NO. lO MORPHOLOGY OF GN ATHOTRICHUS SCHEDL 53 



author and therefore this statement is merely an attemi)t to explain 

 these two parts of the elytra. 



Specific modifications of flic elytra. — While Guathotrichiis shows 

 the strongest development of the deelival sulci, with lateral convexities 

 and granules on the latter, these characters decrease in development 

 in Gn. sulcatus Lee. and are faint in On. materiarius Fitch. There also 

 occur many variations from specimen to specimen so a determination 

 cannot be based on these characters only. 



METATHORACIC WINGS OR HIND WINGS 



The metathoracic wings (figs. 3, 17) are homologous with the meso- 

 thoracic wings but differ considerably in structure and development 

 due to their use as flying apparatus. In the genus Gnathotrichus 

 Eichh. no specific modifications of importance have been observed. 



As in all genera of this family the wings are folded together and 

 covered by the elytra when the beetles are at rest. The hind wings 

 are twice as long as the elytra and three times as long as wide. Near 

 the base on the inner side occurs a deep emargination separating a 

 distinct lobe from the wing — the posterior wing lobe (fig. 3, WL, 

 17, L). The shape of the wings is very much alike in all three species, 

 long oval with the anterior and posterior margins nearly parallel. 



The wings consist of two layers of integument enclosing the tracheal 

 system. In the hind wings both of these layers are membranous ex- 

 cept on the veins and the basal sclerites. Externally they are covered 

 with microscopic hairs, except on the basal heavy parts of the veins 

 and the axillaries (fig. 17). 



Veins. — The venation of the wings shows considerable reduction. 

 The only visible veins are the costa (fig. 3, Co), subcosta (Sco), radius 

 (Ri + Ro), media (Mi, M,) and cubitus (Cui). The radial and the 

 medial veins are split into two branches. 



Costa. — As Hopkins has shown for the genus Dendroctonus, the 

 costal vein (fig. 3, Co, 17, C) is also in Gnathotrichus Eichh. confined 

 to a short basal piece. Apically it does not join another vein but is 

 reduced so that the costal margin is occupied by a membrane only up 

 to the point where the radial vein is bent forward and structurally 

 replaces the costa. The base of the costa is produced into the so-called 

 costal head (fig. 30, CoH). It consists of two parts, the costal condyle 

 (fig. 30, a), and the costal pocket (b). The former articulates with 

 the clavicle process of the metaepisternum (fig. 17, di). From the 

 base of the costal condyle a pocket stretches to the subcosta. At about 

 the center of the pocket a projection of the subcostal head, the sub- 



