Electrophysiological identification of cold receptors on the antennae of the ground beetle Pterostichus aethiops ENNO MERIVEE 1 , ALO VANATOA 2 , ANNE LUIK 1 , MA ¨ RT RAHI 2 ,VA ¨ INO SAMMELSELG 3 andANGELAPLOOMI 1 1 Estonian Agricultural University, Institute of Plant Protection, Tartu, 2 Estonian Agricultural University, Institute of Zoology and Botany, Tartu and 3 University of Tartu, Institute of Physics, Tartu, Estonia Abstract. In single-sensillum extracellular electrophysiological recordings, ter- minal campaniform sensilla at the tip of antennae of the ground beetle Pterostichus aethiops (Pz., 1797) show action potentials of three sensory cells, A-, B- and C-cells, distinguished by differences in their spike amplitudes. Only the A-cell, with the largest spike amplitude, is highly sensitive to temperature fluctuations, showing remarkable changes in its firing rate induced by changes in temperature of 0.1  C. The firing rate of A-cells at 23  C varies from 15–52Hz among different beetles. Mean impulse frequency of A-cells is found to be a function of steady temperature, the firing rate decreasing with temperature increase. A-cells respond to a rapid temperature drop with a strong phasic-tonic reaction; larger decreases in temperature evoke higher peak frequency values. Maximum peak frequencies, varying from 380–630Hz in different beetles, are induced by temperature decreases of 3–10  C, whereas temperature rise strongly inhibits impulse activity of the A-cell. The first manifestation of rapid warming in the nerve impulse sequence is a very long interspike period, followed by diminished activity. Both the length of the long interspike period and the rate of following impulse activity are functions of temperature change; hence, A-cells respond to temperature changes as typical cold receptors, similar to coeloconic and short hair-like sensilla in other insects. Key words. Antennal sensilla, campaniform sensillum, cooling, impulse activity, single-sensillum recording, thermoreceptor, warming. Introduction For animals as small as insects, microclimatic conditions may be quite disadvantageous, if not lethal, in the absence of instant cues about their temperature and humidity. To avoid temperature extremes, and to find and stay in ther- mallyfavourableareas,insectsarewellequippedwithphysio- logically different kinds of thermoreceptors packaged in many different external cuticular structures (Altner & Prillinger, 1980; Altner & Loftus, 1985; Zacharuk, 1985). In beetles, thermoreceptors on the antennae of the cave beetle Speophyes lucidulus have been examined both physio- logically and morphologically (Loftus & Corbie`re- Tichane´, 1981; Corbie`re-Tichane´ & Loftus, 1983). In ground beetles (Carabidae), no physiological or ultrastruc- tural information about thermoreceptors is available. How- ever, in several detailed scanning electron microscopy studies, the morphological types, numbers and distribution patterns of antennal sensilla have been described (Kim & Yamasaki, 1996; Merivee etal., 2000, 2001, 2002), which serve as a firm basis for electrophysiological studies of the Correspondence: Dr Enno Merivee, Estonian Agricultural University, Institute of Plant Protection, 64 Kreutzwaldi Street, 51014 Tartu, Estonia. E-mail: merivee@eau.ee Physiological Entomology (2003) 28, 88–96 88 # 2003 The Royal Entomological Society