Abstract
Differences of urea contents in the hemolymph between the silkworm larvae reared on artificial diet and those on mulberry leaves were investigated during the course of the 5th instar. In the individuals reared on artificial diet, urea-N content in hemolymph was kept at an approximately constant level ranging from 200 to 300μg per ml during the feeding period, but after spinning the level increased markedly up to 700-800μg per ml. On the other hand, urea-N content in the hemolymph of the larvae fed on mulberry leaves gradually decreased from the highest level of 600μg per ml during the feeding stage, to reach a level of below 50μg per ml in mature larvae. Even when larvae fed on artificial diet were subsequently reared on mulberry leaves for only two days before spinning, urea sharply decreased in the hemolymph as in the larvae reared on mulberry leaves throughout. Urea content in the hemolymph of mature larvae tended to increase with the increase of the content of soybean meal or mulberry leaf powder in the artificial diet. When the larvae fed on artificial diet where any of the soybean oil, salt mixture and sucrose components was omitted, urea content in the hemolymph of mature larvae increased more than in the hemolymph of those fed on the complete diet. On the other hand, omission of mulberry leaf powder, soybean meal, potato starch or antiseptic from the diet caused a decrease in the urea content in the hemolymph, though the decrease never reached the level of that observed in rearing on mulberry leaves. When larvae were reared on artificial diet, the urea content in the glandular tissue including sericin of the middle division of the silkgland was almost identical with that in the cocoon shell. These facts strongly suggest that urea in the cocoon shell is excreted with sericin from the silkgland at spinning.
