N most animals so far studied in which sex is genetically determined I there is a chromosomal difference between the sexes. One ses posse~ s~ es two homologous sets of chromoeomes. The other sex has one such complete set, and another set in which one or more inembrrs are absent or replaced by chromosomes of a different type. In some groups the male has only a haploid chromosome set. There are however a number of groups in which sex is wholly or inainly deteriniiied genetically, but in which no chromosomal diflerences between the sexes have been seen. These include all vertebrates except mammals and birds. In the Nematocera, the primitive suborder of the Diptertr Lo which Culex belongs, some families have sex chromosomes, while others have not. According to the review of WOLF (1941), sex chroniosomes are found in the Phrgneiclae, Thaumaleidae, Scatopsidae, and Fungiuoridtre, while no difference between the sexes has been found in