In Israel, activity by the desert-dwelling Blaps sulcata Castelnau normally begins about sunset, reaches its peak toward midnight, and ceases at dawn, but may vary with the weather and with other local environmental conditions. In nature, the beetles spend the daylight hours aggregated in large numbers under thick, flat-bottomed stones, where temperature and humidity are favorable and they are protected from enemies. They are photonegative, and move directly to shady areas upon exposure to direct sunlight. Though thick stones are usually preferred to thin, flat stones as hiding places, the latter proved more attractive under experimental conditions if they were provided with the scent from the anal gland secretion of the insects. The anal glands thus appear to serve 2 functions: (1) as defense mechanisms against predators, when large quantities of the liquid are sprayed suddenly, and (2) for attraction of other members of the species, so that aggregation results. In the latter instance, the exudation is gradual and in very small quantities; a high concentration of the scent was found repellent to Blaps itself. Antennectomized individuals did not respond in this manner, but aggregated in greater numbers under a thick, nonscented stone than under a flat, scented one. However, amputation of both antennae greatly hindered the general activity of these individuals.