The Lateral and Ventral Sclerites of the Insect Thorax

Theories of the origin and homologies of the pleural and sternal sclerites are discussed and the structure of the pleuron and sternum in some primitive and specialized insects is described.The arthropod leg was set primitively in the latero-ventral region of the segment; therefore elements derived from its lower margin lie in the ventral region. These elements, here called the coxosternites, are considered as parts of the sternum. On this interpretation the sternum, apart from the small spinasternite, consists of a pair of coxosternal (subcoxal) sclerites and a mediosternite. The three sclerites are separated by a pair of laterosternal sutures. The furcal pits, orifices of the sternal apophyses, lie in these sutures. A sternacostal suture extends across the mediosternite and usually continues across the coxosternites dividing the sternum into an anterior basisternite and a posterior furcasternite. These primitive relations are clearly shown in some of the lower insects and the interpretation of the sternum given here is based on the assumption that they are constant throughout.Modifications in the structure of the pleuron and sternum resulting from the loss of the primary sutures, appearance of new sutures, expansion or contraction of sclerites, partial desclerotization, and changes in the position of the legs are discussed.