452 MISS A. M. EVANS ON THE STRUCTURE AND OCCURRENCE 



111 the case of CMronomus, however, we have the statement by Miall and 

 Hammond (33. p. 29) that "The salivary ducts pass forwards to open ahove 

 the nientum and behind a minute projection in the floor of the month 

 (lingua)." When viewed from the dorsal surface the lingua (hypopharynx) 

 of CMronomus sp. is seen to consist of a liroad plate bearing at each side a 

 wing-like projection which is inclined at right angles to the floor of the mouth. 

 I have compared this condition with that found in two species of Bibio — 

 B. jyomonw and B. Joliannis. Specimens of the latter species were kindly 

 given me by Mr. H. M. Morris, M.Sc, and my description refers to 

 B. pomonw. The only differences discovered between this species and 

 B. Joliannis with regard to the hypopharynx were those of size. 



Morris (34 a. p. 96) describes the mouth-parts in his paper on the larval 

 and pupal stages of this Nemoceran. I found that the salivary duct ran 

 immediately dorsal to the thick bidentate structure, which is considered to lie 

 a portion of the labium. I therefore conclude that it represents the whole 

 labium. In lateral view it appears not as a simple plate, but of composite 

 nature. Above the end of the salivary duct is the membranous anterior 

 portion of the mouth floor. This has been described as the upper plate of 

 the labium. Its position with regard to the labial duct appears to me to lie 

 conclusive evidence of the fact that it corresponds to the lingua or hypo- 

 pharynx of Chironomus. Laterally it bears a pair of forwardly projecting 

 pointed processes (text-fig. 17, m.) - 2 mm. in length .These are so disposed that 

 together with the hypopharynx they form a membranous crescentic structure. 

 The exterior surface is supported by the arm of a chitinous sclerite s. The 

 ventral portion (v.) of this sclerite is associated by means of a chitinous 

 conuective with the lateral angles of the labium. 



The form of the hypopharynx and its lateral projections recalls in some 

 features that of Ilibius, one of the less specialised Dytiscids, which belong to 

 the most primitive suborder of Coleoptera. Moreover, Morris (1917, p. 104) 

 gives reasons for'regarding the larva of Bibio as primitive amongst Diptera. 

 Thus, if niaxillulse exist in Dipterous larva?, we should expect to find them 

 present in such a form as this. The suggestion is therefore put forward that 

 further investigation may demonstrate the maxillular nature of the processes 

 borne by the hypopharynx in the larva of Bibio, and of the similarly located 

 projections in Chironomus. 



Summary of Conclusions. 



Briefly, then, the presence of maxillula? as a pair of appendages more or 

 less closely associated with the hypopharynx is a characteristic feature of 

 Apterygota. They are in the least modified condition in Machilis. I have 

 emphasised the great degree of reduction which they present in Lepisma. 

 They are well developed in Forficula though partially fused with the 



