358
E. B. EDNEY
adaptations (save perhaps of the sensillae) is not difficult to appreciate.
Pierre recognizes two groups of sand insects: those living near the surface,
which are xerophilic and highly specialized; and those living deeper,
which are hygrophilic and less specialized.
Less clear in their adaptiveness are some features of the larvae of desert
tenebrionid beetles described by Schulze (1969). The structure of the
ninth abdominal segment of these insects is often used as an important
indicator of phylogenetic affinity. Yet Schulze shows very clearly that certain pronounced abnormalities including strong modifications of the shape
of the notum and the replacement of normal setae by knoblike spherical
ones, among other changes, are characteristic of psammophilous species,
no matter what their phyletic associations may be. The function of these
modifications in unknown, but it is a good guess that they are adaptive
in some way.
There has been a good deal of discussion about the significance of surface coloration in desert arthropods, but thus far there is little agreement
on this very interesting topic. Many of the diurnal species—particularly
grasshoppers—are cryptically marked and colored (there is no controversy
here), but so are nondeserticolous forms. Others, as Buxton (1923)
pointed out, are often strikingly black or white, while nocturnal species
are generally pale-colored. Attempts have been made to account for this
pattern of coloration, in terms of radiation load. Black, it has been suggested, prevents the penetration of ultraviolet light to the deeper tissues;
but most insects' cuticles are opaque to ultraviolet whether or not they
are black. The surfaces of some insects have been shown to reflect strongly,
though differentially, in the infrared region,* but infrared radiation can
hardly be responsible for the visible color differences.
Digby (1955) carried out a useful analysis of the effects of visible surface colors on body temperatures of insects. He found an effect, but concluded that since much of the energy of solar radiation is in the infrared
region, and since most insects are differently colored in different parts of
the body, "temperature differences due to color will be of minor importance." Buxton (1922) reported that a dark-colored form of the grasshopper, Calyptamus, was 4.5°C warmer than a light-colored form when both
were exposed to solar radiation, and the same was true of black and green
locust hoppers. But no such differences could be found between dark and
light forms of the grasshopper, Melanoplus, by Pepper and Hastings
(1952). It seems that there is no easy explanation of the black and white
* Rucker (1939) found that carabid beetle elytra reflected 39% of the energy at
1100 nm, 17% at 2150 nm, and 46% at 3000 nm, while Pieris wings reflected 69,
59, and 35%, respectively. Most of the infrared solar energy that reaches the earth's
surface lies in wavelengths between 800 and 2000 nm.
E. B. EDNEY
adaptations (save perhaps of the sensillae) is not difficult to appreciate.
Pierre recognizes two groups of sand insects: those living near the surface,
which are xerophilic and highly specialized; and those living deeper,
which are hygrophilic and less specialized.
Less clear in their adaptiveness are some features of the larvae of desert
tenebrionid beetles described by Schulze (1969). The structure of the
ninth abdominal segment of these insects is often used as an important
indicator of phylogenetic affinity. Yet Schulze shows very clearly that certain pronounced abnormalities including strong modifications of the shape
of the notum and the replacement of normal setae by knoblike spherical
ones, among other changes, are characteristic of psammophilous species,
no matter what their phyletic associations may be. The function of these
modifications in unknown, but it is a good guess that they are adaptive
in some way.
There has been a good deal of discussion about the significance of surface coloration in desert arthropods, but thus far there is little agreement
on this very interesting topic. Many of the diurnal species—particularly
grasshoppers—are cryptically marked and colored (there is no controversy
here), but so are nondeserticolous forms. Others, as Buxton (1923)
pointed out, are often strikingly black or white, while nocturnal species
are generally pale-colored. Attempts have been made to account for this
pattern of coloration, in terms of radiation load. Black, it has been suggested, prevents the penetration of ultraviolet light to the deeper tissues;
but most insects' cuticles are opaque to ultraviolet whether or not they
are black. The surfaces of some insects have been shown to reflect strongly,
though differentially, in the infrared region,* but infrared radiation can
hardly be responsible for the visible color differences.
Digby (1955) carried out a useful analysis of the effects of visible surface colors on body temperatures of insects. He found an effect, but concluded that since much of the energy of solar radiation is in the infrared
region, and since most insects are differently colored in different parts of
the body, "temperature differences due to color will be of minor importance." Buxton (1922) reported that a dark-colored form of the grasshopper, Calyptamus, was 4.5°C warmer than a light-colored form when both
were exposed to solar radiation, and the same was true of black and green
locust hoppers. But no such differences could be found between dark and
light forms of the grasshopper, Melanoplus, by Pepper and Hastings
(1952). It seems that there is no easy explanation of the black and white
* Rucker (1939) found that carabid beetle elytra reflected 39% of the energy at
1100 nm, 17% at 2150 nm, and 46% at 3000 nm, while Pieris wings reflected 69,
59, and 35%, respectively. Most of the infrared solar energy that reaches the earth's
surface lies in wavelengths between 800 and 2000 nm.