SOIL ARTHROPOD SAhII’LING
33
although a number of preliminary investigations, not without promise,
are known to the author. Dr A. J. Nicholson has described, in a personal
communication, the successful use of a tall glass tube containing sugar
solutions of increasing density for the separation of blow-fly resting stages
from frass. The column is filled in stages, using a disc to prevent mixing,
and the un-separated mixture poured in from the top. Different materials
separate at different heights and by tapping the column carefully from
the bottom can be drawn off in turn. An example of a dynamic system
is Ostenbrinck’s (1954) (see also Williams and Winslow, 1955) use of a
tall vertical plastic funnel arranged with its wide opening upwards for
the separation of nematode egg capsules from soil. A constant flow of
water is arranged so that it enters the funnel from below and overflows
over a lip at the top. Soil, containing capsules is emulsified with water
and introduced in an equal stream about halfway up the funnel. The
rate of flow is highest a t the bottom and least a t the top so that particles
are suspended in horizontal layers by the upward flow according to
their rate of sinking; mineral particles are removed at the bottom. In
Ostenbrinck’s technique, since only a single kind of object is to be
separated, the flow rate is arranged to carry the capsules over into the
sieve continuously. However it would appear possible to develop this
device as a means of separating objects of different sinking rates. Floating materials could be passed over into a sieve from the lip at the top
by progressively raising the rate of liquid flow. The reverse process of
separating materials according to rate of sinking in a long vertical tube
and collecting them in a moving trough might well offer a means of
separating small animals from soil and from each other.
Kuhnelt’s use of the centrifuge (1948, 1961) is almost the only sedimentation method that has been operated successfully as a complete
system with arthropods. The crude sample is centrifuged in concentrated calcium chloride solution and the animals decanted from the
surface. It is claimed that by repeated centrifuging with reduced concentrations of calcium chloride solution much of the plant debris, as
well as mineral matter, can be removed. However it is clear from the
description that great care is required in operating this technique and
that separation is not perfect.
3. Differential Wetting
Salt and Hollick’s (1944) extraction method (see below) appears to
have been the first to make use of the differential wetting properties of
insect cuticle and of plant matter. By shaking a mixture of the two in a
vessel containing paraffin (kerosene) and water it is possible to cause
the insects, whose cuticle is wetted by the paraffin, to separate from
the plant matter (which is not) when the two liquids are subsequently
33
although a number of preliminary investigations, not without promise,
are known to the author. Dr A. J. Nicholson has described, in a personal
communication, the successful use of a tall glass tube containing sugar
solutions of increasing density for the separation of blow-fly resting stages
from frass. The column is filled in stages, using a disc to prevent mixing,
and the un-separated mixture poured in from the top. Different materials
separate at different heights and by tapping the column carefully from
the bottom can be drawn off in turn. An example of a dynamic system
is Ostenbrinck’s (1954) (see also Williams and Winslow, 1955) use of a
tall vertical plastic funnel arranged with its wide opening upwards for
the separation of nematode egg capsules from soil. A constant flow of
water is arranged so that it enters the funnel from below and overflows
over a lip at the top. Soil, containing capsules is emulsified with water
and introduced in an equal stream about halfway up the funnel. The
rate of flow is highest a t the bottom and least a t the top so that particles
are suspended in horizontal layers by the upward flow according to
their rate of sinking; mineral particles are removed at the bottom. In
Ostenbrinck’s technique, since only a single kind of object is to be
separated, the flow rate is arranged to carry the capsules over into the
sieve continuously. However it would appear possible to develop this
device as a means of separating objects of different sinking rates. Floating materials could be passed over into a sieve from the lip at the top
by progressively raising the rate of liquid flow. The reverse process of
separating materials according to rate of sinking in a long vertical tube
and collecting them in a moving trough might well offer a means of
separating small animals from soil and from each other.
Kuhnelt’s use of the centrifuge (1948, 1961) is almost the only sedimentation method that has been operated successfully as a complete
system with arthropods. The crude sample is centrifuged in concentrated calcium chloride solution and the animals decanted from the
surface. It is claimed that by repeated centrifuging with reduced concentrations of calcium chloride solution much of the plant debris, as
well as mineral matter, can be removed. However it is clear from the
description that great care is required in operating this technique and
that separation is not perfect.
3. Differential Wetting
Salt and Hollick’s (1944) extraction method (see below) appears to
have been the first to make use of the differential wetting properties of
insect cuticle and of plant matter. By shaking a mixture of the two in a
vessel containing paraffin (kerosene) and water it is possible to cause
the insects, whose cuticle is wetted by the paraffin, to separate from
the plant matter (which is not) when the two liquids are subsequently