Comparative study for function-specific subdivision in the calycal neuropil of the mushroombody in the insect brain.

The mushroombody (MB) of the insect brain is suggested to be a multimodal integration center, and is also implicated in associative learning and memory mechanisms. The calyces (CAs) of the MB receive olfactory, mechanosensory and visual information via input neurons (INs) from the primary center for each sensory modality and thus make synaptic contacts with intrinsic neurons called Kenyon cells, which in turn synapse onto output neurons in the peduncles and lobes. We examined function-specific distributions of INS in CAS of cockroaches, honeybees and crickets. In the honeybee, the calyx is morphologically divided into three concentric zones : the lip, the collar and the basal ring. The collar that mainly contains visual INs is the most massive neuropil area among those three zones, and is over three times as much as the lip containing olfactory INs. Since some of INs from antennal lobe restrictedly localize in the lip, farther subdivision is suggested in the lip. On the other hand, such morphological differentiation has not been reported so far in the CA of the cockroach. We found functional subdivisions in the CA of the cockroach by using intra- or extra-neuronal staining of INs of CAs. The calycal neuropil is subdivided into three concentric zones : the lip, the middle and the base, and into at least two layers : inner and outer layers. Olfactory information mainly inputs into the lip and the middle zones, visual information into the inner layer, and mechanosensory information into the outer layer and the base zone of the calycal neuropil. Olfactory input area occupy over two third of the calycal neuropil, and visual input area is comparatively small. Relatively large volume of input area for a paticular sensory modality might reflects that a given insect species largely depends on the particular sensory modality for its life. Detail about INs in the cricket has not been known yet, however, massive area in the deutocerebrum is observed to be occupyed by antennal mechanosensory inputs.