VI. DESERT ARTHROPODS
365
Concentration occurred as solitary locusts flew into the region by night,
and settled in areas of green vegetation. Such flights were usually upwind,
and may have been initiated by chemical substances released by the vegetation. During the daytime, however, visual clues seemed to keep the
locusts in their habitats. Once within the habitat, conditions were very
favorable for sexual maturation, and further concentration occurred as a
result of small daytime flights and ground movements associated with
roosting, basking, and the search for egg laying and mating sites.
Multiplication occurred as a result of reproduction—each female producing on average 3 egg pods, totaling 400 eggs. There was high hopper
mortality, the causes of which were unknown, but even so the population
of nymphs increased by several times.
The third component, gregarization, resulted in changes in behavior, and
ultimately in physiology and morphology. At low population levels hoppers
tended to avoid contact with each other, at higher concentrations they became indifferent, and finally at the highest concentration they exhibited
mutual attraction and formed large, compact marching bands. Such behavioral changes did take place during the ontogeny of an individual, but
their onset was related rather to population density than to ontogenetic
stage. Thus the hoppers of partially gregarized adults themselves showed
incipient gregarization both in behavior and in color, and adults behaved
differently according to their history as hoppers. Gregarization continued
during the early adult stage as roosting and basking groups were formed,
and finally there occurred small swarmlets of adults. Unlike the night-flying
solitary adults, these locusts flew and migrated by day. They would have
merged to form large swarms, had not control measures prevented this
consummation.
B. ACTIVITY CYCLES
A somewhat different aspect of behavior, daily cycles of activity, which
may or may not be "circadian rhythms" according to whether or not they
continue in constant conditions, have been reviewed in relation to desert
animals by Cloudsley-Thompson and Chadwick (1964; see also Cloudsley-Thompson, 1970). Such rhythms of activity, correlated with one or
another of the main physical parameters of the environment, provide another mechanism ensuring that the animals are out and about at the right
time.
A good example of daily activity is provided by the desert cockroach,
Arenivaga investigata. We have already seen that the upper lethal temperature of this insect is quite high (45.5°-48.5°C according to conditions,
see p. 349), but this level is often exceeded in the insects' environment,
365
Concentration occurred as solitary locusts flew into the region by night,
and settled in areas of green vegetation. Such flights were usually upwind,
and may have been initiated by chemical substances released by the vegetation. During the daytime, however, visual clues seemed to keep the
locusts in their habitats. Once within the habitat, conditions were very
favorable for sexual maturation, and further concentration occurred as a
result of small daytime flights and ground movements associated with
roosting, basking, and the search for egg laying and mating sites.
Multiplication occurred as a result of reproduction—each female producing on average 3 egg pods, totaling 400 eggs. There was high hopper
mortality, the causes of which were unknown, but even so the population
of nymphs increased by several times.
The third component, gregarization, resulted in changes in behavior, and
ultimately in physiology and morphology. At low population levels hoppers
tended to avoid contact with each other, at higher concentrations they became indifferent, and finally at the highest concentration they exhibited
mutual attraction and formed large, compact marching bands. Such behavioral changes did take place during the ontogeny of an individual, but
their onset was related rather to population density than to ontogenetic
stage. Thus the hoppers of partially gregarized adults themselves showed
incipient gregarization both in behavior and in color, and adults behaved
differently according to their history as hoppers. Gregarization continued
during the early adult stage as roosting and basking groups were formed,
and finally there occurred small swarmlets of adults. Unlike the night-flying
solitary adults, these locusts flew and migrated by day. They would have
merged to form large swarms, had not control measures prevented this
consummation.
B. ACTIVITY CYCLES
A somewhat different aspect of behavior, daily cycles of activity, which
may or may not be "circadian rhythms" according to whether or not they
continue in constant conditions, have been reviewed in relation to desert
animals by Cloudsley-Thompson and Chadwick (1964; see also Cloudsley-Thompson, 1970). Such rhythms of activity, correlated with one or
another of the main physical parameters of the environment, provide another mechanism ensuring that the animals are out and about at the right
time.
A good example of daily activity is provided by the desert cockroach,
Arenivaga investigata. We have already seen that the upper lethal temperature of this insect is quite high (45.5°-48.5°C according to conditions,
see p. 349), but this level is often exceeded in the insects' environment,