14 ARKIV FÖR ZOOLOGI. BAND 8. NO 5. 



which are of uniform height throughout and broadly rounded 

 at the top; from the middle of the dorsal edge a narrow bar 

 projects; no arch connects the left and the right half. 



The skeleton of the l:st instar does not resemble that 

 of the parasitic larvse described by Nielsen but agrees, on 

 the other hand, to a considerable extent with that of Ephydra 

 riparia Fall (comp. figs 3 & 4 and 1903 a figs 1, 2 & 6). 



2:nd instar (Figs 5, 6 a and b). 



In this the mouthparts and the cephalopbaryngeal ske- 

 leton has assumed a quite different shape. The W-shaped 

 plate has disappeared. The mandibles are now anteriorly 

 shaped as horizontal blades, the edge of which is beset with 

 sharp teeth laterally as well as on the median side (fig. 5) 

 They have increased considerably in size and articulate directly 

 against the anterior pharyngeal sclerites; the latter are poste- 

 riorly connected by a low, broad arch (fv). 



Of the two pairs of posterior pharyngeal plates, the upper 

 one is longer than the lower one by 1 / 3 and pointed; the lower 

 ones increase slightly in width towards the posterior trun- 

 cate end. Anteriorly they are connected by a narrow arch (bv) 

 who sends out a pair of narrow, horn-shaped appendages 

 backwards; the anterior half of the arch is areolated by small, 

 rounded areas of thinner cuticle. 



3:rd instar (Fig. 7). 



The mandibles are now vertical blades as in the 1st 

 instar, with strong, curved terminal tooth with smooth edge. 

 The anterior pharyngeal sclerite is completely fused with the 

 posterior part of the skeleton. 



The development of the above described parts is cha- 

 racterized by 1) the inconspicuous size of the median tooth 

 in 1st instar, 2) the presence of a W-shaped rasp-plate in 

 the same. It differs from the scheme given by Nielsen in 

 two respects. 1) In the first instar the mandibles are arti- 

 culated to the anterior pharyngeal sclerites and the median 

 tooth by means of two pairs of bar-shaped sclerites; 2) In the 

 3rd instar the anterior pharyngeal sclerites are fused with 

 the posterior ones. 



Sensorial organs (Figs 8, a and b). These are of the 

 same shape in the 2nd and 3rd instar; the anterior one (Fig. 

 8 b) is single-jointed with a small bristle on it; the posterior 

 (Fig. 8 a) is 2-jointed. 



