Ultrastructure and resistance to water loss in eggs of Acanthoscelides obtectus Say (Coleoptera: Bruchidae)

Abstract

The mature oöcyte of Acanthoscelides obtectus is surrounded by three envelopes: an external layer, a chorion and a vitelline membrane. The external layer is secreted by the walls of the lateral oviducts. The chorion and vitelline membrane are secreted by the follicular cells. The vitelline membrane becomes very compact during the hour following fertilization and laying. The chorion is composed of three layers, one of which has a paracrystalline ultrastructure. Mature, unfertilized, chorion-containing oöcytes, whose vitelline membranes are loose, dehydrate rapidly in a dry atmosphere after laying or after removal from the lateral oviducts. Fertilized eggs are quite resistant to desiccation: after 12 days at 25°C and 5% relative humidity, viable larvae are obtained. The compact vitelline membrane is the most effective protection against dehydration. The chorion and the external layer are much less effective in preventing water loss from the egg. The retention of eggs in the lateral oviducts does not seem to lead to any modification of the structure of their envelopes.