16 ARKIV FÖR ZOOLOGI. BAND 6. NIO 1. 



The thoracic segments are lower than those of the ab- 

 domen, almost flat and with only small intersegmental con- 

 strictions. 



The 1st to the 7th abdominal segments on the other hand 

 are on the dorsal side so highly arched, that they appear to 

 ha ve broad conical processes; the ventral side on the cont- 

 rary is almost flat. 



The cuticle has numerous minute cuticular spinulse, 

 which are quite black. The stigmata, which are present on 

 the prothorax and the 1st to the 8th segments are dark brown 

 coloured and consequently conspicuous, contrary to those of 

 O. fagi, which are very difficult to see. 



The head capsule is, as above stated, very dark-colou- 

 red, almost black, and of the same shape as that of O. fagi. 



The clypeus (Fig. 4, Pl. II) is short and by a median 

 incision divided in to two rounded lobes; the sides are strongly 

 chitinous, the anterior margin on the other hand, which over- 

 laps the posterior margin of the labrum, is very thin. A 

 little in front of the middle, and laterally, it has a pair of 

 low, rounded knob-like processi which are thickenings of the 

 cuticle; 3 pairs of minute hyalin hairs, placed two close 

 together near the sides, the 3rd pair nearer the middle. 



The labrum (Figs. 4 & 5, Pl. II) is more than twice 

 as broad as it is long, with only slightly convex anterior 

 edge and the lateral edges converging slightly forwards; the 

 posterior margin has a short median projection with concave 

 sides. 



On the upper side of the labrum there are two pairs of 

 short and blunt, but rather stout, hairs, inserted in deep 

 pores. 



At the anterior edge we notice the same characteristic 

 bristles as in O. populi, but there are 6 pairs of them, and 

 the three median pairs, which are of more even width through- 

 out, blunter and curved more downwards than the other 

 ones, are inserted on the upper side, close to the edge, 

 whereas the three lateral pairs, which are more straight and 

 pointed, are inserted on the ventral side. 



On the ventral side of the labrum (Fig. 5, Pl. II) we 

 notice two short, oblique ridges which bear two pairs of 

 straight stout bristles, which are of even width throughout 

 and almost truncated at the top. 



