Structure of the Specialised and Unspecialised Chorion of the Egg in the Stick Insect Malacomorpha cyllarum (Phasmatodea)

Abstract

The chorion of deposited eggs of the bisexual phasmid Malacomorpha cyllarum (Westwood 1859) was examined by LM, SEM and TEM. The unspecialised chorion consists of three principal layers, from which especially the second one is heavily mineralised. Distinction of sublayers is possible mainly by their different electron density, spongy appearance, and mineralization or simply by phase interfaces, but appears rather arbitrarily in most cases. Specialised regions of the chorion are the operculum and the micropylar plate. The operculum largely shows the same series of layers, but is generally thicker as the unspecialised chorion. Stratification of the highly modified microplyar plate is more complex and characterised by additional spongy modifications, which appear to be derived from the innermost third principal layer. Two micropylar canals, collapsed distally and proximally, run through the micropylar strand. These canals neither open to the exterior nor to the interior of the egg. External and internal closure are achieved by chorion material rather than a secretory plug.