INSECTS : THEIE BEEATHING OEGAJJS. 121 



mence and to proceed along the centre of the column, 

 thence into the abdomen, which they traverse one 

 along each side, sending off slender hranchlets 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 fol- 

 low the tracheal tube 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 per- 

 sonal 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 examraation 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 generally simple organization, 

 make it a more than usually suitable object for investi- 

 gation : besides which, there are the beautiful and in- 

 teresting points in the details of its structure which I 

 have been endeavouring to bring before you. 



liTot less interesting and remarkable is the change 

 in the position of the spiracles, which takes place as 

 soon as this grub arrives at the pupa or chrysalis state. 

 The skin of the active, fish-like larva splits down the 

 back, and out presses an equally active little monster ; 

 6 



