and size and in all these respects are likely to overlap inanimate
materials, especially in the case of those mites, opilionids and insect
pupae which habitually coat themselves with mineral and vegetable
particles. In this case it seems most logical to classify the types of
methods according to the physical principles used despite the fact
that two or more of these are frequently combined in a single extraction
programme.
1. Sieving
Sieving is hardly likely, by itself to provide a means of separating a
wide range of animals from soil, but it is often a most useful adjunct to
other methods either for removing inanimate material outside the size
range which is of interest or for separating animals - perhaps a single
species - which is of economic interest. An example of the first technique is the use of a dry power-driven sieve such as that described by
Lange et al. (1955) and also used by Lloyd for removing leaves in the
litter fauna study mentioned above; in this case all the smaller fraction
is then treated by the special behaviour-type method - which could
not have been applied to material containing large leaves. It is more
usual, however for sieving to be done under water, often with the aid
of powerful jets. This was the basis of Morris’s (1922) wireworm technique in which a battery of sieves was used, stones and roots being retained by the coarser meshes and fine silt and clay passing through the
finer ones. It was shown by Ladell (1936) that this method by itself, is
both laborious and inefficient and he supplemented the use of sieves
with flotation on magnesium sulphate (see below). Both Ladell’s and
later flotation methods have incorporated sieves for the purpose of
primary separation of larger and smaller material.
It is also profitable to sieve the material in alcohol obtained from
Tullgren-type extractors under certain circumstances, eepecially when
it contains appreciable quantities of large plant material or large numbers of animals. In this case by using mesh sizes of about 1 mm and
200 p the counting can be done in two or three stages and removal of
debris of very different sizes leads to greater accuracy.
2. ,Sedimentation
As an alternative to separation on the basis of size there is a number
of possible methods of separation by density. In most cases these can
work on either static or dynamic principles, for example fluids of different densities can be used so that bodies of corresponding densities
also come to rest at definite boundaries, or differences in rates of sinking
can be exploited to achieve separation. Remarkably little effort has
been made to apply these principles to the sepration of arthropods