Mar. iy>D.] MeRCER : DEVELOPMENT OF WlNGS IN LePIDOPTERA. 5 



iniaginal hud there is a slight depression beneath the cuticle (Fig. 



As the larva advances in age during this stage (Figs. 2 and 3) the 

 elongation of the cells is increased, the depression enlarged, until 

 llnally the iniaginal bud (Fig. 3, i/)) is quite distinct and a thickening 

 of the cuticle appears opposite a depression in the imaginal bud. 

 This thickening began to show even in the early part of the stage. 

 Near the trachea and the wing-bud there is in each case a group of 

 cells which bear some resemblance to leucocytes and which were 

 termed by A'erson embryonic cells (^cc); these are believed by 

 Schiiffer ('89) to be a sort of lymphatic gland, a center in which 

 leucocytes are develojied. From the study of these figures it is easily 

 seen that the imaginal bud (//') in this stage is simply a thickening 

 of the hypodermis. 



The Sec(;)xd Larval Stage. (Plate I, Fig. 4.) 



In passing from the first stage to this, the growth of the wing-bud 

 has been gradual and not \ery extensive. The cells have elongated 

 .somewhat and the small pit, referred to in the first stage, has in- 

 creased to a definite invagination into which extends a chitinous plug 

 (Fig. 4, /'j from the cuticle. 



The Third Larval Stage. (Plate I, Figs. 5, 6, and 6a.) 



After the second moult the cells of the wing-bud have increased in 

 length, the invaginating process has been going on until the wing-bud 

 is deeply sunken in the body cavity ; it remains, however, connected 

 with the hypodermis by a thin membrane, the peripodal membrane 

 ( Fig. 5, <?). The term peripodal membrane was proposed by Van Rees 

 ('88) and was suggested by the fact that in the internal development 

 of the legs of flies a similar membrane is formed ; as the leg-buds and 

 the wing-buds closely agree in structure the terms applied to parts of 

 the leg-buds are also applied to the wing-buds. Figure 6 represents 

 a section at a point which shows the peripodal membrane {e) on both 

 sides. The trachea has increased in size. 'Jlie plug of chitin at the 

 opening of the invagination (Fig. 5, /' ) is readily noticed. To get a 

 clear idea of the invagination it is necessary to study a series of sec- 

 tions through it. A section through the center of the invagination 

 will show the chitinous plug (Fig. 5, /') and the opening into the 



