II 



FLIES WITH AQUATIC LARVAE 



135 



covering. 



Soon the new cuticle expands, and in the 

 course of a few hours, or it may be in a few days, 

 becomes as hard as that which has been cast. This 

 periodical operation of moulting gives opportunity for 

 gradual changes of shape. Some Insects, without 

 ever ceasing to run about and procure their food, 

 develop new organs, such as wings, by modification of 

 the shape of the cuticle at certain places. In the 

 later stages of a Cockroach, for 

 example, the edges of two of the 

 thoracic segments are found to 

 project backwards at the sides of 

 the body. Within these small pro- 

 jections wings are formed. They 

 are necessarily crumpled up to 

 allow of their being stowed away 

 into small spaces, and it is only at 

 the next moult that they become 

 expanded. By the same process 

 the long legs of a winged Insect 

 may be developed beneath the un- 

 broken surface of a footless larva. 

 There is a deep inward telescoping of the epidermis, 

 and from the bottom of the fold the limb begins 

 to grow outwards. (See Fig. 42.) 



In a Chironomus larva the head, legs, and wings of 

 the fly are formed by such a process of infolding. 

 The operation occupies a considerable time, and 

 begins soon after the last larval moult, some weeks 

 before pupation sets in. The epidermis is retracted 

 from the larval cuticle, and first secretes a thin, new 

 cuticle, which will form the proper investment of 



FIG. 40. Larva of Chiro- 

 nomus. One of the 

 hind legs, showing its 

 crown of hooks, the 

 retractor muscles, and 

 the formation of a new 

 crown of hooks, prepar- 

 atory to change of skin. 



