316 THE DEVELOPMENT OF THE NYMPH. 



wing discs (w], and below with those of the leg discs (i k /). 

 This I term the ' thoracic involution.' 



The thoracic discs are still enclosed in the disc sacs, but 

 these open into the cavity of the thoracic involution : those 

 of the wings (w) above and laterally ; those of the upper 

 metathoracic discs, which are not represented in the figure, 

 behind and below the orifices of the wing sacs ; and those of 

 the leg discs in order from before backwards on the ventral 

 surface of the invaginated thoracic wall. 



The anterior stigmatic (upper prothoracic) discs are at this 

 period conical invaginations around the main trunks of the 

 anterior stigmatic tracheae of the larva (c). 



The cephalic involution exhibits a roof which consists 

 entirely of paraderm, and a floor composed chiefly of the 

 cephalic discs. These are arranged in the following manner : 

 In front of the stomodseum are the maxillary and lingual 

 discs (e), with the orifice of the sericterial ducts between 

 them. Above and behind the stomodseum is a tongue-like 

 process (/), from which the pro-, meso- and metalabrum are 

 developed. Behind and laterally are the antennal rudiments (g), 

 and the optic discs of Weismann (//) ; the latter rest upon the 

 hemispheres of the neuroblast (s). 



It will be observed that all the imaginal discs are placed in 



DESCRIPTION OF PLATE XIX. 



A sagittal section of the pronymph on the second day of the pupa, constructed from a 

 series of longitudinal and transverse sections. The section represents the median 

 plane except in front of the neuroblast, .?, t, where it is taken through the 

 principal discs and gives the relation of the parts both in the median plane anil 

 about I mm. to the right of the median plane. The stomodreum is in the 

 median plane, and the disc sacs in the plane of the anterior spiracles. 



a, Cephalic involution of the larval integument ; b, anterior larval spiracles ; 

 c, shed intima of the anterior spiracular trachea of the larva ; J, mouth-armature 

 of the larva ; e, labial disc ; /, labrum ; g, antenna ; 7i, optic disc ; t, anterior 

 leg disc ; k, intermediate leg disc ; /, posterior leg disc ; m, sericterial gland of 

 larva ; n, cavity of the mesenteron of the pronymph, containing the intestine and 

 Malpighian vessels of the larva ; 0, dorsal vessel ; />, parablast surrounding the 

 mesenteron, and connecting it with the rectum and stomodseum ; r, rectum ; 

 s, hemisphere ; /, ventral neuroblast ; u, crop ; w, wing disc. 



The dark continuous outline within the chyle stomach of the larva represents 

 the new epithelium of the mesenteron (compare PI. XVIII., Fig. 2, e). 



