Cockroache; Ecology, behavior & history - W.J. Bell

Fig. 3.10 Distribution of Arenivaga sp. in relation to depth below the surface (A,C) and temperature
(B,D). In (A) and (C) the insects are scored according to size: open columns 1st–3rd instar;
striped columns 4th–6th instars; solid columns 7th–9th instars and adults. Adult males
were rarely found below the surface and are not included in the data set. After Edney et al. (1974).
Reprinted by permission of the Ecological Society of America.
nymphs (e.g., Car. lutea—Hubbell and Goff, 1939) are
collected from burrows.
Animal burrows generally offer a more favorable microclimate
than surface habitats. A higher humidity is
maintained by the respiration of the vertebrate occupant
(Tracy and Walsberg, 2002), and because of enhanced air
circulation in burrows, cockroaches that utilize them
avoid the hypoxic conditions that may be encountered
by sand-swimming species (Cohen and Cohen, 1981).
Richards (1971) indicates that animal burrows have a microclimate
that is intermediate between that of caves and
that of surface habitats. Recent studies, however, suggest
that animal burrows are not always cool and humid refugia
from surface conditions. For more than 100 days of
the year soil temperatures rose to over 30C at depths of
2 m in burrows of Dipodomys in the Sonoran desert
(Tracy and Walsberg, 2002).
In a remarkable case of niche construction, at least one
cockroach species mitigates conditions within vertebrate
burrows by building a home within a home. In southeastern
Arizona Arenivaga apacha is a permanent inhabitant
of mounds of the banner-tailed kangaroo rat
(Dipodomys spectabilis) and builds a microenvironment
of small burrows (“shelves”) within the main burrow of
the rat (Cohen and Cohen, 1976). The mini-burrows are
tightly packed with the grasses that were dragged into the
main burrow by the rat for use as nesting material. Although
the rodent burrows extend much deeper, most of
the cockroaches were found 30–45 cm below the sand
surface. Surface temperatures reached as high as 60C,
burrow temperatures reached 48C , but the temperature
of the grass-lined cockroach shelves averaged 16.5C. Humidity
of the burrows was as low as 20%, but the shelves
remained nearly saturated at all times; 91% was the lowest
reading. Conditions within the vertebrate burrow
were nearly as harsh as the open desert and were made
tolerable only by the alterations in the microenvironment
made by the cockroaches; the insects died in 3–5 min if
subjected to temperatures above 40C. These cockroaches
feed on the stored seeds of their host. “With this stored
food available throughout the year and the very stable environmental
conditions, the cockroaches have an ideal
kind of oasis in the midst of a harsh desert environment”
(Cohen and Cohen, 1976).
While A. apacha exhibits striking behavioral strategies
for living in the harsh desert environment, its closely related
congener, the Colorado Desert sand swimming A.
investigata, relies heavily on well-developed physiological
mechanisms. Arenivaga investigata has a higher temperature
tolerance and lower rates of water loss and oxygen
consumption than A. apacha (Cohen and Cohen, 1981).
This is due in large part to the predominance of long
chain wax esters in the cuticle that are effective in waterproofing
the insect (Jackson, 1983). Arenivaga investigata
is also able to tolerate a water loss of 25–30% without
lethal effects (Edney, 1967) and is able to absorb water vapor
from the surrounding air at 82% relative humidity
(RH) (Edney, 1966). This level of RH is available at 45 cm
below the ground surface (Edney et al., 1974). Thus, descending
to that level assures the cockroach a predictable
source of water. Water vapor is absorbed by means of a
unique system of specialized structures on the head and
mouthparts (O’Donnell, 1977a, 1977b). A thin layer of
hygroscopic fluid is spread on the surface of two eversible
HABITATS 55