THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 259 



situated in the front part of the maggot's body. No change is 

 perceptible in these discs during the caterpillar or larval stage of 

 the fly's life ; but when the maggot encloses itself within the last of 

 its skins, which serves it as a chrysalis case or cocoon, the discs 

 begin to undergo a marked development. Each of the lower discs 

 placed in the insect's chest, develops a leg and half of the segment 

 of the body bearing the leg. The upper discs of the joint 

 give origin to the upper halves of the segment and to the wings 

 or their representatives ; and the two foremost discs are responsible 

 for the development of the head and mouth parts of the perfect fly. 

 As development proceeds, 

 we find a complete physio- 

 logical break-down of the 

 chest and head organs of 

 the maggot to be repre- 

 sented. A literally new 

 creature (as to chest and 

 head) is produced and built 

 up from the imaginal discs ; 

 the tail or abdomen of the 

 fly consisting, however, of a 

 mere extension and growth 

 of that of the maggot. 

 There is thus witnessed in 

 the development of the fly 

 a much more complete 

 series of changes than in 

 th,e metamorphosis of a butterfly, and this notwithstanding the 

 fact that the changes undergone by the latter appear to be of 

 more sweeping character than those exhibited by the former 

 insect. Let us bear in mind also the fact, already noted, that the 

 developmental changes in insects may be reduced to a minimum, in 

 respect that many lower insects do not undergo any metamorphosis at 

 all. Even in the cockroach (Fig. 173) belonging to the Orthoptera, 

 or grasshopper and locust group an insect familiar enough to 

 warrant its special mention in the present instance, the young possess 

 eyes, feelers, jaws, and legs before they are hatched. They further leave 

 the egg as minute but active insects, whose organs are really moulded 

 upon the type of those which the perfect cockroach possesses. The 

 young insect then undergoes seven moultings or changes of skin. Its 

 first moult occurs when it leaves the egg, and its second takes place 

 about a month afterwards. This second moult being performed, no 

 further shedding of skin takes place until a year afterwards ; and as 

 but an annual moult subsequently occurs, the insect does not attain 

 maturity till its fifth summer. Thus, in the cockroach, " metamor- 



s 2 



FIG. 173. COCKROACHES. 



(The left-hand figure represents the male ; the female, 

 with rudimentary wings, is represented on the right.) 



