Non-sexual abdominal appendages in adult insects challenge a 300 million year old bauplan

pterygotes (e.g., the tracheobranchial appendages in mayfly nymphs, the tracheal gills of larval gyrinid beetles and those of neuropterids such as Corydalis) usually serve functions other than sexual, i.e., they are either respiratory or locomotory. As a first step to elucidate what drove the evolution of non-sexual abdominal appendages in Bennini, we investigated topology and anatomy of the appendages. Each LASSO consists of a proximal slender, narrow, cylindrical process (the 'rod') and an apical dilation (the 'cup) (Figure C). The long rod-like appendage is formed in part by the epipleurite of segment IV and in part by the membrane connecting it with abdominal sternite IV [4]. Cephally of the proximal margin of epipleurite IV (the rod), a rectangular, medially slightly bent sclerite (presumably the epipleurite of abdominal segment III), shields the membranous connection between sternite IV and epipleurite IV (terminology follows Sweet [5]). From the anterior margin of abdominal sternite IV, an apodeme connects with the proximal end of epipleurite IV, presumably providing muscular attachment. The rod is caudally concave, with the concave part membranous while the cup is strongly sclerotized throughout. The distal dilated part, the cup, usually carries a whitish, apparently waxy 'hood' which encloses a hollow space (Figure D,E; Supplemental information). When this hood is removed, it becomes obvious that the cup is apically slightly concave, in top view horseshoe-shaped, and framed by a rounded fringe except for a small area at the narrower side: from there a broad obtuse ridge extends medially over ca. three quarters of the area enclosed by the outer fringe (Supplemental information). Around the broad median ridge and parallel to the outer fringe runs a row of ca. 70 short peg-like sensillae. Between them and the median ridge a series of narrow sharp radial ridges is located. In all Bennini species examined, two long, filiform setae arise from distinct, subglobular sockets (Figure E; Supplemental information), situated on the median ridge, in a row between centre and exterior fringe. As no obvious wax glands were otherwise observed at the margin of the distal cup-shaped dilation, it seems evident that the waxy hood is the product of the gland underneath the observed filament. The mode of construction of Despite their enormous diversity, the bauplan of adult winged insects (pterygotes) is remarkably conservative since the Lower Devonian: a five-segmented head, a three-segmented thorax with three pairs of walking legs and an eleven-segmented abdomen without any …