56 NATURAL HISTORY OF AQUATIC INSECTS CH. 



When one of the suckers of the fore or mid leg is 

 pressed upon a clean slip of glass, a rather copious 

 deposit of liquid is left behind. It is coagulable and 

 insoluble in water ; it solidifies under water, and is 

 not wetted by water. To this account I may add 

 that it becomes opalescent on exposure to air. Mr. 

 Lowne points out that the adhesion of the suckers 

 suffices to support the weight of the Beetle in vacuo, 

 a proof that atmospheric pressure is not the cause of 

 the adhesion of the sucker. Nor can it be the surface- 

 tension of a thin film of water, for the suckers act 

 perfectly well when applied to a glass plate under 

 water. Since they act in water, in air, and in a 

 vacuum, we can hardly doubt that they act by means 

 of the tenacity of the coagulable secretion. I have 

 tried to exhaust the supply of the secretion by attach- 

 ing and detaching the suckers many times in succes- 

 sion. After thirty such repetitions, which occupied 

 about half an hour, the adhesive power of the suckers 

 was greatly diminished ; when wetted with water, 

 they recovered some of their efficiency, but were 

 much weaker than at first ; after thirty more trials 

 they adhered very feebly indeed. 



The cupules discharge the viscid secretion when 

 pressed against a smooth surface, and it becomes a 

 question of some interest how the secretion is dis- 

 charged at the right moment. The act must be 

 mechanical, not depending upon any nervous stimulus, 

 for the fluid is discharged as freely in a freshly killed 

 Dytiscus as in a living one. It is to be observed 

 that each of the smaller cupules consists of a cylin- 

 drical shaft, perforated by a duct, and surmounted by 



