Distribution and Activity of the Desert Cockroach Arenivaga Investigata (Polyphagidae) in Relation to Microclimate

Arenivaga investigata is abundant in sand dunes of the Colorado desert at the foot of Mount San Jacinto, California. Measurements of temperature and humidity were made in its habitat at various depths in the sand over 24—h periods at least once each month from July 1967 to July 1968. At the driest and hottest times of the year (surface temperature 72°C, RH < 10%), relative humidities of 82% and above were available only 45 cm below the surface, and temperatures below 40°C occurred at —7.5 cm or lower. Since the insects can absorb water vapor from 82% RH or above, their source of water is assured. Onset of subsurface activity towards nightfall may be correlated with the movement downward of the temperature inversion, but this is not proved. The insects are found much more commonly in the sand near or under shrubs than in open sand. Their vertical distribution shows a maximum at —12.5 cm in July, and at —5 cm in November. In July most insects were found at temperatures between 30° and 35°C, whereas in November they were most frequent from 22° to 26°C. This seasonal difference in distribution, which could be attributed to acclimation, awaits further analysis.