178 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



ture ; but comparatively speaking, the variations are very minute, 

 most often concerning the shape and the number of the hypochilan 

 teeth, the shape and the division of the labral chaetae or of the pre- 

 mandible. We cannot here detail these modifications of the buccal 

 organs, which, because of their extreme variability, supply excellent 

 tests for the identification of the genera and even of the species, and 

 are expounded in every work of identification (cf . Goetghebuer 1932, 

 Thienemann 1944).* 



Fig. 2. — Cricotopus gr. silvestris Fabr. 



Part of a cross section concerning the hypochilan region situated immediately 

 before the insertion of the salivary muscle. Relationships of the hypochilum 

 with the lateral cranial wall, the maxilla (Lac), and the mandible. Cf. fig. 4 of 

 this paper and fig. 7 of Gouin (1957). 



IMPORTANT MODIFICATIONS OF THE GRTHOCLADIAN STRUCTURE 



The Diamesan Structure 



Among other characteristics Goetghebuer (1932, p. 145) mentions 

 the presence of numerous hairlike setae implanted on the maxillary 

 lobe and on the labium (hypopharynx auct.). In effect, the labial 



* The adaptative characteristics are rather to be found in the organs of pro- 

 gression, the prolegs. There is a very distinct difference in the manner of 

 motion of standing-water species and of those haunting torrents. The former 

 swim with spiral undulations of their bodies, while the latter move like cater- 

 pillars of the Geometridae, imitating very well the larvae of Tanypinae. 



