114 THE DEVELOPMENT OF WINGS 



As detailed discussions of the tracheation of the wings of nymphs are 

 given in several of the following chapters, it is unnecessary to dwell further 

 on this subject here. 



(5) THE DEVELOPMENT OF THE WINGS OF LARV^ AND OF PUP^ 



Although there are differences in details in the development of the 

 wings in different insects undergoing a complete metamorphosis, the 

 essential features are the same in all. The most striking feature is that the 

 rudiments of the wings, the wing-buds, arise within the body and become 

 exposed for the first time when the last larval skin is shed. This internal 

 development of the wings presents many remarkable phenomena that have 

 attracted the attention of many observers, among whom were Weissmann 

 ('64 and "66), Landois ('71), Ganin ('76), Dewitz ('81), Pancritius ('84), 

 Schaffer ('89), Mayer ('9'6), Gonin ('92), and Mercer ('00). 



Lack of space prevents the giving in this place of an historical account 

 of the growth of knowledge of this subject, for this the writers named above 

 can be consulted. The following account consists largely of a tracing of 

 the development of the wings of a single lepidopterous insect, the cabbage 

 butterfly {Pier is rapes), which is taken as an example. This account is 

 based primarily on the observations of Gonin and the later studies of 

 Mercer, which were made under my direction, to these I have added the 

 results of some of my own studies. 



The development of the wing-buds. — The first indication of a wing-bud 

 is a thickening of the hypodermis; this can be observed by making sections 

 of the body-wall of the wing bearing segments, of very young larvae or even 

 of embryos. In the first larval staditim of Pieris, this histoblast is well 

 marked (Fig. 115, a). In the second stadium, it becomes more prominent, 

 and is invaginated, forming a pocket-like structure (Fig. 115, ^). In the 

 third stadium a part of this invagination becomes thickened and evaginated 

 into the pocket formed by the thinner portions of the invagination (Fig. 

 115, c). In the fourth stadium, the evaginated part of the histoblast 

 becomes greatly extended (Fig. 115, d). It is this evaginated portion of 



trachese in the absence of taenidia; they rarely branch, but often anastonase, which 

 give them a branched appearance (Plate II and Plate III, Fig. 2). 



Each tracheole is of unicellular origin, and is at first intracellular in position; while 

 tracheae are of multicellular origin and the lumen of each is intercellular in position. 

 The development of trachcoles, each coiled within a single cell of the epithelium of a 

 trachea, and the subsequent opening of communication between the trachcoles and the 

 lumen of the trachea, and the uncoiling and stretching out of the trachcoles, so that 

 they reach all parts of the wing, are described later in the account of the development 

 of the wings of Pieris 



It is probable that the trachcoles are the essential organs of respiration, and that 

 the trachea; act merely as conduits, conveying the air from the spiracles to the trachcoles. 

 But a discussion of this question does not fall within the scope of this essay. 1 wish 

 merely to emphasize the importance of distinguishing clearly between trachea; and 

 trachcoles in descriptions of the tracheation of the wings. 



