Egg structure and embryonic development of arctoperlarian stoneflies: A comparative embryo- logical study (Plecoptera)

Abstract

Egg structure and embryonic development of nine arctoperlarian stoneflies from nine families, i.e., Scopuridae, Taeniopterygidae, Leuctridae, Capniidae, and Nemouridae of Euholognatha, and Perlidae, Chloroperlidae, Perlodidae, and Peltoperlidae of Systellognatha were examined and compared with previous studies. The primary aim of this study was to use embryological data to reconstruct the groundplan and phylogeny of Plecoptera and Polyneoptera. Euholognatha has eggs characterized by a thin, transparent chorion, while the eggs of Systellognatha are characterized by a collar and anchor plate at the posterior pole. These features represent an apomorphic groundplan for each group. The embryos form by the concentration of blastoderm cells toward the posterior pole of the egg. Soon after the formation of the embryo, amnioserosal folds form and fuse with each other, resulting in a ball-shaped "embryo-amnion composite" that is a potential autapomorphy of Plecoptera. As an embryological autapomorphy of Polyneoptera, embryo elongation occurs on the egg surface, supporting the affiliation of Plecoptera to Polyneoptera. After its elongation on the egg surface, the embryo sinks into the yolk with its cephalic and caudal ends remaining on the egg surface. This unique embryonic posture may be regarded as an apomorphic groundplan of Plecoptera. Arctoperlarian plecopterans perform three types of katatrepsis: 1) the first type, in which the embryo's anteroposterior and dorsoventral axes change in reverse during katatrepsis, is found in Capniidae, Nemouridae, Perlidae, Chloroperlidae, and Perlodidae, and this sharing is symplesiomorphic; 2) the second one, in which the embryo's axes are not changed during katatrepsis, is found in Scopuridae, Taeniopterygidae, and Leuctridae, and this may be regarded as synapomorphic to them; 3) the third one, in which the embryo rotates around its anteroposterior axis by 90° during katatrepsis as known for Pteronarcyidae, is found in Peltoperlidae, and this type may be synapomorphic to these two families.