18 



eminently characteristic of the Collembola and, with the 

 exception of certain Thysannra, it is found nowhere else 

 among insects. 



The labrum (fig. 13) forms the dorsal or anterior wall 

 of the mouth-cavity. It is seen to be somewhat 

 rhomboidal in form, and on its dorsal surface are 

 distributed some stifiish hairs, whicli are probably of a 

 sensory nature. Situated immediately behind the labrum 

 is the clypeus (clyj).), which is separated from it by means 

 of a flexible suture. This suture functions as a hinge, 

 and admits of a certain amount of movement for the 

 labrum in the vertical plane. Posteriorly, the clypeus is 

 separated from the epicranial region of the head by a 

 shallow groove, while laterally it merges into the sides 

 of the head {j>l.). The posterior or pharyngeal surface of 

 the labrum is lined with a soft membrane; the latter, 

 however, does not exhibit any special chitinizations, 

 either in the form of hairs or gustatory organs, etc., 

 which would constitute an " epipharynx." 



The mandibles (figs. 11 and 15) are elongate tubular 

 structures, which are flattened and somewhat expanded at 

 their apices and provided with five sharp teeth, of 

 which the last is the largest. At the base of each 

 mandible is situated a large opening (Plate II., fig. 15, 

 cav.) through which muscles pass to be inserted on the 

 walls of the mandibular cavity. With the exception of 

 its basal attachment, each mandible lies free within a 

 pocket-like evagination of the pharynx (Plate IV., 

 fig. 35). The mandibles of Anurida differ from the usual 

 Collembolan type in the absence of a masticatory surface. 

 This feature is correlated with the soft and unresisting 

 nature of the food in this species, which necessitates 

 rather the use of a cutting edge than a crushing 

 surface. 



