96 IA T TERNAL ORGANS. 



of fat in the same vicinity, and, like them, are 

 permeated by the minuter extremities of tracheal 

 vessels ; their point of attachment is just above 

 and slightly in advance of the spot where the spi- 

 racle should be ; they are fed by delicate nerves 

 and by tracheal vessels, from the latter of which 

 the permeating threads arise ; but they do not 

 originate, as Landois asserts, from the longitudi- 

 nal tracheae themselves, but from the hypoderm or 

 inner integument of the creature, as Lyonet 

 described it more than a century ago, Herold sixty 

 years since, and Dewitz, Ganin, Graber, and 

 others have latterly clearly shown. These pads, 

 which really originate in the earliest stage of the 

 larva, but are not then readily detected, are com- 

 pressed folds of the hypoderm, which do not pro- 

 trude externally like the other appendages of the 

 body, because they form infolded pockets beneath 

 the outer chitinous covering of the caterpillar ; 

 they resemble the half reversed finger of a glove, 

 so withdrawn that the tip of the finger is on a level 

 with the base. Shortly before pupation, the tip 

 begins to push its way outward beneath the chi- 

 tinous skin, a condition indicated by the swollen 

 sides of the caterpillar at that point, so as to be- 

 come actually rather than potentially external 

 appendages of the hypoderm ; and when the 

 larval skin is removed by pupation, they are for 

 the first time exposed to the eye. 



