22 ARKIV FÖR ZOOLOGI. BAND 6. N:0 7. 



other two larvae ha ve to force their way forward, the head 

 and at least the prothorax acting as a wedge, and any slen- 

 der hairs would inevitably be rubbed off. 



This difference in feeding and locomotion is also noticeable 

 in the shape of the clypeus; in O. fagi and O. quercus there 

 is a pair of knob-like thickenings of the cuticle (Pl. II, fig. 

 1 & 3) which probably serve the purpose to raise the epi- 

 dermis of the leaf; these are not to be found in O. populi. 



Key to the larvae af O. populi, O. fagi and O. quercus. 



The difference between the larvae may be briefly ex- 

 pressed thus : 



Body flattened, with lateral intersegmental constrictions ; 

 black patches on dorsal and ven trål side; pygidium rounded; 

 one prosternal shield ; no cocoon O. populi. 



Body rounded, with dorsal intersegmental constrictions 

 on the 1st to the 7th abdominal segments; no markings on 

 the body; pygidium conical and pointed; 3 prosternal shields; 

 cocoon 2 



I Cuticular spinulse, stigmata and pygidium colourless O. fagi. 

 \ » » » » » da.rk brown 



O. quercus. 



