Fig. 16.

T. castaneum head scheme depicting the major olfactory pathway components. a Head section (dorsal view) showing the brain and the CSNs from the antenna (blue) and the mouthparts (green). b Head section (ventral view) showing the GNG. Section orientation is indicated at the upper right corner (lateral view of the head; V, ventral). Double-headed arrows indicate body (A, anterior ↔ P, posterior; black) and neuro-axis (NA, n-anterior ↔ NP, n-posterior; red). Chemical signals are sensed by about 720 CSNs located in 56 SBas, 87 cSTri, and 11 SCoe on the last three antennal segments. These CSNs express 16 IRs, 62 GRs, 129 ORs, and six SNMPs. Chemosensory information is also perceived in the palps by five IRs, 69 GRs, 49 ORs, and six SMNPs (number in brackets indicates significantly enriched members compared to body). The antennal nerve (AN) projects into the ipsilateral AL, where all except one (light green) of the about 90 GL (dark blue) are innervated. A separate antennal tract (*¹) descends into the GNG (b, blue), where presumably gustatory and mechanosensory information is processed. Incoming olfactory information is processed by a complex network of local interneurons (LNs) in the AL and further relayed by projection neurons forming three ALTs. The medial ALT (mALT) projects to and arborizes in the calyx of the MB formed by about 2700 KCs (orange) to eventually innervate the LH (light blue). The mediolateral ALT (mlALT) and lateral ALT (lALT) directly innervate the LH. From the mouthparts, CSNs project via the maxillary (*³) and labial palp nerves (*⁴) into the GNG, where the gustatory information is processed in the PGC. The olfactory sensory input from the palps is processed in an unpaired glomerularly organized GNG structure, the GOC, as well as in the LG, which receives input from some palpal OSNs via ascending neurons (*²) passing through the GOC. Some of the palp-derived chemosensory information is processed in the single AL glomerulus, which lacks antennal innervation and is, therefore, exclusively innervated by projections from the mouthparts (light green). AL antennal lobe, ALT antennal lobe tracts, AN antennal nerve, CSN chemosensory neuron, cSTri chemosensilla trichoidea, GL antennal lobe glomeruli, GNG gnathal ganglia, GOC gnathal olfactory center, GR gustatory receptor, IR ionotropic glutamate-like receptor, KC Kenyon cells, lALT lateral antennal lobe tract, LG lobus glomerulatus, LH lateral horn, LNs local interneurons, mALT mediolateral lobe tract, MB mushroom body, mlALT mediolateral lobe tract, OR odorant receptor, OSN, olfactory sensory neuron; PGC primary gustatory center, SBas sensilla basiconica, SCoe sensilla coeloconica, SNMP sensory neuron membrane protein