Evolutionary origin and functioning of pregenital abdominal outgrowths in a viviparous insect, Arixenia esau

Although pregenital abdominal outgrowths occur only rarely in pterygote insects, they are interesting from the evolutionary viewpoint because of their potential homology to wings. Our previous studies of early development of an epizoic dermapteran, Arixenia esau revealed that abdominal segments of the advanced embryos and larvae, growing inside a mother's uterus, are equipped with paired serial outgrowths. Here, we focus on the origin and functioning of these outgrowths. We demonstrate that they bud from the lateral parts of the abdominal nota, persist till the end of intrauterine development, and remain in contact with the uterus wall. We also show that the bundles of muscle fibers associated with the abdominal outgrowths may facilitate flow of the haemolymph from the outgrowths' lumen to the larval body cavity. Following completion of the intrauterine development, abdominal outgrowths are shed together with the larval cuticle during the first molt after the larva birth. Using immunohistochemical and biochemical approaches, we demonstrate that the Arixenia abdominal outgrowths represent an evolutionary novelty, presumably related to intrauterine development, and suggest that they are not related to serial wing homologs.