SOIL ARTHROPOD SAMPLING
21
has proved rewarding. The role of light in the Tullgren-type extractor
has never been satisfactorily determined but, since light hardly penetrates any appreciable depth into soil, it seems unlikely that it will do
more than deter animals from escaping upwards a t the start of an estraction. Once the heat gradient is established upward escapes seem
unlikely and are not encountered in practice.
Since most invertebrates are sensitive to chemical vapours (see
Dethier, 1947) it is rather surprising that these have not been more
widely used for selective extraction of soil fauna. Macfadyen (1953)
showed that higher yields of soil aphids and thrips could be obtained by
simply enclosing one end of a soil core in an atmosphere containing
dimethyl phthalate and suspending the other end over an open funnel
so that a vapour-concentration gradient would result. Since then more
effective repellent substances have been marketed for use against
biting flies and it has also been shown that a dry atmosphere is far from
an ideal attractant. It seems likely that better results on a wider range
of organisms could be obtained by exploiting these two advances and
possibly a very simple method could be worked out for field use.
In conclusion, Tullgren-type methods have the great advantages
that they demand relatively simple apparatus, can be used under field
conditions, operate well even in the presence of much plant matter
and require little labour on the part of the operator. Near-perfect
yields can be achieved in particular cases a t the cost of more complicated apparatus and greater labour. On the other hand the variability
of behavioural responses shown by soil arthropods renders it unlikely
that a single Tullgren-type extractor can yield complete extractions
of all groups and the fact that such methods rely on the animals’ own
mobility implies that only mobile stages can be obtained in this way.
c. MECHANICAL SEPARATION METHODS TAKING ADVANTAGE
OF DIFFERENTIAL PHYSICAL PROPERTIES OF ANIMALS
A N D THE SUBSTRATUM
This group of extraction methods is based on mechanical means
of separation, exploiting differences of physical properties between the
bodies of animals and of their surroundings. The powers of movement
of the animals are ignored so that immobile stages such as eggs, egg
capsules and pupae may be extracted - and indeed there is no other
way of extracting them apart from waiting for them to hatch into
mobile stages. A not very important, compensating disadvantage is that
dead organisms will also be extracted and may be confused with live
animals. As with behaviour-type niet,hods it is hardly to be expected
that a single mechanical method will separate all the fauna from the soil.
The reason for this is that animals vary in surface properties, density
21
has proved rewarding. The role of light in the Tullgren-type extractor
has never been satisfactorily determined but, since light hardly penetrates any appreciable depth into soil, it seems unlikely that it will do
more than deter animals from escaping upwards a t the start of an estraction. Once the heat gradient is established upward escapes seem
unlikely and are not encountered in practice.
Since most invertebrates are sensitive to chemical vapours (see
Dethier, 1947) it is rather surprising that these have not been more
widely used for selective extraction of soil fauna. Macfadyen (1953)
showed that higher yields of soil aphids and thrips could be obtained by
simply enclosing one end of a soil core in an atmosphere containing
dimethyl phthalate and suspending the other end over an open funnel
so that a vapour-concentration gradient would result. Since then more
effective repellent substances have been marketed for use against
biting flies and it has also been shown that a dry atmosphere is far from
an ideal attractant. It seems likely that better results on a wider range
of organisms could be obtained by exploiting these two advances and
possibly a very simple method could be worked out for field use.
In conclusion, Tullgren-type methods have the great advantages
that they demand relatively simple apparatus, can be used under field
conditions, operate well even in the presence of much plant matter
and require little labour on the part of the operator. Near-perfect
yields can be achieved in particular cases a t the cost of more complicated apparatus and greater labour. On the other hand the variability
of behavioural responses shown by soil arthropods renders it unlikely
that a single Tullgren-type extractor can yield complete extractions
of all groups and the fact that such methods rely on the animals’ own
mobility implies that only mobile stages can be obtained in this way.
c. MECHANICAL SEPARATION METHODS TAKING ADVANTAGE
OF DIFFERENTIAL PHYSICAL PROPERTIES OF ANIMALS
A N D THE SUBSTRATUM
This group of extraction methods is based on mechanical means
of separation, exploiting differences of physical properties between the
bodies of animals and of their surroundings. The powers of movement
of the animals are ignored so that immobile stages such as eggs, egg
capsules and pupae may be extracted - and indeed there is no other
way of extracting them apart from waiting for them to hatch into
mobile stages. A not very important, compensating disadvantage is that
dead organisms will also be extracted and may be confused with live
animals. As with behaviour-type niet,hods it is hardly to be expected
that a single mechanical method will separate all the fauna from the soil.
The reason for this is that animals vary in surface properties, density