Abstract
This paper constitutes the first report of the chromosome survey undertaken by the author in the Japanese Neuroptera and describes the chromosomes of nine species covering three families of the Osmyloidea, with some considerations on the relationship to taxonomy. The species under study and their chromosome numbers are given in the following table:
All the species herein studied showed that the males possessed the sex-determining mechanism of an ordinary X-Y type, with an exception of Plethosmylus decoratus which was characterized by the formula of an X1X2-Y mechanism in the male. The X and Y usually exhibit remarkable precocious separation in the first meiotic metaphase in eight species under and Y is less remarkable than that in the other species. Probably, this unusual behavior of the X and Y seems to be characteristic to the Sisyridae, because some other species of this family was found also to show a similar condition.
The behavior of the X- and Y-elements was traced in the meiotic nucleus through the growing stages of the first spermatocyte. In the stages ranging from the leptotene to pachytene the X and Y appear as a heteropycnotic chromatic body with a tripartite structure, in close association with a nucleolus. The body shows a weak affinity to stain in the diplotene nucleus. The diakinetic nucleus shows the X- and Y-elements which have separated far apart. It is then apparent that the precocious separation of the X- and Y-elements has taken place in the stage of diakinesis.
