100 



EVENINGS AT THE MICROSCOPE, 



proceeding from one to the other, through the interstices 

 of which the air is filtered. The central plate seems to 

 be quite imperforate. 



The fat, thick-bodied grubs of those beetles called 

 chafers exhibit, in their spiracles, a modification of this 

 structure, rendered still more elaborate. In the case of 



the larva of the common 

 Cockchafer (Melolontlta vul- 

 garis), for example, the cen- 

 tral plate is a projection 

 from one side of the margin 

 of the spiracle ; or, to use a 

 geographical simile, we may 

 say that, instead of being 

 an island in the midst of a 

 lake, it is a promontory. 



Thus the breathing space 

 is a crescent-shaped band, 

 which is crossed in every 

 part by bars passing from 

 the margin to the projecting plate. But, as if the inter- 

 stices left by these bars would be too coarse for the pur- 

 pose, they are made still finer by a membrane, which is 

 stretched across them, and which is pierced with a number 

 of exceedingly minute round holes, through which alone 

 the air is admitted. 



In many of the two-winged flies, which inhabit the 

 water in their earlier stages, there are some interesting 

 contrivances and modifications connected with the organs 

 of respiration. It is necessary that the orifices of the 

 air-tubes should be brought at intervals to the surface of 

 the water, in order to come into contact with the external 

 air; while, at the same time, it is important that as small 

 a portion as possible of the animal's body be exposed to 

 danger, by being protruded from its sheltering element. 

 An example in point you may see in this vase. 



Wll"" 

 SPIBACLE OF COCKCHAFER-GBUB. 



