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AQUATIC BEETLES 57 



a nearly hemispherical umbrella, in the centre of 

 which the duct opens. We can roughly imitate its 

 form by taking half of a hollow india-rubber ball and 

 applying to the centre of its convex surface the end 



I of a glass tube. Pass the tip of a pen-knife through 

 the india-rubber to represent the orifice of the duct. 

 If this rough model is placed on the table, with the 

 edge of the hemisphere downwards, the glass tube 

 may be pressed against the centre of the convex 

 surface until the hemisphere is deeply sunk in the 

 middle. It will then be found that the slit made by 

 the pen-knife gapes, and I believe that this con- 

 trivance is employed to discharge the contents of the 

 gland, though direct observation of the action of the 

 natural organ is hardly practicable. 



Mr. Lowne finds that the cupules are occasionally 

 torn off by the cementing action of the secretion. On 

 examining with a lens four females taken at hazard, 

 I found that one of them had a number of cupules 

 firmly adhering to the prothorax.^ 



The suckers are no doubt chiefly employed to 

 retain the female, as their restriction to the male sex 

 indicates. They are, however, occasionally used to 

 hold the prey, though this is generally grasped by the 

 middle legs, and torn to pieces by the fore legs. Mr. 

 W. F. Baker tells me that he has seen a male Dytiscus 



^ In the Cypris stage of all Cirripedes the antennule bears an 

 organ which has something in common with the cupules of the 

 male Dytiscus. It takes the form of a cup, and adheres by 

 means of a coagulable secretion, which attaches the animal to 

 its future support. In Cirripedes, however, the cup is a modi- 

 fied joint, whereas the cupules of Dytiscus are modified hairs. 



