INSECTS: THEIR BREATHING ORGANS 127 



clearest book-descriptions, or even the best and most elab- 

 orate 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 greater 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 gener- 

 ally simple organization, make it a more than usually suit- 

 able object for investigation: besides which, there are the 

 beautiful and interesting points in the details of its struct- 

 ure which I have been endeavoring to bring before you. 



Not 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; which, if you did 

 not know it, you would never think of connecting with 

 the grub from which it has proceeded; so totally different 

 is it in form, in structure, and in motions. 



We shall easily find some in our basin that have passed 

 into this stage. Yes, here is one, which will please to take 

 its place in the glass trough with its younger brothers. 

 How strange the transformation! It reminds us of a lob- 

 ster, though, of course, the resemblance is only cursory. 

 With the naked eye we see that the thorax is greatly en- 

 larged, not only actually, but proportionally; that it forms 

 an oval mass, occupying some five-sixths, at least, of the 

 entire animal; the rest apparently being taken up by a 

 slender, many -jointed abdomen, which curves round' the 



