24
A. M A C F A D Y E N
left to re-form as two immiscible layers. Subsequently Raw (1955) introduced the use of benzene which has a freezing point above that of
water. By standing the vessel containing the two liquids in a refrigerator
and freezing the benzene (melting-point 5” C) the latter could be picked
off as a solid lump and then melted; the animals being easily freed in
this way from particles in the water fraction. The paraffin-water interface technique has also been used by Davies (1955) in conjunction with
a motor-driven “vacuum cleaner’’ for separating arthropods from
litter in the field. A stream of air carried the animals and mineral
matter from the soil and projected the material a t the two liquids; this
caused everything to be wetted, the arthopod-containing pa.rafin
layer was retained and the plant litter and soil which came to rest in
the water layer were rejected.
Undoubtedly the principle of differential wetting at an interface has
enormously increased the efficacy of mechanical extraction techniques. It is not, at present, a universal method because many kinds
of arthropods, among them woodlice, myriapods and some groups of
mites, are not wetted by the chemicals used. Furthermore even the
groups which are wetted are inefficiently extracted from large proportions of plant matter presumably because the animals cling tightly
or are hopelessly entangled (Evans, 1953).
The use of these methods has, therefore, been mainly confined to
separation from arable soils containing much mineral matter and
especially clay. Under these circumstances, however, the funnel methods
are least efficient and the two types of technique thus complement ea.ch
other.
I n the field of mineral separation the art of controlling wetting properties of different metals has been developed to a very high degree
and by the use of surface active substances under controlled conditions
of salt concentration and pH, given minerals can be made to float or
sink a t will. It seems that this is a field in which further experimentation based on a methodical plan could greatly improve the effectiveness
of separation processes for biological material and perhaps also permit
the sorting of different groups of animals.
Some groups of arthropods are particularly prone to float even in
pure water as a result of wax layers and outstanding among these are
the Collembola. W. G. Hale (to whom the author is greatly indebted
for an account of his method, which is to be published shortly) has
developed a method for extracting Collembola from peaty soils eniploying the principle that vegetable material sinks in both water and
magnesium sulphate solution, when boiled at room temperature under
reduced pressure. About half the Collembola remain floating on the
surface, due to the hydrophobic nature of their cuticles, and the rest
A. M A C F A D Y E N
left to re-form as two immiscible layers. Subsequently Raw (1955) introduced the use of benzene which has a freezing point above that of
water. By standing the vessel containing the two liquids in a refrigerator
and freezing the benzene (melting-point 5” C) the latter could be picked
off as a solid lump and then melted; the animals being easily freed in
this way from particles in the water fraction. The paraffin-water interface technique has also been used by Davies (1955) in conjunction with
a motor-driven “vacuum cleaner’’ for separating arthropods from
litter in the field. A stream of air carried the animals and mineral
matter from the soil and projected the material a t the two liquids; this
caused everything to be wetted, the arthopod-containing pa.rafin
layer was retained and the plant litter and soil which came to rest in
the water layer were rejected.
Undoubtedly the principle of differential wetting at an interface has
enormously increased the efficacy of mechanical extraction techniques. It is not, at present, a universal method because many kinds
of arthropods, among them woodlice, myriapods and some groups of
mites, are not wetted by the chemicals used. Furthermore even the
groups which are wetted are inefficiently extracted from large proportions of plant matter presumably because the animals cling tightly
or are hopelessly entangled (Evans, 1953).
The use of these methods has, therefore, been mainly confined to
separation from arable soils containing much mineral matter and
especially clay. Under these circumstances, however, the funnel methods
are least efficient and the two types of technique thus complement ea.ch
other.
I n the field of mineral separation the art of controlling wetting properties of different metals has been developed to a very high degree
and by the use of surface active substances under controlled conditions
of salt concentration and pH, given minerals can be made to float or
sink a t will. It seems that this is a field in which further experimentation based on a methodical plan could greatly improve the effectiveness
of separation processes for biological material and perhaps also permit
the sorting of different groups of animals.
Some groups of arthropods are particularly prone to float even in
pure water as a result of wax layers and outstanding among these are
the Collembola. W. G. Hale (to whom the author is greatly indebted
for an account of his method, which is to be published shortly) has
developed a method for extracting Collembola from peaty soils eniploying the principle that vegetable material sinks in both water and
magnesium sulphate solution, when boiled at room temperature under
reduced pressure. About half the Collembola remain floating on the
surface, due to the hydrophobic nature of their cuticles, and the rest