DEVELOPMENT OF MOUTH PARTS 



47 



adult insects of incomplete metamorphosis. There remains to determine the 

 relations of the larval mouth parts of Simulium with its very different imaginal 

 mouth parts. 



" In selecting flies for the study of the postembryonic development of the 

 mouth parts I have chosen two which in the imaginal condition possess all the 

 parts possessed by any fiy, and these parts in as generalized condition as is to be 

 found in the order, and which also possess in the larval stage a similarly full 

 complement of mouth parts. Such larvge as those of the Muscidge, with their 

 problematical hooks and lack of other parts, and such imagines as the muscid 

 flies, with no parts left except proboscis and maxillary palpi, are impossible for 

 the determination of the relation between larval and imaginal parts. From the 

 mouth parts of the imaginal Simulium and of other nematocerous forms it is 



7nd. 

 md 



umx. 



Fig. 1. — Sagittal section tiirough head of old larva of Simulium sp., showing 

 forming imaginal head parts within. 



I.e., larval head wall ; i.d., imaginal derm ; l.a., larval antenna ; i.a., Imaginal 

 antenna ; i.e., imaginal eye ; l.md., larval mandible ; i.md., imaginal mandible ; l.mx., 

 larval maxilla ; i.mx., imaginal maxilla ; l.li., larval labium ; i.U., imaginal labium. 



not difficult to trace the evolution to the specialized muscid conditions, and if 

 the mouth parts of Simulium and similarly equipped flies can be interpreted, 

 the various members of the dipterous series culminating in the muscids can. So 

 in Simulium and Blepharocera I have found suitable forms for study ; both with 

 females possessing the so-called mandibles, both with maxillas and labium well 

 developed in both sexes, and both with larvae equipped with biting mouths with 

 unmistakable mandibles, maxillre, and labia, and in one case, that of Simulium, 

 with the embryonic development of the larval mouth parts fully traced and the 

 homologies certainly * determined. 



" Simulium sp. — In the female imago f the mouth parts consist of a short 

 liplike labium composed of a short basal sclerite and three terminal lobes, being 

 the two large paraglossse and a median short membranous lobe, the fused glossge ; 

 a pair of maxillae, each consisting of a basal sclerite, a long five-segmented 

 palpus, and a single pointed, bladelike terminal lobe reaching nearly to the end 



* Metschnikov, E. Embryologische St.udien an Insekten. Zeitschr. f. ioiss. Zool., vol. xvi. 

 1866 ; embryonic development of mouth parts of Simulium described on pp. 392-421. 



t " In describing the adult mouth I shall assign to the various parts those names which, from 

 my earlier studiy of the comparative anatomy, seem correctly used, and the use of which is 

 confirmed by the results of this ontogenic study." 



