Conserved ultrastructure but divergent connection patterns of the larval Malpighian tubules in Protaetia brevitarsis and Holotrichia diomphalia (Coleoptera: Scarabaeidae).

Abstract

Malpighian tubules are key excretory and osmoregulatory organs in insects, yet their structural organization and hindgut associations remain poorly documented in scarab larvae. In this study, the morphology and ultrastructure of the Malpighian tubules were examined in third-instar larvae of the saprophagous Protaetia brevitarsis, a cetoniine scarab, and the phytophagous Holotrichia diomphalia, a melolonthine scarab, using light and transmission electron microscopy. In both species, four Malpighian tubules are present, originating posterior to the third circle of gastric caeca and arranged in a conserved pattern consisting of two laterally positioned tubules and two ventrally adjacent tubules. The epithelial organization and ultrastructural characteristics of the Malpighian tubules are highly similar between the two species, with principal and stellate cells exhibiting comparable cellular features. In contrast, clear differences were observed in the spatial connection patterns between the Malpighian tubules and the rectum. In P. brevitarsis, the distal ends of the tubules bent posteriorly to form two bilateral pairs connecting to the rectum, whereas in H. diomphalia the tubules joined the rectum at distinct, non-adjacent sites. These findings demonstrate a conserved ultrastructural organization of Malpighian tubules in scarab larvae alongside species-specific variation in hindgut connection patterns, providing new morphological insights into the diversity of rectal complex organization in Coleoptera larvae.