122 EVENINGS AT THE MICROSCOPE 



This you may observe with the unassisted sight, and 

 you may mark, also, how, from time to time, a portion, 

 more or less, of the bubble of gleaming air is inhaled or 

 expired by the animal, causing a diminution or increase 

 of its volume; and this of itself would convince you that 

 it is the spiracles of the animal which are thus protected. 



The application of a low magnifying power, say from 

 thirty-five to fifty diameters, for we can hardly use a higher 

 magnification than this to the animal while alive, will reveal 

 a few more of the details. 



We see, then, that the extremity of the last segment 

 forms a circular disk hollowed in the centre, where it is 

 perforated with the two orifices of the air-pipes. The 

 margin of this disk carries about thirty stiff but slender 

 spines or bristles, some of which are branched in a forked 

 manner. Each bristle bears, on its two opposite sides 

 viz., on those aspects which face the next bristle on either 

 hand two series of not very close-set branchlets, set like 

 the plumes of a feather, or the pinnae of a fern-leaf, which 

 give it the elegant plumose appearance which the unas- 

 sisted eye recognizes. The bristles have a granulose sur- 

 face near the extremity, and terminate in fine points. 



The curious faculty of repelling water, which the in- 

 terior surface of this plumy coronal possesses, is of the 

 highest value in the economy of the insect; for, on the one 

 hand, it permits the breathing orifices to be brought into 

 contact with the air, even when nearly a quarter of an 

 inch below the surface; and, on the other hand, it allows 

 the volume of air enclosed within the funnel to be perfectly 

 isolated and carried securely away, as a reservoir for the 

 wants of the animal, when engaged in its avocations of 

 necessity or pleasure, in the recesses of its sub- aquatic 



