Central projections of the antennal cold receptor neurons and hygroreceptor neurons of the cockroach Periplaneta americana.

The central projections of the cold receptor axons were examined by filling two types of cold receptive sensilla with cobalt lysine--a cold and hygroreceptive (C/H) sensillum and a cold receptive and olfactory (C/O) sensillum--on the antennae of the cockroach, Periplaneta americana L. When the dye filled a single C/H sensillum, four axons were stained. Three of these axons terminate in the ipsilateral antennal lobe, while the other branches in the ipsilateral dorsal lobe. One of the branches passed through the tritocerebrum to terminate in the suboesophageal ganglion, while the other branches end in the lobe. When a single C/O sensillum is dye filled, all axons of the four receptor neurons terminate exclusively in the ipsilateral antennal lobe. One axon from the C/H sensillum and one axon from the C/O sensillum terminate in a particular glomerulus in the ventroposterior region of the antennal lobe. Each of these axons also has a tuft in separate glomeruli situated just dorsal to the glomerulus in which both axons terminate. This set of three glomeruli have indistinct boundaries and appear to form a complex of glomeruli similar to the macroglomerular complex of male moths. Assuming modality-specific convergence of antennal afferents, these axons appear to belong to the cold receptor neurons, and the set of glomeruli appear to function in cold reception. Two other neurons stained from C/H sensilla always terminate in the glom-eruli distinct from the set of glomeruli mentioned earlier. These neurons are assigned to the pair of hygroreceptor neurons, and their glomeruli are thought to function in hygroreception.