On the Behaviour of Wireworms of the Genus Agriotes Esch. (Coleoptera, Elateridae)

1. The general humidity behaviour, the humidity receptors and the orientation mechanisms have been studied in the larvae of Agriotes. 2. Wireworms avoid dry air, the intensity of avoidance being greatest when the alternatives are close to saturation . Within this humidity range a difference of 7.5 % R.H. in the alternative chamber (at 17 ° C.) is sufficient to ensure the successful avoidance of the lower humidity by nearly every individual, while statistically significant reactions are obtained with differences as small as 0.5 % R.H. At lower ranges of humidity the same differences yield progressively less intense reactions; at low humidities the response is entirely eliminated. 3. The intensity of the reaction is in better accord with the humidity differences when these are expressed as saturation deficiencies rather than as relative humidities. This suggests that the reaction is initiated by the evaporation of water (‘evapori-meter’ receptor) and not by the operation of receptors which function hygrometrically (‘hygrometer’ receptors). 4. The humidity ‘receptors’ lie on the head. Amputation experiments indicate that the relevant sites of evaporation are distributed between the antennae, maxillary and labial palps; the structure of these appendages is compatible with such a function;- on the other hand, sensilla which might conceivably have a hygroscopic function are absent. 5. Orientation is achieved by the operation of two mechanisms. First, larvae are more active in dry than in moist air (low hygro-kinesis); secondly, larvae show a directed response to low humidities (klinotaxis); this is displayed as a powerful backward recoil if the wireworm crosses a steep gradient from moist into dry air. No tropo-tactic component is involved. The failure of the reaction at low humidities is explained by the great activity of the larvae which interferes with the operation of the klino-tactic mechanism. It is suggested that the kinesis is maintained by a relatively constant rate of water loss from the head appendages, while the klino-taxis is initiated by any sudden increase in the rate of evaporation. 6. The behaviour can readily be correlated with the humidity conditions prevailing in the soil; the necessity for such a sensitive response is possibly dictated by the permeability of the cuticle which renders wireworms peculiarly liable to water loss in unsaturated atmospheres.