THE DEVELOPMENT OF THE NYMPH. 319 



the underlying paraderm is removed by phagocytes, which 

 exist in great numbers near the edge of the growing disc. 



During this extension, however, the disc also increases in 

 size by the growth and multiplication of all its cells, whilst the 

 paraderm undergoes a corresponding contraction in its whole 

 extent. In this way the edges of the discs are ultimately united 

 with each other, first in the ventral region, and later, on the 

 dorsal aspect of the nymph. 



The Tipper Prothoracic Discs (PL XIX., c and Fig. u) are 

 seen as pouch-like involutions of the anterior edge of the 

 cephalic involution, surrounding the shed intima of the pro- 

 thoracic spiracular tracheal trunk of the larva. 



These discs subsequently undergo evolution, i.e., their inner 

 surface, that next the shed intima of the tracheae, becomes 

 external, and they thus become the horn-like stigmata of the 

 pronymph (stigmatic cornua). The rudimentary prothoracic 

 tergum (dorsum of the prothorax), which is far larger in the 

 pronymph than it is subsequently, is also developed by the 

 extension of the edges of the anterior stigmatic discs (PL XX., 

 Fig. 4). 



The Stigmatic Cornua of the pronymph (PL XX., Fig. 3, s c) 

 have a simple trumpet-like orifice, rapidly become highly 

 chitinized, and acquire a yellow colour. They are subsequently 

 shed, and are replaced by intersegmental spiracles developed 

 between the pro- and mesothorax. These are the anterior 

 spiracles of the nymph (PL XX., Fig. 3, s). 



The Intersegmental Anterior Spiracles of the nymph resemble 

 the anterior spiracles of the larva in having digitate extremities, 

 but differ in possessing only four or five digitations. Weis- 

 mann confounded them with the stigmatic cornua, and it is 

 only recently that I discovered that the two are distinct and 

 exist simultaneously. 



The coexistence of two spiracles, one on, and the other 

 behind, the prothorax, is of extreme interest, setting at rest 

 questions as to the nature of the imaginal spiracles and the 

 morphology of the thorax which have already been discussed, 

 and bearing out the views I have expressed on the subject. 



