800 MR. J. G. n. PllEW ON THE LARVAL 



ceplialic imaginul sacs wliich contain the inifigiiial discs anil a 

 well-marked though narrow cavit3\ The cephalo-pharyngeal 

 hand extends oiitwiirds under the median region of their \ entral 

 surface, and its lateral margins are then produced downwards to 

 form a lateral flap {l.f.) on each side. Slightly more posteriorly 

 the imaginal sacs separate completely from the ceplialo-pluuyngeal 

 band, tlieir posterior ends being attacheil to the dorsal lobes of 

 tlie brain by nervous tracts. 



The lateral flaps are not deep anteriorly, but as they pass pos- 

 teriorly their depth increases so that they hang down close .ngainst 

 the sides of the oesophagus, their ventral edges lying slightly 

 below the level of the ventral surface of the a\sophagus, and 

 being attached to two small longitudinal trache;e uhicli will be 

 mentioned later. There is thus a well-defined sinus whose floor 

 is formed by the cc.sophagus, and whose roof and sides are formed 

 by the ceplialo-pharyngeal band and its lateral ilaps respective!)'. 

 At about the anterior limit of the brain the dorsal vessel jiasses 

 into this supracesophageal sinus, its dorsal and lateral walls fusing 

 with the cephalo-pharyngeal band. Tlie vessel ends by opening 

 into the sinus at the point marked Y in text-fig. 11. Slightly 

 posterior to the opening of the ve.ssel into the sinus the dorsal 

 regions of the lateral flaps approach one another below the vessel 

 and fuse, forming a vertical median plate (m.pl.). The vertical 

 plate is short in the antero-posterior direction, and posterior to 

 its termination the ventral jiartsof the flaps form an aiched mem- 

 brane over the tesoi^hagus. As it passes backwards this membrane 

 becomes less and less extensive and more and more closely 

 applied to the ojsophagus, and where the latter pa.'-ses beneath 

 the transverse commissure its doi'.sal and lateral regions are closely 

 sui-rounded by the ijiembrane, the ventral edges of which are 

 united to the membrane covering the dor.sal surface of the sub- 

 ittsophageal ganglion and to the two small longitudinal tracheni 

 already mentione<l. The ventral parts of the latei-al Haps do not 

 extend posteriorly beyond the transverse commi.'-sure. 



The two small longitudinal trachea; mentioned above arise in 

 the head region and pass backwards, one on each side of the 

 oesophagus, until they come into relation with the ventral edges 

 of the lateral fla|)S along which they paf-s to the dorsal surface of 

 the svibcesophageal ganglion. 



As the larva approaches maturity the cephalic imaginal sacs 

 and their contained discs increase enormously in size. The two 

 sacs extend upwards above the cephalo-pharyngeal band until 

 they are in contact along the median liiie; probably in a very 

 late stage the two sacs communicate with each other as is the 

 case in Melophat/us ovhms (16), but this stage h.-is nor- been 

 ob.served. Each sac when almost fully developed consists of an 

 antero-ventral i-cgion containing the antennal disc and a postero- 

 tlorsal legion which contains the eye rudiment and also tlie rudi- 

 ments of certain regions of the head capsule. The eye rudiment 

 is innervated from the posterior region of the corresponding 



