Structural patterns in the corpora pedunculata of orthoptera: a reduced silver analysis

The calyx region and pedunculus of the corpora pedunculata ("mushroom bodies") were studied comparatively in reduced silver preparations of the brain from 16 species of Orthoptera representing four families (Acrididae, Gryllidae, Tettigoniidae, and Gryllacrididae). In the acridid grasshopper Melanoplus femurrubrum (de Geer), on which emphasis was placed, the concave primary calyx is bilayered and exhibits a special central zone. Globuli cell axons occur within both layers. The bulbous accessory calyx is unlayered and sends anterior extensions beneath the primary calyx. The main input tracts into primary and accessory calyx, respectively, are the tractus olfactorio-globularis and tritocerebral tract. The pedunculus consists of one barrel with three major fiber columns, of which two originate in the primary calyx and one in the accessory calyx. Its fibers display a coaxial arrangement, superimposed on the tripartite organization. Structural conditions in other acridids are similar. In the other families the calyx region similarly includes a bilayered primary calyx and unlayered accessory calyx. The latter, variable in form, is closely associated with the base of the primary calyx in tettigoniids and gryllacridids. The calyces receive the same major tracts as in acridids. The pedunculus is coaxially organized. These features are theorized to have originated as follows. In the progenitors of Orthoptera the corpora pedunculata included two mutually equivalent, bilayered calyces and a "double-barreled" pedunculus. The orthopteran primary calyx arose through coalescence of these calyces. Concomitantly, the two peduncular barrels fused into one. The accessory calyx originated at the base of the primary calyx, from the class of globuli cell axons of the latter's external layer. Probably this occurred in response to increased functional importance to tritocerebral input.