798 ME. J. G. H. FREW ON THE LARVAL 



gut. From their point of junction the posterior tubes pass bade- 

 wards in the dorsal region of the body, having a somewhat wavy 

 course but no convolutions or sharp bends. The anterior tubes 

 are almost twice the length of the posterior ones ; from their 

 point of junction they pass forwards in the dorsal region of the 

 body to about the level of the proventriculus. They then turn, 

 abruptly posteriorly. The distal limb of each tube lies lateral to- 

 the proximal one, and has a somewhat greatel' diameter. The 

 distal limbs extend backwards to a point slightly posterior to the 

 point of origin of the malpighian tubes from the gut. 



The proximal limbs of the anterior tubes are transparent and' 

 appear dark by reflected light ; their lumen is very narrow. 

 The distal limbs are opaque, glistening white by reflected light> 

 and have a relatively wide lumen which contains a frothy mass. 

 At present no more precise details can be given concerning this 

 diflerence in structure, and presumably function, of the two- 

 regions of the antei'ior malpighian tubes, but it is hoped shortly 

 to investigate the matter more fully. All that can be said at 

 present is that the frothy mass in the distal limbs of the anterior 

 tubes does not appear to consist of CaCOn which does occur in the 

 malpighian tubes of certain Cyclorrhaphous larva; (11). The 

 posterior tubes resemble in structure the proximal limb." of the 

 anterior tubes. The lumen of the malpighian tubes is in all places- 

 intracellular. 



Central Nervotis System — Cephalic Imaginal Discs and 

 related jjarts (text-fig. 11). 



The cerebral ganglia are rounded lobes lying i]i the first ab-. 

 dominal segment. They are separated from each other by a 

 longitiidinal doi-sal fissure, except in their antero- ventral regicms, 

 where they are united by a short, and moderately thick, trans- 

 verse commissure. Each is united by a, broad a!Soi)hageal 

 commissure to the ganglionic mass lying below tiie (esophagus.. 

 This ganglionic mass (called hereafter the subcesophageal ganglion) 

 is a more or less cone-shaped body ; it lies mainly in the first 

 abdominal segment, but its posterior end extends a short distance 

 into tlie second abdominal segment. It represents the fusion 

 of the subujsophageal, thoracic, and abdominal ganglia of less 

 specialised insects. 



The a'sophagus passes backwards from the pharynx along the 

 dorsal surface of the subQ3Rophageal ganglion, l^or pait of its 

 comse it lies at the base of the fissure separating the dorsal lobes 

 of the brain, and the transverse commissure {t.c.) uniting the 

 two lobes lies innnediately dorsal to it. The dorsal vessel (d.v.) 

 dips down from the dorsal body- wall and jiassos through the 

 dorsal region of the fissure separating the dorsal lobes of the 

 brain. Between the posterior ends of the dorsal lobes it is 

 suTTOunded liy Weismann's Ring (w.r.).i\n oblique cellular collar, 

 of which the dorsal part is anteiior, the ventral part posterior. 

 The ring is relatively thick at the .sides but less so dorsally, where 



