STUDIES ON THE POSTEMBRYONIC DEVELOPMENT OF HYALELLA AZTECA (SAUSSURE)

¹ Catholic University of America, Washington, D. C.

Hyalella asteca grows well under laboratory conditions. Like other amphipods, the frequency of its moulting depends upon several factors, chief of which is temperature. At low temperatures ecdysis is delayed. Frequency of moulting in developing animals is no indication of sex, as it occurs at varying time intervals in both sexes. The young moult more often than do the adults. In reaching the time for first pairing, the number of moults is more important than the age, in days, of the animal. First pairing generally occurs in the eighth instar.

In general, growth at first is slow. After the secondary sex characters first become apparent (Instar 6) growth becomes more rapid.

Centers of more active growth are present in certain regions, such as the second gnathopod of the male, and the proximal joint of the flagellum in both pairs of antennae. In the female the fifth thoracic segment appears to be a "growth center," for eggs begin forming in this region. No doubt, there are regions of active growth throughout the body, but the above are the most prominent.

Animals during any instar are not the same size. Males particularly vary in size, although they may mature after the same moult. Retarded individuals, animals that pair much later than the normals, are fairly common. It is possible that some of them never become sexually mature.

The external secondary sex characters, lamellae and gnathopods, make their appearance several moults before the animals pair. Lamellae grow gradually, and reach their full size and become bordered with hooked hairs when the first eggs are released.

First gnathopods in both sexes are similar in size and form. Second gnathopods of the female retain their juvenile form, although they increase in length. Second gnathopods of the male develop a very large propodus with a strong dactylus and the palm of the propodus changes from chelate to transverse and then finally to subchelate.