RT1JI)IER 03 THE INSECT FAUNA ON SCOTCH BROOM
189
density have been -noted several times in these studies. Attacks by
birds were proportional to the availability of the scolytids and also of
Leucuptera larvae in the mines, and the numbers of spider webs were
proportional to the density of Leucopteru adults in the field.
It is worth noting that with t8he application of the precipitin test we
have gained a much clearer understanding of the great impact of
arthropod predation on insect populations. But in further studies much
closer analytical observations on the behaviour of both vertebrate and
invertebrate predators will be needed.
IX. DISPERSAL B Y FLIGHT OF THE POPULATIONS OF
BROOM INSECTS
In recent years the adaptive significance of dispersal by flight in
insects has been clarified in a series of publications by Johnson (1960,
1963, 1966, 1966). I n his 1966 paper he says that “the periodic disintegration of the population pattern in its breeding habitat is the procew of dispersal and migration, and more of it is adaptive and less
accidental than has been thought”.
I n the studies of the broom insects at Silwood it became abundantly
clear that no population study ia complete without some measurement
of dispersal and that the dispersal by flight plays an important part in
the population dynamics of most insect species. The degree to which
populations undergo “periodic disintegration” varies greatly in different
species and in different generations within the same species, but this
innate behaviour pattern, which occurs in the life of a species both at
high and at low densities, can be likened to a self-regulatory mechanism
the effect of which may increase exponentially with the rise in density.
As Johnson points out the immediate effects of dispersal lead to the
redistribution of local populations over a wider area within which populations mix. This must increase the boundaries and the extent of the
genetic pool and also lead to the relaxation of pressure imposed by the
various degrees of crowding. Experimentally, and on a small scde, redistributions of local populations resulting in higher overall survival,
have been demonstrated by Huffaker (1958).
Johnson (1963, 1966) considers that the basic mechanism of migratory flight in insects is associated with an endocrine deficiency syndrome in early life and ovarial immaturity and structural polymorphism.
Structural polymorphism is of course well known in aphids and in
the study of Acysthosiphon spartii, Smith (1966a) concluded that one of
the main factors responsible for checking the rise in population of the
aphid was dispersal by alatae which quickly left the area before reproducing. Mary dimorphism was also found in two broom beetles, Sitom
189
density have been -noted several times in these studies. Attacks by
birds were proportional to the availability of the scolytids and also of
Leucuptera larvae in the mines, and the numbers of spider webs were
proportional to the density of Leucopteru adults in the field.
It is worth noting that with t8he application of the precipitin test we
have gained a much clearer understanding of the great impact of
arthropod predation on insect populations. But in further studies much
closer analytical observations on the behaviour of both vertebrate and
invertebrate predators will be needed.
IX. DISPERSAL B Y FLIGHT OF THE POPULATIONS OF
BROOM INSECTS
In recent years the adaptive significance of dispersal by flight in
insects has been clarified in a series of publications by Johnson (1960,
1963, 1966, 1966). I n his 1966 paper he says that “the periodic disintegration of the population pattern in its breeding habitat is the procew of dispersal and migration, and more of it is adaptive and less
accidental than has been thought”.
I n the studies of the broom insects at Silwood it became abundantly
clear that no population study ia complete without some measurement
of dispersal and that the dispersal by flight plays an important part in
the population dynamics of most insect species. The degree to which
populations undergo “periodic disintegration” varies greatly in different
species and in different generations within the same species, but this
innate behaviour pattern, which occurs in the life of a species both at
high and at low densities, can be likened to a self-regulatory mechanism
the effect of which may increase exponentially with the rise in density.
As Johnson points out the immediate effects of dispersal lead to the
redistribution of local populations over a wider area within which populations mix. This must increase the boundaries and the extent of the
genetic pool and also lead to the relaxation of pressure imposed by the
various degrees of crowding. Experimentally, and on a small scde, redistributions of local populations resulting in higher overall survival,
have been demonstrated by Huffaker (1958).
Johnson (1963, 1966) considers that the basic mechanism of migratory flight in insects is associated with an endocrine deficiency syndrome in early life and ovarial immaturity and structural polymorphism.
Structural polymorphism is of course well known in aphids and in
the study of Acysthosiphon spartii, Smith (1966a) concluded that one of
the main factors responsible for checking the rise in population of the
aphid was dispersal by alatae which quickly left the area before reproducing. Mary dimorphism was also found in two broom beetles, Sitom