160 THE PHYSIOLOGY OF INSECT SENSES 



relation between locomotory behaviour and rate of evaporation has 

 clearly been demonstrated (Wellington, 1949; Green, 1954). No in- 

 formation is available with regard to the receptors involved. It is 

 possible that the response is mediated by rather general changes in the 

 internal milieu rather than by specific sense organs. 



The identity of humidity receptors is even less firmly established 

 than that of olfactory receptors. In Tenebrio responses to humidity are 

 aboHshed when the antennae are extirpated or completely covered. 

 The most common sensilla are pits and pegs, the latter being confined 

 to the seven distal segments. The intensity of response is greatly de- 

 creased when the seven distal segments are removed, but complete 

 abolition requires removal of the additional segments which bear only 

 pits. Asymmetrical amputation has shown that a threshold number of 

 pits is required (Pielou, 1940). In Tribolium but not in Sitophiius, 

 similar ablation experiments have shown that there is a correlation 

 between the number of thin-walled sensilla and the intensity of res- 

 ponse (Roth and WilHs, 1951b). The sensilla are simple or multiply 

 branched pegs. On the antennae of the human louse there are humidity 

 receptors which are distinct from the olfactory receptors (Wiggles- 

 worth, 1941). They are tufted sensilla; each consists of a minute cone 

 bearing four delicate apical hairs. A number of neurons are associated 

 with each sensillum. Similar sensilla are the humidity receptors of the 

 larva of Musca domestica (Hafez, 1950, 1953) and of Drosophila 

 melanogaster (Benz, 1956). In grasshoppers the sensilla coeloconica 

 have been suggested as humidity receptors (Aziz, 1957 b; Riegert, 

 1960), but there is no supporting experimental evidence (Slifer, 1961). 

 By studying antennaless mutants of Drosophila Begg and Hogben 

 (1946) came to the conclusion that the antennal receptors mediated 

 positive responses to wet and that responses to dry were mediated by 

 receptors at other unknown locations on the body. Perttunen and 

 Syrjamaki (1958) came to exactly the opposite conclusions and pro- 

 posed an explanation of the discrepancy. Kuwabara and Takeda 

 (1956) conditioned honeybees to extend the proboscis when water 

 vapour was held near the antennae and then amputated various lengths 

 of antennae. They concluded that the sensilla ampullacea were most 

 probably the hygroceptors. Bursell (1957) found that the normal 

 orthokinetic response of the tsetse fly Glossina morsitans was abolished 

 when the branched hairs guarding the thoracic spiracles were 

 removed. 



How the hydroreceptors operate is still a mystery. They could res- 

 pond directly to water molecules striking the surface; they could 



