THE EXO-SKF.LETON OF THE PROBOSCIS. 149 



the dorsal surface is lengthened by the elongation of the meso- 

 labrum and the palpigerous scales. The oral lobes are also pro- 

 portionately longer than at a later period. 



At a still earlier period of development in the nymph (Fig. 32), 

 the labrum is less displaced in relation to the theca than in the 

 newly-escaped imago. The head is then smaller, and it is 

 apparently the unequal growth of the head which pushes the 

 labrum towards the distal extremity of the proboscis. 



During the evolution of the proboscis, after the escape of the 

 imago from the pupa, the ventral (upper) surface of the proboscis 

 is gradually elongated, and the discophore is converted into a 

 portion of the dorsal surface of the oral sucker, whilst the 



Fig. 32. — A longitudinal median section of the proboscis of a nymph, about the 

 middle of the second week : a, boundary of air space between the hypodermis 

 and the pseudotracheic of the disc ; /', aggregations of embryonic cells ; ,-,, folds 

 of the rostrum ; d, the discophore ; / /;, the pharynx ; in I, tlie mcso- and / /, the 

 prelabrum ; sg, labial gland ; s s, the sheath. 



theca assumes its position above the labrum. At the same 

 time the cylindrical portion of the rostrum becomes shortened 

 and conical, whilst the pro.ximal part is converted into a 

 membranous cone. 



d. The Comparative Anatomy of the Proboscis in the Diptera. 



The form of the proboscis in the immature imago of the 

 Blow-fly affords an easy transition between the most diverse 

 types of the dipterous mouth. The various modifications of 



10 — 2 



