MECHANORECEPTION 75 



RESPONSES TO GRAVITY 



As the foregoing discussions have impHed, insects lack receptors 

 designed specifically for the detection of gravitational force. Infor- 

 mation required for maintenance of primary orientation is derived 

 secondarily by the central nervous system from many receptors whose 

 primary function may have little to do with responses to gravity. The 

 precision with which some insects respond to gravity and the import- 

 ance of a critical evaluation suggests that there may be some receptors 

 whose information is critical. Buckmann (1954) has demonstrated, for 

 example, that the staphylinid beetle BUdius bicornis Grm., which 

 burrows in sea sand, responds to gravity with a precision of the order 

 of one angular degree. This precision is not inferior to that exhibited 

 by other animals which possess statocysts (Buckmann, 1955 a). 

 Centrifugation experiments have indicated that neither light nor 

 surface features are cues. Unfortunately the nature of the required 

 sensory information is not known. In any event, the antennae are not 

 required (BUckmann, 1955 b). 



Another insect for which a precise response to gravity is critical is the 

 honeybee. In order to be able to communicate the direction of food 

 sources by transposing the angle of flight with respect to the sun to an 

 angle of dancing with respect to gravity in a dark hive, the bee must 

 be able to assess very accurately the direction of gravitational force. 

 Vowles (1954), working with ants, had shown that the antennae were 

 of importance in these species. Iron filings were cemented to various 

 parts of the bodies of ants climbing a vertical surface. The ants were 

 then suddenly subjected to a magnetic force. Only when the filings 

 were on the funiculi of both antennae did the ants suddenly change 

 orientation. This result plus the knowledge that different orientations 

 with respect to gravity normally impart different rotational forces on 

 the funiculus led to the hypothesis that the Johnston's organs are 

 involved in geotaxis. 



With the honeybee, however, the antennae are not so critical. 

 Instead, hair plates in the neck region and at the articulation of the 

 thorax and abdomen play a prominent role (Lindauer and Nedel, 

 1959). When the bee is standing on a horizontal surface the hairs of the 

 pro-thorax are in even contact with the back of the head. When the bee 

 crawls up a vertical surface the head will incline towards the sternum 

 because of its low centre of gravity. In this position the more ventral 

 hairs of the hair plate will experience maximal shearing forces. The 

 less the substrate is inclined towards the vertical, the smaller the 



