34 INJURIOUS AND USEFUL INSECTS 



the suckers is very great, and they will easily support the 

 whole weight of the beetle in vacuo, which incidentally 

 proves that they do not depend upon atmospheric pressure. 

 It has been fully demonstrated that the cupules discharge a 

 viscid secretion, as do the adhesive organs of a fly's foot. It 

 is easy to see that a coagulable fluid is left behind when 

 one of the suckers of a Uytiscus is pressed upon a clean 

 slip of glass. The restriction of the suckers to the male 

 beetle suggests that they are used to hold the female. They 

 are also used to hold the struggling prey, but this cannot 

 be their main function, since they are not found in the 

 female at all. It is to be noted that the basal joints of the 

 tarsus of the middle leg are enlarged and bear cupules, so that 

 this leg also is serviceable in holding. 



The elytra of the female are furrowed lengthwise, while 

 those of the male are smooth. This has been interpreted as 

 an additional provision for holding the female, but doubt 

 is cast upon the conclusion by the following well-ascertained 

 facts: — (i) Furrows on the elytra diminish instead of increas- 

 ing the holding power of the suckers, which act best on smooth 

 and polished surfaces ; (2) the suckers of the male are not 

 applied to the furrowed surface of the elytra ; (3) females 

 with smooth elytra are common in some districts, and the 

 male can hold these just as well as the others. 



The equilibrium of the body of the beetle (and we shall 

 find that the same is true of the larva) is so adjusted that 

 when the beetle rises through water without exerting itself, 

 the tail-end comes up first. The largest pair of spiracles, 

 upon which the insect mainly relies for replenishing its 

 supply of air, are situated far back. This arrangement 

 not only prevents needless exertion in assuming the attitude 

 most convenient for breathing, but gives in particular emer- 

 gencies an extra chance of life. Suppose that the insect 

 is stunned, or in some other way temporarily disabled ; it 

 will rise by its own buoyancy to the surface in just that 

 attitude which exposes the spiracles to the air. If it is still 

 in a condition to breathe, it can do so without changing its 

 position. 



When the beetle dives, it takes down a considerable supply 

 of air lodged beneath its wing-covers. The air clings to 

 the felted hairs which cover the top of the abdomen, and 



