Olfactory sensory neurons (OSNs) are embedded in nasal mucosa in mammals and antennal sensilla in insects. Even with distinct structures, olfactory information is processed in a similar way in the primary olfactory center for both classes. These primary olfactory centers are compartmentalized into spheroidal glomeruli, in which a large number of OSNs synapse onto a moderate number of secondary interneurons. Since it is possible to track neural events within a network of fully characterized neurons in insects, the primary olfactory center, antennal lobe (AL), provides a valuable model system for examining the neural basis of olfaction. The cockroach Periplaneta americana has excellent capabilities of olfactory discrimination and olfactory associative leaning. However, in contrast to rich accumulation of knowledges about neural pathways and processings of specific odors, such as sex-pheromones, relatively little is known about those of general odors. To address this question, we focused on the glomerular organization of the cockroach AL. We unambiguously identified 205 glomeruli and classified them into ten glomerular groups (T1-T10 glomeruli) based on the detailed innervation patterns of sensory tracts. More especially, we found that the axons of OSNs housed in perforated basiconic sensilla selectively projected to the antero-dorsal T1-T4 glomeruli, whereas those in trichoid, grooved basiconic and capitular sensilla innervate the postero-ventral T5-T10 glomeruli. Since distinct sensillum types are tuned to distinct subsets of odorant molecules, the AL is functionally compartmentalized. Next, using voltage sensitive-dye imagings and intracellular recordings, we revealed spatio-temporal activity patterns of glomerli and found that T1-T4 glomeruli and T5-T10 glomeruli responded to different subsets of odorants with different temporal activity patterns. In addition, we observed axon terminals of uniglomerular projection neurons from these two glomerular groups are also segregated in the secondary olfactory centers. In conclusion, we revealed that information about general odors is processed by two parallel pathways from pheripheral to higher brain centers.

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Melanin is a brown-black pigment that is distributed to variety of terrestrial organisms. In the case of insects, melanin is synthesized not only for the formation of body color pattern but also in the situation where immune reactions occur. In insect cuticle, two types of enzymes, laccase and prophenoloxidase, are involved in melanin synthesis. They both are copper-containing enzyme and have a role to oxidize phenolic compounds such as L-dopa and dopamine. Laccase and prophenoloxidase belong to distinct family of proteins, three-domain multi-copper oxidase and type-III copper protein, respectively. It has been reported that the RNAi experiments of multiple insect species have shown that laccase is involved in brown-black pigmentation and cuticle hardening during the ecdysis process. In contrast, the knockdown of prophenoloxidase gene does not affect the process of cuticle pigmentation. In contrast to laccase, prophenoloxidase is thought to be responsible for melanin formation at wounded part of the cuticle and at around the invading parasites. This indicates that the melanin formation by the protein is important for defense reactions in insects. Here, this review describes about the systems for melanin synthesis in insects by focusing mainly on the two phenol oxidizing enzymes. It also discusses about the possible history of the systems for melanin synthesis during the evolution of insects.

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