70 THE PHYSIOLOGY OF INSECT SENSES 



swimming occurs. Destruction of the hair plates interferes with normal 

 swimming. 



By analogy with marine invertebrates, especially Crustacea, it might 

 be expected that statocysts would be common in aquatic insects. Such 

 organs are ideally constructed for responding to gravity in a medium 

 where cues are few. Aside from a series of questionable cases studied 

 by Wolff (1922) and Studnitz (1932), however, no true statocysts have 

 been reported. In the larvae only of many Limnobidae (Diptera) there 

 is located on the terminal segment a pair of small sacs, open to the 

 outside, and equipped with muscles by which water can be pumped in 



Fig. 50. The organ of pressure sense of Nepa. T, trachea ; N, nerve ; M, 

 membrane of overlapping expanded margins of scale sensilla; C, 

 closed spiracle ; S, sensory papilla. (Redrawn from Thorpe and Crisp, 

 1947.) 



and out (Fig. 51). Invariably each pouch contains a few minute par- 

 ticles either formed by the larva itself or picked up from the outside. As 

 the muscles of the sac contract and relax, the particles are rattled 

 about the interior. 



In the interior of the sac are usually two sensory hairs, one at the 

 blind end or deepest part of the sac, the other, in a lateral position. 

 Wolff (1922) was convinced that these organs were not static in 

 function. Studnitz (1932), on the other hand, after proving that uni- 

 lateral or bilateral destruction of the organs abolished the positive 

 geotaxis characteristic of the normal larvae, concluded that they were 

 indeed statocysts. 



In water bugs (A^e/?^ and ^/j/ze/oc/ze/rw^) there are elaborate mechano- 

 receptive organs which appear to be designed to respond to pressure 



