120 EVENINGS AT THE MICROSCOPE 



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 (Melolontha vulgaris), for 

 example, the central plate is a projection from one side of 

 the margin of the spiracle 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 interstices left by these bars would be too coarse 

 for the purpose, they are further sublimated by a mem- 

 brane, which is stretched across them, and which is per- 

 forated with a number of excessively 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. 



Here is a slender worm, an inch and a half in length, 

 thickest a little behind the head, and tapering gradually 

 to a lengthened tail, the twelve segments of the body 

 being very conspicuous. It swims up and down or to and 

 fro in the clear water, with a not very rapid, wriggling 

 movement, throwing its body alternately from side to 

 side in the form of the letter S. 



