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E. B. EDNEY
at lower temperatures (Beament, 1964 and other papers); however, the
precise nature of the change in the lipid layer is by no means clear (Bursell, 1970) and the matter deserves further consideration (Hackman,
1971; Gilby, 1965 and quoted by Hackman, 1971; Edney and McFarlane,
1973).
The temperature above which permeability increases rather rapidly
varies among species. In many, such as the blood-sucking bug Rhodnius,
the mealworm Tenebrio, or the pupa of Pieris, this is well above the lethal
temperature of the insect concerned (from 50° to 60°C) (Wigglesworth,
1945). In others the transition temperature is a good deal lower, about
30°C in cockroaches (Beament, 1958; Ramsay, 1935). Often the transition temperature is higher in insects with generally lower cuticular permeabilities; for example, Ahearn (1970a) found transition temperatures
of 40°, 47.5°, and 50°C for the three desert tenebrionid beetles, Eleodes
armata, Cryptoglossa verrucosa, and Centrioptera muricata, respectively,
and the beetles stood in that order of decreasing cuticle permeability (see
Table II). Again, three Saharan desert arthropods studied by Délye
(1969) (Eremiaphila, a cockroach; Prionotheca, a tenebrionid beetle; and
Othoes, a solfugid) all have low transpiration rates, and high transition
temperatures insofar as the latter are determinable. But even in the very
xeric firebrat Thermobia, the transition temperature, 28°C, is apparently
below that of the insect's preference temperature (Beament et ai, 1964).
Thus the ecological significance is not clear, although the adaptive value
of the lipid-mediated impermeability itself is of course very evident.
The situation is probably more complex than the above statement suggests. For example, Loveridge (1968a) believes that in Locusta, permeability of the cuticle is, affected by the relative humidity to which the insect
is exposed. His measurements show that in dead locusts with blocked
spiracles the rate of water loss through the cuticle increases from about
0.080 to 0.097 mg/gm/hour/mm Hg as the relative humidity increases
from 0 to 2 5 % . Such an effect would be highly adaptive for an arthropod
exposed periodically to low humidity. The fact that the cuticle is differentially permeable in different areas, as Makings (1968) showed for Slifer's
patches in the desert locust, is an added complication.
There is, in addition, great variation among species in what may be
called the "basal" transpiration rate, that is the rate below the transition
temperature, at about 20°-30°C, and it is clearly of importance to know
whether these values are related to conditions to which the species are
normally exposed. As Table II shows, in general, they are so related. Furthermore, large differences in basal rate of transpiration are often observed
between different developmental stages of the same species—eggs and
pupae, for example, are often much less permeable than larvae or adults,
E. B. EDNEY
at lower temperatures (Beament, 1964 and other papers); however, the
precise nature of the change in the lipid layer is by no means clear (Bursell, 1970) and the matter deserves further consideration (Hackman,
1971; Gilby, 1965 and quoted by Hackman, 1971; Edney and McFarlane,
1973).
The temperature above which permeability increases rather rapidly
varies among species. In many, such as the blood-sucking bug Rhodnius,
the mealworm Tenebrio, or the pupa of Pieris, this is well above the lethal
temperature of the insect concerned (from 50° to 60°C) (Wigglesworth,
1945). In others the transition temperature is a good deal lower, about
30°C in cockroaches (Beament, 1958; Ramsay, 1935). Often the transition temperature is higher in insects with generally lower cuticular permeabilities; for example, Ahearn (1970a) found transition temperatures
of 40°, 47.5°, and 50°C for the three desert tenebrionid beetles, Eleodes
armata, Cryptoglossa verrucosa, and Centrioptera muricata, respectively,
and the beetles stood in that order of decreasing cuticle permeability (see
Table II). Again, three Saharan desert arthropods studied by Délye
(1969) (Eremiaphila, a cockroach; Prionotheca, a tenebrionid beetle; and
Othoes, a solfugid) all have low transpiration rates, and high transition
temperatures insofar as the latter are determinable. But even in the very
xeric firebrat Thermobia, the transition temperature, 28°C, is apparently
below that of the insect's preference temperature (Beament et ai, 1964).
Thus the ecological significance is not clear, although the adaptive value
of the lipid-mediated impermeability itself is of course very evident.
The situation is probably more complex than the above statement suggests. For example, Loveridge (1968a) believes that in Locusta, permeability of the cuticle is, affected by the relative humidity to which the insect
is exposed. His measurements show that in dead locusts with blocked
spiracles the rate of water loss through the cuticle increases from about
0.080 to 0.097 mg/gm/hour/mm Hg as the relative humidity increases
from 0 to 2 5 % . Such an effect would be highly adaptive for an arthropod
exposed periodically to low humidity. The fact that the cuticle is differentially permeable in different areas, as Makings (1968) showed for Slifer's
patches in the desert locust, is an added complication.
There is, in addition, great variation among species in what may be
called the "basal" transpiration rate, that is the rate below the transition
temperature, at about 20°-30°C, and it is clearly of importance to know
whether these values are related to conditions to which the species are
normally exposed. As Table II shows, in general, they are so related. Furthermore, large differences in basal rate of transpiration are often observed
between different developmental stages of the same species—eggs and
pupae, for example, are often much less permeable than larvae or adults,