Tritocerebral tract input to the insect mushroom bodies.

Insect mushroom bodies, best known for their role in olfactory processing, also receive sensory input from other modalities. In crickets and grasshoppers, a tritocerebral tract containing afferents from palp mechanosensory and gustatory centers innervates the accessory calyx. The accessory calyx is uniquely composed of Class III Kenyon cells, and was shown by immunohistochemistry to be present sporadically across several insect orders. Neuronal tracers applied to the source of tritocerebral tract axons in several species of insects demonstrated that tritocerebral tract innervation of the mushroom bodies targeted the accessory calyx when present, the primary calyces when an accessory calyx was not present, or both. These results suggest that tritocerebral tract input to the mushroom bodies is likely ubiquitous, reflecting the importance of gustation for insect behavior. The scattered phylogenetic distribution of Class III Kenyon cells is also proposed to represent an example of generative homology, in which the developmental program for forming a structure is retained in all members of a lineage, but the program is not "run" in all branches.