Response characteristics of two types of cold receptors on the antennae of the cockroach,Periplaneta americana L.

1.	There are two types of cold receptors on the antennae of Periplaneta americana. One type of cold receptor is present in a thermo- and hygroreceptive sensillum along with a moist receptor and a dry receptor, and the other is in a thermoreceptive and olfactory sensillum along with olfactory receptors. These two types of cold receptors were investigated using stimulations of constant temperature (T), stepwise decrease of temperature (T), and constant rates of temperature decrease (dT/dt).
2.	For cold receptors in the thermo- and hygroreceptive sensilla (C-H cells), steady-state activities are higher at lower constant temperatures (T) within the range of 16–31 °C examined (n = 7). On the other hand, those of the cold receptors in the thermoreceptive and olfactory sensilla (C-O cells) have individually a broad activity maximum in the range of 18–27 °C (n = 7).
3.	The differential sensitivity to downward temperature steps (T) of C-H cells was in the range of - 5.4 to - 12.4 (impulses/s)/°C (n = 11). It was higher at lower adapting temperature, from which changes were initiated, in the range of about 21 to 31 °C, that is, it was reflected by the steady-state activity at the initial and adapting temperature. On the other hand, the differential sensitivity of C-O cells was in the range of - 10.7 to- 15.2 (impulses/s)/°C (n = 9), and was little reflected by the steady-state activity.
4.	Responses to decreasing temperature in C-H cells (n = 14) are negatively related to the rate of temperature decrease (dT/dt) in the range of about 0 to - 0.16 °C/s at given temperatures, and are also affected by instantaneous temperature (T) in the range of 18 to 30 °C. This dependence is also observed in C-O cells (n = 10), but less pronounced than in C-H cells.