152
V. G. DETHIER
receptive processes. The various sign stimuli required for the release
of an instinctive activity co-operate according to the rule of heterogeneous summation. These facts led us to the postulation of Innate
Releasing Mechanisms, one of which is possessed by each separate
reaction. Apart from releasing stimuli, directing stimuli play a part,
enabling or forcing the animal to orient itself in relation to the environment. The internal causal factors controlling, qualitatively
and quantitatively, the motivation of the animal may be of three
kinds: hormones, internal sensory stimuli, and, perhaps, intrinsic or
automatic nervous impulses generated by the central nervous system
itself. Instinctive 'reactions' are of varying degrees of complexity;
even the simplest type, the 'fixed pattern/ depends on a system of
muscle contractions which is of a configurational character.
"These results are incomplete in more than one respect. First, the
evidence is still very fragmentary, and the generalizations are still of
a very tentative nature. Second, the work done thus far has been
mainly analytical, and no attempt has yet been made to combine
the separate conclusions into a picture of the causal structure underlying instinctive behaviour as a whole. We have, however, gained
one thing: we are realizing more and more clearly that the physiological mechanisms underlying instinctive behaviour are much more
complicated than we were able to see at the start. Previous attempts
at synthesis, such as Pavlov's reflex theory and Loeb's tropism
theory, now appear to be grotesque simplifications" (see also
Lorenz, 1950).
Fortunately, the announcement of Thompson and Parker stimulated
rather than discouraged attempts to understand host-parasite relationships. As a result, some elucidation of the problem has been achieved.
By way of a beginning, Murr (1930) was able to demonstrate convincingly that the small parasite wasp Habrobracon
juglandis Ashm.
could successfully locate its host and recognize it sufficiently well merely
on the basis of odor to begin oviposition. Wasps were able to locate the
host from short distances by host odor disseminated through the air or
equally well by following an odor trail on the substrate. Salt (1935,
1937), using as an experimental animal the polyphagous chalcid wasp
Trichogramma
evanescens, essayed to settle the question. He subdivided
the process of parasitization into three actions which occur naturally and
in sequential order: host-finding, host-selection, appraisal of host suitability. In the normal sequence of events the wasp perceives its host,
an egg, from a short distance, approaches, touches it with its antennae,
climbs upon the egg, subjects it to a minute examination by tapping it
with the antennae, drills into it, and then lays an egg. Salt concerned
V. G. DETHIER
receptive processes. The various sign stimuli required for the release
of an instinctive activity co-operate according to the rule of heterogeneous summation. These facts led us to the postulation of Innate
Releasing Mechanisms, one of which is possessed by each separate
reaction. Apart from releasing stimuli, directing stimuli play a part,
enabling or forcing the animal to orient itself in relation to the environment. The internal causal factors controlling, qualitatively
and quantitatively, the motivation of the animal may be of three
kinds: hormones, internal sensory stimuli, and, perhaps, intrinsic or
automatic nervous impulses generated by the central nervous system
itself. Instinctive 'reactions' are of varying degrees of complexity;
even the simplest type, the 'fixed pattern/ depends on a system of
muscle contractions which is of a configurational character.
"These results are incomplete in more than one respect. First, the
evidence is still very fragmentary, and the generalizations are still of
a very tentative nature. Second, the work done thus far has been
mainly analytical, and no attempt has yet been made to combine
the separate conclusions into a picture of the causal structure underlying instinctive behaviour as a whole. We have, however, gained
one thing: we are realizing more and more clearly that the physiological mechanisms underlying instinctive behaviour are much more
complicated than we were able to see at the start. Previous attempts
at synthesis, such as Pavlov's reflex theory and Loeb's tropism
theory, now appear to be grotesque simplifications" (see also
Lorenz, 1950).
Fortunately, the announcement of Thompson and Parker stimulated
rather than discouraged attempts to understand host-parasite relationships. As a result, some elucidation of the problem has been achieved.
By way of a beginning, Murr (1930) was able to demonstrate convincingly that the small parasite wasp Habrobracon
juglandis Ashm.
could successfully locate its host and recognize it sufficiently well merely
on the basis of odor to begin oviposition. Wasps were able to locate the
host from short distances by host odor disseminated through the air or
equally well by following an odor trail on the substrate. Salt (1935,
1937), using as an experimental animal the polyphagous chalcid wasp
Trichogramma
evanescens, essayed to settle the question. He subdivided
the process of parasitization into three actions which occur naturally and
in sequential order: host-finding, host-selection, appraisal of host suitability. In the normal sequence of events the wasp perceives its host,
an egg, from a short distance, approaches, touches it with its antennae,
climbs upon the egg, subjects it to a minute examination by tapping it
with the antennae, drills into it, and then lays an egg. Salt concerned