THE DEVELOPMENT OF THE NYMPH. 



321 



a remarkable palisade-like structure (PI. XVIII., Fig. 8), 

 which Van Rees has mistaken for the developing discs. The 

 large cells of the paraderm give off processes and form new 

 cells beneath them, which enclose a network of blood sinuses. 

 These reticular septa are well developed between the abdominal 

 segments when the discs are quite small, and they are not 

 replaced by permanent structures until the development of 

 the nymph is far advanced. 



The Proboscis is developed from two median processes 

 (PI. XIX., e and/). The upper and larger of these, /, repre- 

 sents a layer of imaginal cells, which, in the larva, lie within the 

 pharyngeal sinus (p. 44), and which is withdrawn from the 

 sinus by the shedding of the cephalo-pharyngeal sclerite. It 



FIG. 45. A median section of the pupa on the fifth day, showing the proboscis, 

 neuroblast, dorsal muscles, archenteron, and dorsal vessel of the young nymph. 



becomes the pro-, meso- and metalabrum. The lower and 

 smaller process, c, is formed by the coalescence of the four 

 appendicular discs of the head (p. 82), and it is from this 

 that the rostrum, pseudolabium and ligula are developed. 



The maxillary discs (p. 82, Figs. 8, 2, 1 : 13, mx, and 

 PI. XV.. Fig. i, z 3 ), which are first situated on the inner surface 

 of the stomal disc of the larva, increase rapidly in size during 

 the first few hours of the pupa stage, when they lie one on 

 either side of the cephalo-pharynx. After the latter is shed, 

 the discs unite with each other and with two small groups of 

 imaginal cells, one on either side of the orifice of the united 

 sericterial ducts, to form the lower median process, e 

 (PI. XIX.). 



