Histopathology of intersexuality in mosquitoes.

Masculine expressivity in several species of mosquitoes is dependent upon low larval rearing temperatures. At elevated temperatures femininity prevails in both sexual genotypes. This paper is concerned with the histological development of organs of genotypic males of Aedes stimulans (A. stimulans) and ix Aedes aegypti (A. aegypti) before, during and after their determination to a specific sexual form. Intersexuality of homologous and nonhomologous organs was shown to arise in a manner that has not been previously reported in insects. Homologous organs develop similarly in the two sexes up to a critical stage, after which divergent morphogenesis commences. This critical period occurs at an earlier stage of ontogeny in males than it does in females. At elevated temperatures and during their period of plasticity, organs of genotypic males developed in a feminine manner. Determination began approximately at the time of divergent development. If the temperature remained high at this time, organs continued their feminine development, but if the temperature was lowered before feminine specialization, masculinity quickly ensued. Nonhomologous organs that are found only in males were present during ontogeny of larvae which were reared at elevated temperatures. These organs reached a high degree of juvenile masculine specialization during the larval period, but then histolyzed during pupal development. Nonhomologous organs that are present only in females were also studied. Primordia of the median oviduct, spermathecae and related feminine parts were the ones selected. In genotypic males which were reared at high temperatures, the primordia appeared at the same time and developed in the same manner as did those of a normal female. When larvae were reared at elevated temperatures just long enough to activate these primordia and were then exposed to low temperatures, the discs developed into masculine genital appendages of abdominal segment eight.