330 THE DEVELOPMENT OE THE NYMPH. 



dorsal disc on each side much smaller than the others, but I 

 have found no trace of it. It cannot be doubtful that the two 

 pairs in each segment correspond with the dorsal and ventral 

 discs of the thoracic segments, from which they only differ in 

 having no appendages and in their smaller size. 



The union of the abdominal discs occurs from before back- 

 wards, and in the ventral median line, long before they unite 

 in the dorsal region, so that even on the sixth day of the pupa 

 the dorsal vessel can be seen pulsating beneath the transparent 

 parablast, which still covers a considerable portion of the dorsal 

 surface of the abdomen. 



e. The Pupa Sheath. 



On the fifth day the nymph may be regarded as completely 

 formed. The wings, legs, segmentation of the body, and for- 

 mation of the head, thorax and abdomen as distinct cavities, 

 foreshadow the form of the imago, and resemble those of a 

 lepidopterous nymph. But these parts exhibit, as it were, a 

 rude outline of the perfect insect ; the joints are as yet only 

 indicated by furrows, none of the seta; are developed, and the 

 embryonic epiblast of the imaginal discs has only partially 

 replaced the paraderm in the dorsal region. The discs from 

 which the abdomen is developed have not as yet coalesced with 

 each other. 



Amongst the small epithelial cells of the discs other and 

 larger cells have made their appearance ; these are the tricho- 

 genic cells from which the setae are afterwards developed. 



At this stage the whole surface of the disc epithelium is seen 

 to be covered by a thm chitinous cuticular layer, which during 

 the next day or two separates from the underlying epithelium, 

 and forms a loose sheath, which is not cast off until the fly 

 emerges from the pupa-case. 



The formation of the nymph is, therefore, accompanied by 

 two virtual ecdyses, that of the larval cuticle and that of the 

 pupa-sheath, after which the cuticle of the imago is developed. 

 Thus, as Weismann observed, ' we find three cuticular skins. 



