I. TRÄGÅRDH, LEPIDOPTEROUS LEAF-MINERS. 25 



4. In Phyllocnistis, on the other hand, the function of 

 the labrum seems to be quite different. Its great size and, 

 more particularly, its shape which enables it to completely 

 cover the mandibles even when they are fully open at once 

 suggests that its function is to protect the mandibles, or, to be 

 more exact, to protect the epidermis of the leaf from the 

 lateral edges of the mandibles. 



The reason for this extraordinary caution being exercised 

 becomes evident, when we take into consideration the fol- 

 lowing features in the biology of the larva. 



It is, at least in so far as is yet known, the only larva 

 which during its whole period of feeding * is as a p-f eeder, conse- 

 quently during the låter half of its growth is is comparatively 

 and absolutely the largest of those larvae mine-roof of 

 which only consists of the epidermis of the leaf. And this 

 circumstance would to all appearance render it extremely 

 difficult for the larvae not to cut through the epidermis which 

 w r ould be pressed close to the head, if the size and shape of 

 the labrum did not prevent the mandibles from touching it 

 at the sides where it is already separated from the rest of 

 the leaf-tissue. As it now is, it is only at the anterior margin, 

 where the leaf is yet intact, that the finely serrated edge 

 of the mandibles can reach it. 



And the perpendicular hairs on the dorsal side of the 

 labrum seem to serve the purpose of lessening the danger 

 of the sides of the mandibles striking against the detached 

 epidermis. 



There are, in two other genera, structures which may, 

 at least to some extent, serve the same purpose. 



Thus in Ornix (fig. 30), one pair of the hairs is large and 

 perpendicular, whereas the others are almost atrophied. 



7. Mandibles. 



In the externa! feeders the mandibles are triangulär or 

 quadrangular plates, convex on the exteriör, concave on the 

 interiör side; they are placed vertically, and meet in the 

 median line when closed, the teeth of one mandible fitting 



1 The last instar is almost rudimentary, and does not feed (conip. pag. 



45—47). 



