CHEMORECEPTION AND THE BEHAVIOR OF INSECTS
153
himself primarily with the processes of selection and appraisal of suitability. He discovered that selection is based overwhelmingly on the
size of the host. Larger hosts, whether suitable or not, are preferred.
False hosts, that is, inanimate objects may be attacked even in the
presence of suitable natural hosts if the latter are smaller. Here then
was an indication that the process of selection was amenable to analysis
in terms of the physical nature of the environment.
If a host were of a size which was acceptable to the wasp, the next
step in selection involved discrimination between parasitized and unparasitized hosts. The wasp can discriminate accurately at two different
steps in the process of parasitization, namely, when it subjects the host
to minute examination and when it drills. The preliminary discrimination is carried out by contact chemoreception. There is some indication
that the tarsi rather than the antennae are involved, because the discrimination is made after the wasp has walked upon the egg, not when it
first touches the egg with its antennae. The wasp can discriminate between a clean egg and one which has been walked upon by another
wasp. Washing removes the chemical trace. If the water in which an
egg has been washed is placed upon another egg, the latter is treated
by the wasp as though it were already parasitized. Tests with various
dilutions of the wash water showed that discrimination by the wasp
decreased with dilution. Under ordinary circumstances the chemical is
volatile and disappears from the surface of the egg with time. If a
parasitized egg is carefully washed, a wasp will commence drilling; but,
once the ovipositor has penetrated, the wasp becomes aware that the egg
is already parasitized and the ovipositor is withdrawn without an egg
having been laid. The circumstances of this phase of discrimination
strongly suggest perception by means of chemoreceptors upon the
ovipositor.
A further attack upon the problem of ovipositional behavior of Trichogramma was launched by Laing (1937), who was concerned more with
the initial stages of host-finding. He showed that the parasite was first
attracted to a particular environment where the host exists. The observation that insects seek out a circumscribed environment in which their
host, prey, or food exists is now well documented. Arriving in this
environment Trichogramma
moves at random (Varley, 1941); it eventually finds the host egg by perceiving chemical traces deposited by the
late owner of the egg. As already described, further steps in the behavior are regulated by contact chemoreception. At its departure from a
host egg the wasp now no longer travels in a random fashion but pursues
a twisting and winding path which increases its chances of encountering
another egg in the immediate vicinity.
153
himself primarily with the processes of selection and appraisal of suitability. He discovered that selection is based overwhelmingly on the
size of the host. Larger hosts, whether suitable or not, are preferred.
False hosts, that is, inanimate objects may be attacked even in the
presence of suitable natural hosts if the latter are smaller. Here then
was an indication that the process of selection was amenable to analysis
in terms of the physical nature of the environment.
If a host were of a size which was acceptable to the wasp, the next
step in selection involved discrimination between parasitized and unparasitized hosts. The wasp can discriminate accurately at two different
steps in the process of parasitization, namely, when it subjects the host
to minute examination and when it drills. The preliminary discrimination is carried out by contact chemoreception. There is some indication
that the tarsi rather than the antennae are involved, because the discrimination is made after the wasp has walked upon the egg, not when it
first touches the egg with its antennae. The wasp can discriminate between a clean egg and one which has been walked upon by another
wasp. Washing removes the chemical trace. If the water in which an
egg has been washed is placed upon another egg, the latter is treated
by the wasp as though it were already parasitized. Tests with various
dilutions of the wash water showed that discrimination by the wasp
decreased with dilution. Under ordinary circumstances the chemical is
volatile and disappears from the surface of the egg with time. If a
parasitized egg is carefully washed, a wasp will commence drilling; but,
once the ovipositor has penetrated, the wasp becomes aware that the egg
is already parasitized and the ovipositor is withdrawn without an egg
having been laid. The circumstances of this phase of discrimination
strongly suggest perception by means of chemoreceptors upon the
ovipositor.
A further attack upon the problem of ovipositional behavior of Trichogramma was launched by Laing (1937), who was concerned more with
the initial stages of host-finding. He showed that the parasite was first
attracted to a particular environment where the host exists. The observation that insects seek out a circumscribed environment in which their
host, prey, or food exists is now well documented. Arriving in this
environment Trichogramma
moves at random (Varley, 1941); it eventually finds the host egg by perceiving chemical traces deposited by the
late owner of the egg. As already described, further steps in the behavior are regulated by contact chemoreception. At its departure from a
host egg the wasp now no longer travels in a random fashion but pursues
a twisting and winding path which increases its chances of encountering
another egg in the immediate vicinity.