ANURIDA. 369 



which are situated not far from the bases of the hairs. 

 They are the trichogenous or hair-forming" cells ; their 

 positions are easily recognised by their very large 

 nuclei. 



The basement membrane (b.m. in figs. 16 and IT) is 

 an extremely thin and apparently structureless layer ; it 

 is easily seen in preparations stained with methyl-blue- 

 eosin, and appears bright blue. 



3. The Mouth Parts. 



The structure and development of the mouth-parts of 

 Anurida have formed the subject of an elaborate paper 

 by Folsom (TO), and much of the following account is 

 based on his memoir. 



They consist of (1) the labrum or upper lip, (2) the 

 mandibles, (3) the maxillulae, (4) the lingua or tongue, 

 (5) the first maxillae, and (6) the second maxillae or 

 labium. 



The principal mouth-parts are remarkable in that they 

 are deeply insunk within the head-capsule and in that they 

 are capable of being partially protruded from it when 

 in use (Plate V., fig. 52). This deep-seated position of 

 the mouth-parts is a secondary condition, and has been 

 brought about in the following manner. In the embryo 

 the sides of the facial region of the head develop from 

 two lateral evaginations of the germ band. These 

 evaginations eventually fuse with the developing 

 fundaments of both the labrum and labium, and in this 

 way form a kind of enclosing box which, by further 

 growth, comes to surround all the remaining mouth- 

 parts. By this means, the latter appear in the adult 

 insect as if they were deeply pushed back into the cavity 

 of the head. This condition of the mouth-parts is 



AA 



