The limiting factors that determine whether man-made structures will provide suitable habitats for cockroaches are favorable temperature and availability of water and food. The range of temperatures that man provides for his own comfort and protection fosters the rapid increase of cockroach populations indoors. Gunn (1934, 1935) has demonstrated that the preferred temperature range (zone of indifference) of _Blatta orientalis_ is 20-29° C. The upper limit of the preferred temperature of _Blattella germanica_ and _Periplaneta americana_ is 33° C. (Gunn, 1935). The lower limits of temperature tolerance were not sharply defined in Gunn's work. However, less than optimum temperatures, if they last for only short periods, are not necessarily lethal. The 24-hour mortality for _P. americana_ that had been held for one hour at 0° C. was only 2±2 percent (Knipling and Sullivan, 1957). Gunn (1934) observed that _Blatta orientalis_ would not settle at temperatures above 33° C. and would react violently against higher temperatures (e.g., 39° C.) by running away; thus the thermotactic behavior of cockroaches might be presumed to bring them into favorable environments within structures. Thermal death points have been determined for the above three species by Gunn and Notley (1936). It is common knowledge among those who rear cockroaches experimentally that, unless the water content of the food is high, fluid water is essential in the insects' dietary. Ten species of domiciliary cockroaches have been shown to be unable to survive as long on dry food alone as they could on food and water at 36-40 percent relative humidity (Willis and Lewis, 1957). _Blatta orientalis_, when in a state of normal water balance, usually spent more time in the drier part of a humidity gradient; but desiccated insects tended to become hygropositive (Gunn and Cosway, 1938). We presume that other domiciliary species behave similarly. If water is available nearby, it may be presumed that partially desiccated cockroaches could locate a source through the mediation of a humidity sense. Hygroreceptors have been demonstrated on the antennae of _Blattella germanica_ (Roth and Willis, 1952a) and suggested for _Blatta orientalis_ (Gunn and Cosway, 1938).