Developmental Changes in Weight and Hemolymph Total Solids in the Sixth Instar of Peridroma saucia (Lepidoptera: Noctuidae)

Abstract

In spring and winter generations of the variegated cutworm, Peridroma saucia (Hubner), female larvae are heaviest at the fourth day after the fifth ecdysis, their weight decreasing subsequently, but in summer generations their weight continues to increase until the sixth day. Spring and winter males show weight changes similar to those of females of the same generations, but the summer males, in contrast, maintain a constant weight from the fourth to the sixth day after ecdysis. In spring and winter generations, the hemolymph of female larvae has a higher refractive index than that of male larvae at the fourth and subsequent days after the fifth ecdysis, but in summer generations this difference is not evident until the sixth clay. The hemolymph refractive index increases during the prepupal stage. This is at variance with findings on various silkworm species which characteristically incorporate hemolymph proteins and amino acids into silk at this time. It is consistent with previously published work and the analysis of the present findings, to associate the variation in total wet weight and plasma refractive index with the interaction between progressive larval differentiation and metamorphosis phenomena. Upon these are superimposed fluctuations ascribed to sex, genetic change, and host-plant composition. The data obtained in this study suggest that, as a gauge of larval age in populations of healthy insects, plasma refractive index measurements may be more reliable than measurements of wet weight.