The Respiratory Mechanisms of Some Insect Eggs

Abstract

ABSTRACT By the use of the cobalt sulphide injection technique the distribution of air in the shell of a number of insect eggs has been studied. Air is usually confined to an inner layer of porous protein, connected with the atmosphere through pores of varying type which are likewise filled with spongy material. In Rhodnius the ‘resistant protein layer’ which lines the shell is the porous structure and the ‘pseudomicropyles’ connect this layer to the exterior. The arrangement in Cimex is similar. In Oncopeltus the spongy walls of the ‘sperm cups’ convey air to a porous inner layer. After laying, the lumen of each cup (the micropylar canal) is occluded with solid cement. In Dixippus the so-called ‘micropyle’ in the ‘scar’ of the egg is the respiratory pore. It is filled with spongy protein containing air and conducts the air to the spongy inner layer of the endochorion. As the egg develops and its contents are reduced in volume, free air collects between the two layers of the endochorion in the region of the pore. In Blattella an elaborate stigmatic apparatus which is moulded in the crista of the oötheca conveys air to a spongy process at the upper pole of the egg and so to a thin porous air-filled layer which lines the chorion. In Bombyx and Ephestia a thin porous inner layer of the chorion containing air communicates with the exterior through scattered pores containing air-filled spongy material. In the eggs of Diptera the chorion consists of tapering columns with spongy walls which unite the cement-covered outer layer to a spongy inner layer containing air. The horns on the Drosophila egg and the dorsal folds on the Calliphora egg provide respiratory outlets for this system. The spaces between the columns contain liquid in Calliphora and Drosophila-, in Syrphus these spaces are greatly enlarged and contain air. The spongy layers may become filled with air in eggs which are still bathed in fluid in the oviduct, or in which water is present in adj acent parts of the shell. The mechanism of filling is discussed. In the case of Rhodnius there is quantitative evidence that the system will provide for the respiratory needs of the egg.