41 



upper part of the thoracic segment, whilst a tongue-shaped rudiment 

 becomes developed into the wing. 



It is, however, at the time just previous to pupation, and at the 

 actual time of pupation, that the great development of the embryonic 

 wing takes place. Previous to this the wings have been folded into 

 a lateral cavity ; but at this stage the sheath of the rudimentary wings 

 is drawn back, blood is forced in, and the wings are thus everted 

 from the cavities in which they have hitherto been confined. This 

 seems to be one of the many important changes that take place in 

 the larva during the quiescent period preceding pupation. At the 

 actual change into the pupa, however, the imaginal buds extend 

 rapidly along their edges, and the pupal wing is developed as a closed 

 sac, the future growth of the wing taking place during the pupal stage 

 of the insect. 



Before entering into a somewhat full consideration of the further 

 development of the wing, and the scales with which some wings are 

 covered, it may be well to notice here that the general' principle of 

 wing-development just outlined has been shown to be correct, not 

 only for Lepidoptera and Diptera, but also for the Hymenoptera, 

 Trichoptera, Coleoptera, and Neuroptera. In the ant-lion (Myrme- 

 leon formicarius) Pancritius found no rudiments of the wings in 

 larvae a year old, but they were detected in the second year of larval 

 life, and are said not to differ much histologically or in shape from 

 those of Lepidoptera. In the Coleoptera and Hymenoptera the 

 imaginal buds appear rather late in larval life, yet their structure is 

 like that of those in Lepidoptera. 



During the pupal period the wing becomes much corrugated and 

 folded, and scales are developed on the wing surface. How then, it 

 may be asked, does the small, thick, corrugated, sac-like wing, which 

 in the nymph or pupa (or even at the time of emergence of the 

 imago) consists of a series of closely compressed folds, become the 

 large, thin, fiat, leaf-like structure which we see in the perfect insect? 



In the pupal wing considerable growth and development take 

 place in various ways. Among other structures, the inner surfaces 

 of the upper and lower layers of the wing membrane give rise to a 

 number of vertical prolongations, which finally meet each other in 

 what is termed the ground membrane of the wing. These vertical 

 fibres, then, stretching from one membrane to the other, prevent the 

 two surfaces from separating widely from each other, and also main- 

 tain them at a fixed distance. When, therefore, the hsemolymph or 

 blood is forced between the membranes of the wing by the insect, it 

 does not cause the wing to swell out so as to form a balloon-shaped 

 sac or bag ; but the processes holding the two membranes of the 

 wing closely together cause the blood to be spread equally and the 

 folds to be gradually unrolled from the base outwards by the pressure 

 exerted by the blood. When the wings have attained their full size 

 the hremolymph coagulates, the upper and lower layers are still more 

 closely united, and the nervures are solidly buried between them. 



