A strain of the almond moth, Cadra cautella (WALKER), which had been maintained under a constant temperature (30°C) for about eight years, was bred under several combinations of temperature-humidity conditions. Many adult males bred at 25°C had abnormal copulatory organs. Variation of humidity had no influence on the occurrence of such abnormal males.<br>At a temperature of 25°C, the percentage emergence of males became lower and the occurrence of such abnormal males became frequent. The males seldom emerged below 20°C.<br>Preimaginal development was remarkably retarded when the 3rd and the 4th instars of males and the 3rd, 4th and 5th instars of females were maintained at the temperature of 20°C for about 5 days. When the 1st and 2nd instars of females were bred under such condition their speed of development was faster than that expected by the law of total effective temperature. Many abnormal males appeared when the 1st and the 3rd instars, prepupa and pupa were bred under such conditions. But no such effect was observed after treatment in egg stage.<br>From these results it is assumed that the effect of low temperature is different on males and females. Male gonad development and the formation and growth of male copulatory organs are especially influenced.<br>Why did these abnormalities of development and of sexual character occur at the temperature of 25°C which is nearly optimum for development? The strain used in this experiment has been maintained for many generations at the constant temperature of 30°C and may have become acclimatized to this constant condition, with the result the strain has become sensitive to such slight fluctuation of temperature.<br>The abnormal forms of the male copulatory organs are as follows: abnormal formations of vinculum and of the valva of one side; degeneration of valva and of aedoeagus; pseudohermaphroditic male with a bursa copulatrix; and a super male having a supernumerary valva.<br>The ostium bursae of the pseudohermaphroditic male is located between the 8th and 9th sternites. This differs from the prevailing theory of the situation of the ostium. In the pseudohermaphroditic male, one side of the uncus, the same side of the gnathos, the valva of that side, and other associated structures are underdeveloped. The ostium (opening) of the bursa is on this reduced side and is situated on the intersegmental membrane between the vinculum and the 8th abdominal sternite. In the normal female, however, the ostium of the bursa is located on the sternite of the 8th abdominal segment or on the intersegmental area between the 7th and 8th abdominal segments. Therefore the position of the opening of the bursa is not homologous in the intersex and in the normal female. This fact suggests that the morphological position of the opening of the bursa has been interpreted incorrectly for female genitalia of the generalized lepidopterous adult with the exception of Monotrysia. The fundamental position of the opening is on the intersternite area between 8th and 9th abdominal segments. Therefore the ostium must have then advanced into the previous sternite during evolutionary development. Otherwise ovipositor and uncus seem to be homologous as shown in this pseudohermaphroditic male.