Studies on the Developmental Physiology of the Silkworm, Bombyx mori in Environmental Control. III.

Using the F1 hybrid larvae of the bivoltine silkworm races (Gunko×Banri), the effect of photoperiod on the larval duration, cocoon weight (pupal weight plus cocoon-shell weight) and cocoon-shell weight were investigated. The used larvae had been previously incubated during their embryonic development at three different temperatures (25°C, 23°C and 16°C) . They were reared at nine different photoperiodic conditions such as OL-24D, 4L-20D, 8L-16D, 1OL-14D, 12L-12D, 14L-1OD, 16L-8D, 20L-4D and 24L-OD at 25°C, fed with two kinds of artificial diets with different nutritional values.1) In the 25°C-incubated group, comparatively shorter larval duration (29-30 days) was observed when the larvae were reared under the short-day conditions from 4L-20D to 12L-12D, though the long-day conditions over 14L-10D prolonged the larval duration up to 32 days. Thus a short-day type (“J”-shaped curve) of photoperiodism was seen. Whereas the duration of final instar was little affected by the photoperiod given through the larval stage.2) In the 23°C-incubated group, another type (“W”-shaped curve) of photoperiodism was found in both sexes. Namely, comparatively shorter larval durations (32 days) were found at 4L-20D in the female, 8L-16D (31 days) in the male, and 14L-1OD (33 days in the female, 31 days in the male) . And the prolonged larval durations (up to 35 days) were observed at OL-24D, 12L-12D and the long-day conditions over 20L-4D in both sexes. In the final instar, the photoperiodical response curve somewhat differed from “W” shap, namely the duration at 4L-20D in the female was rather longer and also the duration at 8L-16D in the male became longer than at OL-24D.3) In the 16°C-incubated group, prolonged larval duration (up to 34 days) was seen under the short-day conditions from OL-24D to 14-1OD, while it was shortened by the long-day conditions over 20L-4D. Thus a long-day type (“L”-shaped curve) of photoperiodism was found in both sexes. The similar photoperiodical response in the duration of the final instar was observed in both sexes4) As to the photoperiodical regulations of both cocoon weight and cocoon-shell weight, regardless of the difference of the temperature of egg-incubation, they indicated rhythmically fluctuated response curves having three maximal weights of cocoon and cocoonshell at 4L-20D or 8L-16D, 16L-8D and 24L-OD. In these three photoperiodic conditions, heaviest weights of cocoon and cocoon-shell were always observed at 24L-OD, and lightest weight was seen at OL-24D or 12L-12D.The above mentioned results clearly demonstrated that both weights of the pupa and of the silk-products do not always relate clearly to the length of the developmental period.