rsrcHE. 



23 



holl()\v, that its lumen is a continuation 

 of the parapodial space, as shown in Fig. 

 4, and that both lumen and parapodial 

 cavity are lined by a fine cuticula. He 

 asserts that this discover)' is anatomi- 

 cal proof that the chord, the parapodial 

 membrane, and the disc itselfare all parts 

 of a single invagination of the einbiyonic 

 ectoderm. Graber,* however, in 1SS9, 

 supported by certain observations on 

 Calliphora, a muscid, asserts that the 

 beginnings of both the thoracic and 

 cephalic imaginal discs are not ec- 

 todermic invaginations, but ectodermic 

 thickenings towards the inside, fol- 

 lowed by delaminations. 



There exists in fact at the present 

 time no embryological evidence on the 

 origin and first stages of development 

 of the thoracic and cephalic imaginal 

 discs in the brachyceran diptera, al- 

 though all the later writers except 

 Graber, supporteii bv the fact of their 

 undoubted ectodeiinic origin in the 

 larva of Coretiira, and also by the ana- 

 tomical evidence adduced by Van Rees, 

 are of the opinion that thev arise as in- 

 vaginations of the embryonic ectoderm. 



I have been .for some time studying 

 the embryonic development of Melo- 

 phagus ovinus, and, although the studv is 

 not 3'et completed, I am able to give a 

 detailed account of the origin and early 

 liistory of the imaginal discs in this in- 

 sect. Melophngus is a cyclorrhaphic 

 brachvceran. Leuckartt early showed 



* Graber. Vergl. Stud, iiber d. Embry. d. Insecten. 

 Denks. ."Vcad. Wiss. z. Wieii. Bd. 56. p. 306. 



the Striking similarity of its larval 

 and pupal forms, and of the pupipars 

 in general, to those of the muscids ; in 

 an earlier portion of this paper, I have 

 emphasized the same fact : Mtiggenburgt 

 has described the moutliparts of almost 

 all the ]5upipars and homologised 

 them with the parts of the fly's proboscis ; 

 Brauer§ considers the pupipars de- 

 generate flies. The relationship between 

 the pupipars and the muscids is 

 undoubtedly a very close one, and obser- 

 vations on the former must be conclusive 

 in determining the method of origin of 

 imaginal discs in tlie latter, as well as in 

 all the higher dipters. 



In Melophagus the cephalic and thor- 

 acic imaginal discs first appear as local 

 thickenings followed bv invagination of 

 the embryonic ectoderm. The cephalic 

 discs make their appearance first, and 

 very early in the ontogeny of the insect. 

 The ventral plate, as in other dipters, 

 is not confined to the ventral side of 

 the embryo, but it encircles the anterior 

 end and covers the anterior third of 

 the dorsal side, as is shown in Fig. 5. 

 The stomatodeum appears as an ecto- 

 dermic depression on the dorsal side 

 at some distance from the anterior end 

 (S, Fig. 5), the proctodeum as a cor- 

 responding depression on the dorsal 

 side, slightly distant from the poste- 

 rior end (P, Fig. 5). These are not, 

 however, the permanent positions of 



t Leuckarl- Pie Fortpfia 

 1858. 



zung u. Entwick. d. Pupiparen. 



% Muggenburg. Der Riissel d. Diptera pupipara 

 Archiv f. Naturgeschiciite. Jahrg. 58, iSs>2, p. 2S7. 



§ Brauer- Systemat.-zool. Studien. 91 Bd. d. Sitzungsbr 

 d. Kais. Acad. d. Wiss. ist Abt. 1885, p. 405. 



