MECHANORECEPTION 71 



changes and are presumed to have a static function. The organs in 

 Nepa, situated adjacent to the spiracles of the third, fourth, and hfth 

 abdominal segments, consist of groups of peg-shaped sensilla alter- 

 nating with umbrella-shaped sensilla (Fig. 50). According to Hamilton 

 (1931), the whole is covered by a thin membrane which docs occlude 

 the adjacent spiracle, but Thorpe and Crisp (1947) considered the 

 membrane to consist of the overlapping umbrella-like portions of the 

 sensilla which do indeed cover the spiracle. Baunacke (1912) con- 

 sidered these organs to be hydrostats. After he ascertained that a 

 blinded Nepa placed on an underwater seesaw would reverse its 

 direction of crawling as the inclination was reversed, he eliminated the 



Fig. 51. Statocyst in the larva of an aquatic dipteran (Limnobiidae). The 

 black objects are particles in the cavity. (Redrawn from Wolff, 1922.) 



organs. Elimination abolished the bug's response to the change of 

 inclination. Oevermann (1936) and Thorpe and Crisp (1947) con- 

 firmed these observations. Oevermann, however, demonstrated that 

 there must be proprioceptive information from the legs, because a 

 weighted insect can still make position corrections. 



Baunacke (1912) believed that the three pairs of organs act as a 

 system and respond to relative changes in pressure. In other words, the 

 lowest pair (on the fifth sternum) are under greater pressure than 

 the upper pair (third sternum) when the bug is in certain attitudes. For 

 this system to work the gas-filled cavities of all of the sense organs 

 must be in communication with one another via the tracheal system. 

 According to Hamilton, who described the organs as covered with a 

 membrane sealing them from the trachea, this is not so, but Thorpe 



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