INSECTS. 471 



number of species the same individual differs so materially 

 at the different periods of life, both in its internal and 

 external conformation, in its habits, locality, and kind of 

 food, that it becomes one of the most interesting investi- 

 gations of the physiologist to ascertain the manner in 

 which these changes are -effected, to trace the successive 



Fig. 232. Tipula, Crane-fly (Female.) 



steps by which that despised and almost unnoticed larva 

 that but a few days before lay grovelling in the earth, 

 with an internal organisation fitted only for the reception 

 and assimilation of the grossest vegetable matter, has 

 had the whole of its external form so completely changed, 

 as now to have become an object of admiration and delight, 

 and able to ' spurn the dull earth/ and wing its way into 

 the open atmosphere, with internal parts adapted only for 

 the reception of the purest and most concentrated aliment, 

 which is now rendered absolutely necessary for its support, 

 and the renovation increased energies demand. 



The heads of insects are fit objects for the microscope ; 

 fig. 243 shows the head of a Gnat, detached from the 

 thorax, magnified about 50 diameters ; the eyes cover 



