THE EXO-SKELETON OF THE PROBOSCIS. 141 



The sclerites, of the under surface of the labrum, form a 

 complete tube, the prepharyngeal tube (Fig. 28, ./, />/>), by uniting 

 with the base of the ligula. By means of this tube the mouth 

 communicates with the pharyngeal section of the alimentary 

 canal. It curves towards the ventral surface of the proboscis. 



The Pseudolabium. As that part of the haustellum which I 

 have called the theca, together with the oral sucker, is a 

 morphologically distinct part, it will be convenient to adopt a 

 term for its designation. As has been already observed, it is 

 usually regarded as the labium, or lower lip. It forms the floor 

 of the mouth, is connected on its oral surface with the root of 

 the tongue, and terminates in a pair of lobes united behind 

 the oral sucker ; these lobes have been alternately regarded as 

 modified palpi and paraglossse. Although functionally a lower 

 lip, its manner of development and its relation to the rostrum 

 show that it is not a labium, and that it is not developed from 

 the second pair of maxillae, therefore I propose the term 'pseudo- 

 labium.' 



The greater part of the labium of a generalised insect consists 

 of the galeae of the second pair of maxillae. My contention is 

 that, in the flies, the second pair of maxillae are exceedingly 

 rudimentary or entirely absent, and that their place is taken 

 by the first pair, the united galeas of which form the pseudo- 

 labium. 



The Sclerites of the Pseudolabium. I have been unable to find 

 any names for these sclerites, in published works, which are 

 suitable for their designation ; I am therefore obliged to 

 suggest the following new terms : 



I term the large ventral sclerite of the theca (Fig. 30, ^), 

 the ' thyroid' from its form. It articulates at its distal extre- 

 mity with the ' furca,' by which name I distinguish Kraepelin's 

 ' Untere Chitingabel ' (Fig. 28, 5). 



The oral sclerites of the theca are three in number. They 

 lie in the groove which forms the mouth cavity ; a median 

 sclerite, the hypoglossa (Fig. 28, st), and two lateral rods, the 

 paraphyses (PI. VI., pa), one on each side of the hypoglossa. 



The distal ends of the paraphyses articulate with a pair of 



