30 ARKIV FÖR ZOOLOGI. BARD 8. N:0 9. 



mandibles. In a normal external feeder it is obviously yery 

 essential that the opposite edges of the two mandibles should 

 meet in the median line, in order to cut out a portion of 

 the leaf. But as the mandibles are not straight but concave 

 at the inner, convex at the exteriör, side, this result can 

 only be obtained by their edges being convex, which is the 

 case, when the teeth increase in size towards the apex of 

 the mandibles. 



In the leaf-miners whose mandibles have been modified 

 into horizontal blades such a shape, with the longest teeth 

 in the middle would be very uneconomical, because it would 

 simply mean that the teeth outside the longest would not 

 have any function to fulfil, the tissue of the leaf where they 

 pass being already cut by the longer teeth. The ideal type 

 would instead be one where every part of the cutting edge 

 could be brought into action against the leaf, and this is 

 achieved in some degree in Nepticula (fig. 40), and still more 

 so in Cemiostoma (fig. 41). 



In these genera the thin, dorsal, blade-shaped part of the 

 mandible present in Gracilaria (fig. 44), Eriocrania (fig. 38) 

 and Tischeria (fig. 39) is very poorly developed. 



In the sap-feeders, on the other hand, this part plays a 

 prominent part, and its development leads to a gradual re- 

 duction of the teeth which we can follow in the series Ornix 

 (fig. 42), Gracilaria 1 (fig. 44), Lithocolletis (fig. 43), and Phyllo- 

 cnistis (fig. 45), and this is due to the circumstance that in 

 the same degree as the former develops, the latter become 

 useless and therefore reduced. 



8. Maxillae and Iabium. 



a) Tissue-feeders. 



Eriocrania. 



The name of the species I have not been able to ascertain, 

 since I did not succed in rearing it; it was found in the 

 leaves of an oak, and corresponds perfectly to Stainton's 

 description and drawings of E. subpurpurella, which has the 

 same mode of life as our larva. 



1 In this genus we can follow the development of the blade from the 

 lst to the 2nd instar (comp. figs- 44 and 32). 



