Structure and ecdysial process of the larval spiracles of the scarabaeoidea, with special reference to those of Lepidoderma

Abstract

The structure of the larval spiracles of the scarabaeid beetle, Lepidoderma albohirtum Waterh., has been examined by means of serial sections and stereoscan electron micrographs of whole and dissected specimens. Gas exchanges with the ambient atmosphere are effected through aeropyles in the spiracular plate. The aeropyles are of two types: (1) slit-like aeropyles about 4 µ long and 0.1-0.5 µ wide; and (2) round or oval aeropyles that are usually about 0.2-0.3 u wide. The ecdysial tube is often open in preparations, and this has led to the commonly expressed view that it is the chief or only opening into the spiracular atrium. The open condition of the ecdysial tube in preparations is an artifact. The formation of a new spiracle is briefly described. Cuticular struts project inwards from the peritreme and branch at their apices. Then the apical branches divide and those from different struts anastomose to form the plate and the network immediately below the plate. All of these structures are secreted by very long cytoplasmic projections. The nuclei of the epidermal cells remain in the region of the peritreme and do not enter the main columns of cytoplasm that in due course secrete the cuticular struts. The cytoplasmic columns that form the largest struts consist of projections from many epidermal cells. The chief differences in the structure of the spiracles within the superfamily Scarabaeoidea are briefly noted. A closing apparatus is present in the Trogidae, Lucanidae, Passalidae, and Glaphyrinae but is absent in the Geotrupidae and Scarabaeidae. The ecdysial process in primitive Scarabaeoidea is of the typical elateroid type, whereas specialized Scarabaeoidea have a bulla that carries the ecdysial tube. The modified elateroid process of specialized forms is here named the pseudopanorpoid type. The spiracles of some species of Trox are biforous. Those of other species of Trox and all other Scarabaeoidea examined are cribriform. The cribriform type of spiracle has been evolved on at least two occasions within the superfamily.