378 INSECTA. 



more than the external surface of the permanent head, and the most 

 important parts of the latter (antennae, eyes, rudiments of the 

 proboscis) can already he recognised on them. It only remains for 

 the cephalic vesicle to he evaginated through the aperture of the 

 pharynx ( + to + ), so as to produce the completed head of the 

 pupa. By this evagination of the parts formerly invaginated, that 

 which was before the aperture of the pharynx becomes the neck- 

 region (Fig. 186 B, + to + ), which now connects the head with 

 the thorax (Van Rees). The protrusion of the " cephalic vesicle," 

 which was directly observed by Weismann, seems to be caused by 

 the increase of pressure from within, brought about by the con- 

 traction of the posterior parts of the body. In accordance with the 

 conformation of the imaginal head thus produced, the anterior end 

 of the oesophagus becomes ventrally flexed. 



We have pointed out above that the so-called pharynx is nothing more 

 than an invaginated portion of the external surface of the larval head. The 

 " brain-appendages " must be regarded as diverticula of this invagination, in 

 which the separate parts of the head appear as rudiments in an invaginated 

 condition. They may thus be compared throughout with the rudiments of the 

 thoracic limbs. All these "imaginal discs," according to their origin, have 

 to be derived from invaginated portions of the external surface of the body. 

 It is difficult to reconcile with this hypothesis the accounts of Graber (No. 

 28), who in a later embryonic stage of Galliphora [Musca of most authors) 

 observed the rudiments of the imaginal discs lying as simple epithelial plates 

 in the interior of the body. Since Graber left uninvestigated the preceding 

 and the subsequent ontogenetic stages, we can only record his statement, and 

 must leave the problem to be solved by future researches. 



B. Development of the Internal Organs of the Imago. 



It has already been mentioned that most of the organs of the 



larvae of the Muscidae (and of most Diptera, Lepidoptera, Coleoptera, 



a and Hymenoptera) undergo 



disintegration through the 

 action of the blood-corpuscles 

 (leucocytes), and that their 

 reconstruction proceeds from 

 certain embryonic cell-groups, 

 the imaginal discs. Disin- 

 tegration and reconstruction 

 take place during the pupal 

 stage so gradually that in 



Fig. 187.— Diagram illustrating the formation of """a*-- & j 



the imaginal hypodermis in the abdomen of the many cases the continuity 



Muscidue (from Lang's Text-book), hi, imaginal . . . ,. , , , 



discs of the hypodermis ; Ui, larval hypodermis. 01 the Organ IS not disturbed 



