SOME ASPECTS OF MOISTURE ABSORPTION AND LOSS IN EGGS OF MELANOPLUS BIVITTATUS (SAY)

Abstract

The penetration of moisture into or out of eggs of Melanoplus bivittatus (Say) is assisted or retarded by a variety of envelopes. Proceeding inwardly these are, in the newly laid egg, the chorion, the primary lipoid layer (probably waxy), and the vitelline membrane; later, to these are added the secondary lipoid layer (probably oily), the yellow cuticle, a bonding material, the white cuticle, and the serosa, which secretes all the others of this group. The primary lipoid layer waterproofs the egg in its earliest stages, but its function is soon taken over by the secondary lipoid layer. A specialized moisture-permeable area, the hydropyle, is formed at the posterior pole. Moisture is absorbed from the environment chiefly through the hydropyle, reaching a peak of absorption on the eighth or ninth day at 25 °C. and then decreasing because of the limited expansion of the cuticular layers. The original moisture content of the egg is approximately doubled. Moisture loss in young eggs, up to seven days of age, is restricted at first by the primary lipoid layer and later by the secondary lipoid layer. From the eighth day to the end of anatrepsis, on the 14th day, the desiccation rate is greatly increased. Most of the loss occurs through the hydropyle, which at this time is normally absorbing water; however, the relationship between loss and gain through the hydropyle is not close. After blastokinesis, or after the 14th day of incubation at 25 °C., the desiccation rate is extremely variable. Some of the moisture loss takes place through the hydropyle, and when the rate is low all of it does so; however, in many eggs the general cuticular surface becomes permeable (to desiccation but not to absorption), and the losses are erratic and unpredictable. With the gradual onset of diapause after the 21st day, the desiccation rate declines. Changing the moisture conditions during incubation of the eggs results in changes in the desiccation rates. The variability of the rates in older eggs was not caused by retardation of development, irregular moisture absorption, intermittent drying, or laboratory techniques, but was probably the result of variable cuticular permeability. Most M. bivittatus eggs, regardless of age, survived losses up to one-third of their moisture content, but few survived more than a two-thirds loss. After more than a one-third loss, the collapsing action of the egg frequently produced distorted or abnormal embryos. Though these often lived for some time, they never hatched. Those individuals and those age groups with the lowest desiccation rates naturally survived dry conditions longest.