154 GLIMPSES OF NATURE. 



As a maggot it is active, while as a chrysalis it is 

 quiescent this, in short, is the characteristic feature 

 of the perfect development. 



There is, however, much more in fly-development 

 than meets the eye of the popular naturalist. Ordi- 

 narily, or at least in many insects, the organs and parts 

 of the grub are gradually transformed into those of the 

 adult. Long ago, Newport taught us how the cater- 

 pillar's long and diffused nerve-chain grew into the 

 much more concentrated nervous system of the butter- 

 fly. The flies, however, seem to illustrate a metamor- 

 phosis which is much more complete and sweeping 

 than that just indicated. When the maggot, white 

 and legless, leaves the egg of the mother-fly, there are 

 found within its body certain curious masses, arranged 

 with tolerable regularity, and called imagined discs. So 

 long as the maggot stage continues, these discs remain 

 in statu quo. The grub eats voraciously, changes its 

 skin to accommodate the increasing growth of its body, 

 and finally, after thus laying up a store of material in 

 which development is to work its own sweet will, it 

 becomes the chrysalis. 



Then these discs come to the front of things. They 

 enlarge, and out of their substance are formed legs, 

 wings, head, proboscis, and, in short, the belongings 

 of the adult fly. The organs which served the maggot 

 disappear and dissolve away, and only the tail of the 

 grub seems to take part in the formation of that of the 

 full-grown insect. This study makes it clear once again 

 that the fly is an insect with a history. It has come 

 to be the fly of to-day, as the result of a very ancient 

 process of evolution ; and although this fact may not 

 reconcile us to its worrying habits, it may perchance 

 invest our enemy with a new interest in our eyes. 



