36 AN ECONOMIC ENTOMOLOGY. 



shield, beneath which the air must enter, or some other variety 

 of screen, — all intended to sift out of the air the foreign particles 

 which might injure the insect. Yet it is through these spiracles 

 that most of the contact insecticides must act, as will be more 

 fully detailed hereafter. In many heavy insects there are at- 

 tached to the tracheae numerous air-sacs or bladders, the object 

 of which is to reduce the specific gravity of the insect to enable 

 it to fly more readily. A common example is the well-known 

 " May-beetle" or "June-bug," which has the entire body cavity 

 filled with such little bladders. If one of these beetles be watched 

 when it emerges from its hiding-place early in the evening, it 

 will be noted that it first crawls to the top of some blade of grass 

 or twig, or even upon a stone, and rests there a few minutes. 

 If it be closely observed it will be seen to be pumping in air, — 

 that is to say, the body pulsates rapidly, and the abdomen seems 

 continually to increase in size. This is because the insect is fill- 

 ing the air-bladders, and, as soon as this has been accomplished, 

 it tries its wings, causing the preliminary ' ' whirr' ' which is so 

 familiar to the observer in the early evening of a June day. After 

 two or three trials the insect becomes confident of its ability to 

 sustain itself, and flies off heavily, with a steady hum. If such 

 ^ beetle be found at mid-day and thrown into the air it will fall 

 heavily, and will be absolutely unable to sustain itself in flight 

 until it has had an opportunity to inflate its air-sacs. 



While the vast majority of all insects are air-dwellers, yet many 

 live underground, a very large number in water, and a consider- 

 able percentage in decaying or excrementitious matter, v>-here 

 pure air is very difficult or impossible to obtain. All sorts of 

 devices to enable them to breathe under such conditions have 

 been developed, and in all cases without any essential modifica- 

 tion of the system. Underground dwellers usually find air sufifi- 

 cient for their needs in the earth in which they live, and the 

 development here is principally in the direction of more perfect 

 coverings for the spiracles to prevent the entrance of dirt. 



Among insects living in or under water, or spending a portion 

 oi their life there, we note a great variety of modifications. 

 Many beetles, and some insects of other orders, have the wing- 

 covers dome-shaped, and so closely fitted that they are able to 

 carry a little supply of air between them and the abdomen. 



