An ‘exaggerated’ trait in insects: the prothoracic skeleton of Stictocephala bisonia (Homoptera: Membracidae)

The prothoracic skeleton of Stictocephala bisonia was investigated in adults and fifth‐instar nymphs on a gross morphological (SEM, maceration) and light microscopic level. In both nymphs and adults, the prothoracic skeleton consists of the pronotum, episternum, epimeron, precoxale, sternum, trochantin, and two endoskeletal characters (furcal arms and pleural apophyses). In nymphs, the entire pronotum is a single‐layered outgrowth of the integument communicating with the body cavity and filled with hemolymph and fat body cells (“spine”); the dorsal and ventral processes and the suprahumeral bud are extensions of this single‐layered integument. In adults, the pronotum is composed of (1) a proximal, single‐layered part, and (2) a larger, distal, double‐layered part (“posterior reduplication”) with two cuticular layers separated by a thin lumen. The posterior reduplication is elevated above the body and forms hollow (air‐filled) extensions (e.g., suprahumeral horns). Its two cuticular layers are connected through cuticular columns that appear on the external surface as pits. The lumen between these layers communicates with the body cavity and contains nerves and tracheae. In the lumen of newly eclosed adults, intercellular space, epidermal cells with long processes, and hemocytes with nonlipid granules are present. In the lumen of sclerotized adult pronota, the intercellular space has disappeared, together with definite cell boundaries. Several structures are associated with the external cuticle: two types of innervated sensilla trichodea that articulate in the center of external pits, sensilla campaniformia, sensilla coeloconica, and cuticular canals with exterior openings. The morphogenetic implications of pronotal construction, various aspects of adult prothoracic anatomy, and the value of glands and sensilla for an adaptive interpretation of the pronotum are discussed. J. Morphol. 238:157–178, 1998. © 1998 Wiley‐Liss, Inc.