Abstract
Certain Muscid eggs are shown to change their shape almost simultaneously with changes in the relative humidity of the air in their immediate vicinity. Evidence is produced that these shape changes are caused by the effect of humidity on the chorion. The net effect of the shape changes appears to be the setting up of increasing strain in the chorion, making it progressively easier for the larvae to rupture their shells, with increasing humidity. These shape changes therefore provide an explanation for certain well-known humidity effects on the hatching of Muscid eggs.