106 EVENINGS AT THE MICROSCOPE. 



We have not yet, however, examined the origin of this 

 air-breathing system. There is but one entrance to the 

 air, or rather two placed close together, at the end of 

 that round column, which is sent off from the eighth 

 segment of the abdomen. This column, which is rough- 

 ened all over with minute points, and fringed with rows of 

 hairs, ends in a horny, conical point, which seems entire 

 while under water, but no sooner does it come to the sur- 

 face, than it is seen to split into five triangular pieces, 

 which open widely, and expose a hollow, at the bottom of 

 which are the two spiracles. 



From these the two main air-pipes are seen to com- 

 mence, and to proceed along the centre of the column, 

 thence into the abdomen, which they traverse one along 

 each side, sending off slender branchlets all along, and 

 becoming more and more attenuated themselves ; till, at 

 length, we trace them into the thorax, and thence through 

 the slender neck into the head itself, until they terminate 

 in fine points close to the back of the mouth. It needs, 

 it is true, a very transparent specimen to follow the 

 tracheal tubes thus through their entire course ; but in 

 such it can be done without difficulty. And it is very 

 instructive to do so, inasmuch as one such personal 

 examination of an insect under a good microscope will 

 make you far more familiar with the peculiarities of its 

 physiology than the clearest book-descriptions, or even 

 the best and most elaborate plates, alone. 



Perhaps you may think I have kept you too long over 

 these gnat-grubs, but my reason for being more minute 

 in the examination of this creature is, that its extreme 

 abundance in every place, and through the greatest part 

 of the year, puts it in the power of every one to procure 

 a specimen alive and healthy almost whenever he chooses ; 

 and, therefore, it is peculiarly available for microscopic 

 study ; while the transparency of its tissues, and its gene- 

 rally simple organisation, make it a more than usually 



