NO. 3 INSECT HEAD SNODGRASS 1 25 



it is homologous with the preclypeal area of the psocid (fig. 46 G, 

 Adp). The terminal piece in the cicada (H, Lm) that closes the 

 groove in the upper part of the labium would appear to be the labrum 

 by comparison with Heteroptera. The "mandibular plates" (Md) 

 on the sides of the head must be the true bases of the mandibles. Their 

 upper ends (c, c) have the same relations to the surrounding parts 

 that the anterior mandibular articulations have in biting insects. The 

 mandibular bristles are chitinous outgrowths from the ventral pos- 

 terior angles of the plates, and the protractor apparatus of each bristle 

 in the adult is differentiated from the posterior margin of the mandib- 

 ular plate, as the writer has elsewhere shown (1927). 



In the larvae of Lepidoptera, a somewhat different type of modi- 

 fication has produced an unusual distortion in the relation between the 

 frons and the clypeus. The caterpillar head shows no essential varia- 

 tion within the order, but the homologies of the facial structures are 

 clear if interpreted by the characters w^hich serve as identification 

 marks in the other orders. The triangular facial plate (fig. 50 A) 

 thus becomes the clypeus, because the suture (cs) bounding it is identi- 

 fied as the epistomal suture by the origin of the anterior tentorial arms 

 from its lateral parts (fig. 50 I, AT). Upon this plate arise the 

 muscles of the buccal region of the stomodeum. The median part of the 

 frons is invaginated and forms the thick internal ridge (Fr) dorsal 

 to the apex of the clypeus, which is to be identified as the frons by 

 the origin of the labral muscles upon it. The so-called " adfrontals " 

 (A, fr) are probably lateral remnants of the frons at the sides of the 

 clypeus, and the " adfrontal " sutures are the true frontal sutures 

 (fs). That the relations of the plates of the caterpillar's head, as 

 thus established, are identical with those in other insects is made clear 

 in the diagram given at E of figure 47. The clypeus (Clp) has simply 

 extended into the area of the frons, and the median part of the latter 

 plate (Fr), bearing the origins of the labral muscles, has been in- 

 flected, while its distal parts, the so-called " adfrontals " (fr), maintain 

 the original lateral ground of the primitive frontal area. The lower 

 ]:»art of the clypeus is sometimes strengthened between the ])ases of 

 the jaws by a secondary thickening forming a submarginal ridge (h) 

 on its inner surface. 



MODIFICATIONS IN THE POSTERIOR VENTRAL REGION OF THE HEAD 



The structural changes in the posterior parts of the head described 

 here are associated with an elongation of the postgenal regions, re- 

 sulting in the production of a long interval between the foramen 



