I. TRÄGÅRDH, METAMORPHOSIS AND BIOLOGY OF ORCHESTES. 21 



poses, and the presence of the thin-walled areas, witli a fine 

 outicular fold, at the top of them suggests, that they act as 

 a kind of retractile feet. The thin-walled areas probably 

 enable the larvae to retract the projections to some extent 

 and, at the same time, when protracted, act as a kind of 

 suckers. 



The thoracie segments on the other hand are flat, taper 

 gradually forward and the prothoracic shields are quite 

 smooth, which suggests that the thorax acts as a wedge, to 

 make room for the body. 



b. Adaptions for feeding purposes. 



The occurrence of a double terminal tooth in the man- 

 dible, one upper and one lower, enables the larvse to de- 

 tach a larger portion of the parenchyma, than they other- 

 wise would be able to do. 



On the leaves of the oak and the beech, as a matter of fact, 

 the parenchyma is entirely consumed in the mines, whereas 

 on the willow, the leaves of which are thicker, only a layer 

 is consumed (comp. p. 9). 



Furthermore, the two ridges in the roof of the labrum, 

 with their bristles, are adaptions, probably for the purpose of 

 assisting the mandibles to detach the parenchyma. 



In this respect the three species agree essen tially, but 

 while in O. fagi and O. quercus (Pl. II fig. 3 & 5) the ridges 

 are provided with only two pair of straight and blunt brist- 

 les, in O. populi there are 3 pairs of pointed and curved 

 bristles. 



The dorsal side of the labrum exhibits a feature which, 

 although to all appearence small and unimportant, yet forms 

 a conclusive proof, that in O. populi on one hand and O. 

 fagi and O. quercus on the other the mode of feeding and 

 consequently also locomotion is different. 



We notice that on the labrum of the latter two species 

 the four liairs are exceedingly short and scarcely project 

 above the surface (Pl, II, fig. 1 & 4) whereas in O. populi 

 (Pl. II, fig. 11) they are long and slender. 



The reason for this difference is obvious: the latter 

 larva is more stationary and does not move forward; the 



