Growth and differentiation of the testes in the large milkweed bug, Oncopeltus fasciatus (Dallas)

During the fourth instar there is a five‐fold increase in testis volume, caused by mitotic proliferation of spermatogonia that leads to accumulation of many layers of cysts of resting spermatocytes. At the beginning of the fifth instar the basal cysts undergo meiotic division, and a wave of meiosis followed by spermateleosis progreses apically. There is little or no spermatogonial multiplication; the three‐fold increase in testis size involves primarily spermatocyte differentiation. The wave of differentiation continues at a slower rate in the young adult; most of the spermatocyte cysts are transformed into about 250,000 spermatozoa by the twelfth day. Multinucleate apical cells that stain deeply with Feulgen reagent are present in the early fourth instar, enlarge in the fifth, and persist in the adult testis. Spermatocyte differentiation is not affected by treatment of nymphs with a synthetic juvenile hormone to analog during the fourth or fifth instar, although the treatment causes moderate to high juvenilization at the metamorphic molt. Since late fourth‐instar testes transplanted into mature male or female adults also undergo extensive transformation of spermatocytes into spermatozoa, molting is not required for differentiation. Treatment of late fourth or early fifth instar nymphs with the chemosterilant tretamine causes high mortality at the metamorphic molt, but the wave of spermatocyte differentiation during the fifth instar proceeds normally, although there is evidence of severe chromosome damage. Comparisons with development in the female of Oncopeltus and in other insect species suggest that there is a profound change in the biochemical milieu at the molt from fourth to fifth instar, many processes preceding this molt tending to be larval in character and to diminish or disappear afterward while processes normally occurring after the molt tend to be adultoid.