278 



INVERTEBEATA 



CHAP. 



forms a deep pouch with wide opening from the bottom, out of which 

 the wing grows up. This condition is found in the more primitive 

 Diptera, such as Culicidae and Tipulidae, which also have long 

 antennae. The Lepidoptera agree with the Chrysornelidae in their 

 mode of wing development. It is obvious that the only difference 

 between the development of a Buprestid beetle and an Orthopteran is 

 that the Orthopteran is active at all stages of larval life till the adult 

 condition is attained, whereas in the Buprestid a quiescent stage, the 

 pupal condition, intervenes between larval and adult stages. 



On the other hand, the development of Muscidae among Diptera, 

 a group to which our house- and blue-bottle flies belong, shows, as 

 has been already mentioned, very complicated conditions, which have 



oc.c 



vn.c. 



FIG. 224. Two diagrams illustrating the metamorphosis of the head of Musca vomitoria. 



(After Van Rees.) 



A, early pupa, in which the head region is still invaginated. B, later pupa, in which the head 

 region is evaginated. db, abdominal region ; cut, cast-off cuticles of previous moults ; ceph, head region ; 

 m.g, mid-gut ; o, mouth ; oc.c, compound eye ; prob, rudiment of proboscis of adult ; storn, stomodaeum ; 

 th, thoracic region ; v.n.c, ventral nerve cord. The crosses mark the boundary between head region 

 and thoracic region. In A the head region is outlined by a very thick line, the thoracic region by a 

 line of medium thickness, the abdominal region by a very fine line. 



been elucidated chiefly by the labours of Kowalevsky (1884) and 

 Van Eees (1889). In the first place the larva is a worm-like maggot 

 or grub, devoid not only of abdominal limbs but of visible thoracic 

 limbs, of eyes, antennae, and jaws. The region which will form the 

 head of the fly is represented by a pouch of ectoderm opening in 

 front (ceph, Figs. 223 A, 224 A), and into this pouch, about half-way 

 back, the mouth or stomodaeum opens. 



Imaginal discs are found on the ventral surfaces of the three 

 thoracic segments. These consist of pouches of ectoderm connected 

 with the surface only by strings of ectoderm cells. On the segments 

 of the abdomen both dorsally and ventrally, there are also imaginal 

 discs, but these are just areas of thickened ectoderm which are not 

 invaginated. Finally on the inner surface of the portion of the head 



