323 THE DEVELOPMENT OE THE NYMPH. 



The small groups of imaginal cells at the orifice of the seric- 

 terial ducts represent the labium of the larva in which they are 

 developed, and they form the ligula of the imago (p 147) ; 

 hence, as has been already stated, I regard the pseudolabium, 

 which is developed from paired maxillary discs, corresponding 

 with the maxillae of the embryo, as representing the galeae of 

 the maxillae, and not the true labium, as is usually held. 



On the fourth day of the pupa the mouth organs closely 

 resemble those of an hemipterous insect (Fig. 45). The upper 

 and lower processes lie upon the pectus and form a labrum and 

 haustellum, between the basal portions of which the fulcrum is 

 subsequently developed. The oral lobes do not appear at this 

 stage, but are subsequently developed from the sides and apex 

 of the haustellum, and by the seventh or eighth day the organ 

 has assumed its definitive form (PI. XXI.). 



Weismann's account [2] of the manner in which the proboscis 

 is developed is exceedingly indefinite. He says : ' Its several 

 parts originate in a manner which is totally unlike that in 

 which the mouth organs of the larva are formed ;' and, after 

 describing the origin of the latter from paired rudiments 

 (lateral appendages), continues: 'The parts of the proboscis 

 first appear in the pupa as structures similar to those which 

 are ultimately developed from them ; thus the underlip does 

 not originate from paired rudiments, like the under-lip of the 

 larva, but as a hollow, grooved process.' This is undoubtedly 

 my lower process c, the origin of which I have traced to the 

 coalescence of the paired maxillary rudiments. Weismann, 

 moreover, admitted that he was unable to observe the earlier 

 stages of the development of the proboscis. 



Menzbier [50], on purely theoretical grounds, concluded that 

 the head is developed from six pairs of discs, and he supposed 

 that three pairs unite to form the proboscis ; Kunckel d'Hercu- 

 lais, as has been already mentioned, described the maxillary 

 discs in Volucella, and these are the only facts which have 

 been previously recorded as to the manner in which the pro- 

 boscis is developed in the Diptera. 



The Antennae are developed as hollow processes of the great 



