802 MR. J. a. 11. FUKW ON Til a LARVAL 



side three stout nervous tracts one supplying eacli leg disc. 

 Each imaginal leg sac is pear-shaped with its broad posterior 

 end receiving the nerve from the ganglion, its narrow anterior 

 end being produced forwards into the peduncle of the sac. 



The prothoracic leg sacs and their contained imaginal discs lie 

 in the median region of the third thoracic segment. In all 

 specimens examined the peripodal cavities of the sacs communi- 

 cated with each other, but this union would probably not be 

 found if sufficiently young specimens were examined ; it is quite 

 certain that the prothoracic leg discs leach a given stage of 

 development sooner than do the meso- and metathoracic leg 

 discs. This point is refen-ed to again below. TJie peduncle of 

 each sac passes forwards close to the ventral body-^v•a]l to end in 

 the ventral hypoderm of the prothoracic segment slightly to one 

 side of the median line. At the point where the peduncle 

 reaches the hypoderm th^ chitin of the body-wall is traversed by 

 a minute fissure elongated transversely to the longitudinal axis of 

 the body. The fissure in the chitin is, of course, due to the dis- 

 continuity of the hypoderm over a small area brought about by 

 its invagination to form the peduncle of the sac. The fissure 

 passes obli(]uel3^ through the chitin, so that in a longitudinal 

 section through the peduncle and fissure the two form a practi- 

 cally continuous line. I am unable to offer any exphmation of 

 the obliquity of the fissure. Similar fissures occur opposite the 

 peduncles of the other leg discs and opposite those of the wing 

 .and haltere discs. 



One specimen which, when killed, was approaching its time of 

 pupation, showed the method of evagination of the prothoiacic 

 legs. The united imaginal sacs had grown forwards between 

 the peduncles of the sacs so that the inner two-thirds of the 

 peduncles were united by two closely approximated horizontal 

 hypodernial sheets, a doi-sal and a ventral, the space between the 

 two sheets being continuous posteriorly Avith the cavity of the 

 united imaginal sacs. The central cavities of the inner two- 

 thirds of the peduncles had not remained separate, but were 

 probably represented by the extreme lateral regions of the space 

 between the dorsal and ventral hypodermal sheets. There can 

 be little doubt that eventually the whole length of the peduncles 

 becomes united by the dorsal and ventral hypodermal sheets; 

 thus, eventually, there is only one imaginal sac with a single 

 broad, dorso-ventrally flattened, peduncle, and containing the two 

 prothoracic leg discs. Probably, when evagination is to take place, 

 the dorsal and ventral walls of this flattened peduncle sej)arate, 

 and the sac evaginates through the space between tliem. In the 

 .specimen from which the above particulars weie obtained the 

 meso- and metathoracic imaginal leg sacs showed no sign of union, 

 though they almost certainly undergo the .same process at a later 

 stage. In all larva? examined the degree of development was 

 greatest for the prothoracic leg discs, less for the raesothoracic, 

 -and least for the metathoracic. , 



