German cockroaches Blattella germanica Linne have been known to inhabit regularly definite types of properties such as office buildings, restaurants, hotels, shops and so on, but quite few also inhabit Japanese style wooden houses in Japan. In order to clarify the reason for this behavior, experiments and observations were made on the populations bred in several different environments. 1. Experiments of adaptability to three different structural materials In order to evaluate the adaptability of cockroaches to different structural materials which they inhabit, cockroaches were bred on the surfaces of wood, metal and plastic, at an insectarium kept at 25°±1℃. The periods of larval development were 51.9,56.1 and 59.9 days in male and 53.1,58.8 and 60.1 days in female, on wood, metal and plastic, respectively. It seems that wood is more favorable to German cockroaches than metal and plastic. 2. Comparison of developmental periods of larvae in seven different enviroments The larval periods of breeding groups bred in four different types of properties and under three different constant temperatures were compared during the periods from June to August, 1972. The larval periods were 52.4 days in a concrete office building, 63.6 days in a wooden apartment, 68.9 days in a concrete apartment, 70.0 days in a wooden house and 59.0 days under a constant 25℃, respectively. First instar larvae failed to develop into adults when kept constantly at 15℃ or at 10℃. 3. Comparison of life spans in three different environments Successive breedings were conducted in three different environments, in a concrete office building in Kawasaki, a wooden house in Yokohama and in the insectarium kept at 25°±1℃, and the pattern of life span under natural conditions throughout two years were observed. At the insectarium, six generations were passed successively during the two years without any change of characteristics. For instance, the average of developmental characteristics were as follows : embryonic period 21-28 days, larval period 59.0 days, the period prior to oviposition 9.8 days, frequency of oviposition 7 times in max. and 4.6 times in average, number of hatched larvae per ootheca of the first oviposition 36.8,and longevity of the male 90.0 days and the female 164.6 days. In summer, the cockroaches developed in a concrete building, as well as at the insectarium kept at 25℃. Of four successive generations observed for two years, two failed to survive the winter due to low temperature, because of lack of central heating. On the other hand, when there was central heating a generation was successful in overwintering and reproduced again beyond the winter. And in another one, some individuals overwintered successfully, but no reproduction occurred beyond then. Although they were able to breed normally in Japanese style wooden houses in summer, it seemed unlikely that they could establish themselves regularly there, because of severe low prevailing temperature as compared with concrete building centrally heated, in where room temperature was kept above ca. 15℃ even at midnight of winter. It was concluded, finally, that establishment of German cockroaches to definite types of properties depends principally upon the temperature conditions. Generally speaking, in Japan, they are able to inhabit concrete buildings regularly that there is central heating, on the other hand it seems to be very difficult to establish themselves in Japanese style wooden houses. But, if there are any continuously heated places such as refrigerator or boiler, it might be possible to do so.
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