NO. I THE INSECT HEAD SNODGRASS 21 



early stage. In the adult insect, as in the larva, ridge-forming grooves 

 on the head sometimes resemble the cleavage line, and have been 

 mistaken for the "epicranial suture." On the adult head of the water 

 beetle Hydrophilus a Y-shaped groove exactly duplicates a typical 

 cleavage line, but it is formed by a midcranial ridge that meets the 

 ridge of the angulated epistomal sulcus. 



IV. THE POSTERIOR HEAD STRUCTURE 



The modifications of the insect head hardest to understand, and 

 the most confusing to taxonomists, are those that affect the posterior 

 surface, particularly when this surface becomes ventral in prognathous 

 species. 



The head of an acridid grasshopper is a good example of the 

 primitive structure of the head and its position on the thorax, since 

 the subgenal margins are approximately horizontal (fig. 6 E) and 

 the occipital foramen occupies a large part of the posterior head sur- 

 face (F). The labium hangs from the neck between the posterior 

 tentorial pits. By contrast, in most of the higher orders of insects 

 the foramen is much contracted by shortening from below (fig. lo). 

 In a simple hypognathous head of this type (fig. 9 A) the hypostomal 

 margins of the cranium have been drawn upward on the rear surface 

 of the head. The hypostomal sulci (hs) extend to the tentorial pits 

 (pt) as usual and become continuous with the postoccipital sulcus 

 (pos) over the occipital foramen. The postocciput and the hy- 

 postomata thus form a continuous marginal band of the cranium. 

 The labium still hangs from the neck approximately between the ten- 

 torial pits, but both the labium and the maxillae are now suspended 

 from the back of the head. An example of this type of head struc- 

 ture is seen in the hymenopteron Xyela (B), except that the base of 

 the labium has lost its association with the tentorial pits. Other less 

 diagrammatic examples of the same essential structure are seen in 

 the beetle larvae Popillia (C) and Melandrya (D), and in an adult 

 Myrmelionid (E). 



On the figures accompanying the following discussions it may 

 seem inconsistent that the basal plate of the labium in some cases is 

 labeled the postmentum (fig. 9 B, D, E, Pint), in others the submen- 

 tum (C, Smt). The labial sclerotization fundamentally consists of 

 a prementiim and a postmentum, but the postmentum is often subdi- 

 vided into a mentum and a suhmentum. The basal plate, therefore, 

 may be either a postmentum or a submentum. The prementum is 

 always to be identified by the attachment on its base of a median 



