202 AN INTRODUCTION TO ENTOMOLOGY 



ment follows a course quite similar to that of the other appendages of 

 the body. 



In insects with a complete metamorphosis the genital appendages 

 are represented in the larva? by invaginated histoblasts ; the develop- 

 ing appendages become evaginated in the transformation to the pupa 

 state and assume their definitive form after the last ecdysis. 



III. THE DEVELOPMENT OF THE HEAD IN THE 



MUSCID/E 



In the more generalized Diptera the head of the larva becomes, 

 with more or less change, the head of the adult; the more important 

 of these changes pertain to the perfecting of the organs of sight and the 

 development of the appendages, the antennas and mouth-parts. 



But in the more specialized Diptera there is an anomalous retard- 

 ing of the development of the head, which is so great that the larvae 

 of these insects are commonly referred to as being acephalous. This 

 retarded development of the head has been carefully studied by Weis- 

 man ('64), Van Rees ('88) and Kowalevsky ('87). The accompanying 

 diagrams (Fig. 220) based on those given by the last two authors illus- 

 trate the development of the head in Musca, which will serve as an 

 illustration of this type of development of the head. 



The larvae of Musca 

 are conical (Fig. 219); and 

 the head-region is repre- 

 sented externally only by 

 the minute apical segment 

 Fig. 2 19 Larva oi : the house-fly, Musca of the con i ca l bodv. It 

 domestica (After Hewitt). 



will be shown later that 



this segment is the neck of the insect, the developing head being 

 invaginated within this and the following segments. This invagina- 

 tion of the head takes place during the later embryonic stages. 



In Figure 220 are given diagrams, adapted from Kowalevsky and 

 Van Rees, representing three stages in the development of the head of 

 Musca. Diagram A represents the cephalic end of the body of a 

 larva; and diagram B and C, the corresponding region in a young and 

 in an old pupa respectively; the parts are lettered uniformly in the 

 three diagrams. 



The three thoracic segments (1,2, and 3) can be identified by the 

 rudiments of the legs (/', I' 2 , and F). In the larva (A) the leg-buds 

 are far within the body, the peripodal membrane being connected with 



