DISTINGUISHING CHARACTERS OF INSECTS. 2S 



fly-grubs, and the like, are but young and imperfect 

 insects, bearing indeed something of the same relation 

 to those full grown as does the tadpole to the frog. 



They are no arbitrary characters which thus separate 

 insects from nearly allied races. Thus, the spider, ex- 

 cluded from the list of insects by its eight legs and its 

 head and thorax being in one mass instead of two, is also 

 separated from them by far more important internal cha- 

 racters; as, for instance, the possession of true lungs. 

 Thus also the many-legged woodlouse, shrimp, &c., with 

 the spider, are more widely separated from the six-legged 

 insect by the absence of metamorphosis, than by any 

 difference in the number of their limbs. 



These non-changing animals attain the perfect state 

 merely by increasing in size and in the perfection, some- 

 times also in the number, of their parts. From time to 

 time they cast their skin, as it becomes too small to 

 contain them, but they undergo no essential change of 

 form or character after their exclusion from the egg. 



Insects, on the contrary, as has been said, undergo a 

 series of " metamorphoses" or changes, more or less 

 complete, before arriving at the perfect or winged state. 

 It is true that these metamorphoses are but develop- 

 ments, and that the chrysalis, for instance, is not changed 

 into a butterfly, but that it is itself a butterfly in a husk ; 

 still the difference between the perfect and the imperfect 

 insect is, as a rule, so great, and the stages of develop- 

 ment are so marked, that the word metamorphosis may 

 fairly be applied. 



