THE DEVELOPMENT OF THE NYMPH. 331 



one over the other, in the ripe pupa — the pupa-case, the pupa- 

 sheath, and the epidermis of the imafjo.' The pupa-sheath is 

 not, however, separated at once, but remains attached to the 

 nymph at the inflections between the segments, and more 

 especially between the head and thorax and the thorax and 

 abdomen ; it becomes greatly thickened in these folds before 

 its final separation (see PI. XXI.). 



Weismann first described the manner in which the pupa- 

 sheath is formed. Reaumur was aware of its existence, but he 

 thought that the rudimentary appendages are enclosed in it on 

 their first appearance. Weismann correctly stated that when 

 they first appear on the si'.rface they are not covered by any 

 sheath. 



f. Changes in the Alimentary Canal. 



The changes which the alimentary canal undergoes in the 

 first two days of the nymph stage (part of the third, fourth, 

 and fifth days of the pupa) are of so extraordinary a character 

 that I am greatly surprised to find that they had never been 

 observed by anyone previously to my having undertaken their 

 investigation. 



Kowalevski correctly described the formation of a new 

 epithelial layer in the chyle stomach by the union of the 

 imaginal rudiments discovered by Ganin, but his investigations 

 cease with the beginning of the third day of the pupa state. 

 He gives a description of a section in which a new epithelial 

 layer surrounds the so-called corpus luteum [145, Fig. 21], 

 which he regards as the shed epithelium of the larval chyle 

 stomach ; but he fails to show what becomes of the remainder 

 of the larval intestine, or in what manner the great saccular 

 mesenteron of the nymph is formed. 



There are 34 mm. of hind-gut, metenteron, in the larva, and 

 30 of these do not reappear in the imago, in which the meten- 

 teron only measures about 4 mm. Yet no one seems to have 

 considered the impossibility of the total disappearance, not 

 only of 30 mm. of intestine, but also of the great Malpighian 

 tubes of the larva, without leaving a trace of their ever having 



