Ultrastructure of the eggshell and micropylar apparatus in Somatochlora metallica (Vander L.), Orthetrum cancellatum (L.) and Sympetrum sanguineum (Müll.) (Anisoptera: Corduliidae, Libellulidae)

Eggshells are studied using light, scanning electron and transmission electron microscopy. The nomenclature of the eggshell layers is revised. The vitelline envelope (vitelline membrane) is the innermost layer. Its thickness is dependent on the length of the egg stage, and it is believed to play the dominant protective role in the eggshell. The endochorion is thin and multilayered. The exochorion is filamentous and synonymous with the jelly-layer. The jelly has two main functions, one is to camouflage the egg, the other is to enhance the protection provided by the vitelline envelope. Non-expanded jelly is dense and found in the overwintering eggs of S. sanguineum. The expansion of the jelly is water-dependent, but the jelly in S. sanguineum is unable to expand, regardless if water is present. Expanded jelly seems to be of about the same thickness in many species, even when the size of their eggs differ. The thin (24 nm) threads in the jelly are of the same size and structure in all three species investigated, and are therefore believed to be a basic structure in odonate egg-jelly. No micropylar atrium or pedicel is present. The micropylar chutes lead the sperm directly into the oocyte, and no sperm-storage chamber exists. No extrachorionic material is present in the eggshells.