NO. 3 INSECT HEAD SNODGRASS I27 



serves a median suture, but in the honeybee (C) the line of union 

 between the postgenal lobes is obliterated, and the bridge presents a 

 continuous surface in the space between the foramen magnum and 

 the fossa containing the bases of the labium and maxillae. In an 

 adult tenthredinid {Ptcronidca) , on the other hand, the foramen 

 magnum, though greatly reduced in size by the development of a 

 wide occipito-postgenal area, is still " open " below, that is, it is closed 

 by a narrow remnant of the neck membrane between the approximated 

 angles of the postgenae. The labium, however, is displaced ventrally 

 and united with the bases of the maxillae. 



In the Hymenoptera, then, there can be little question as to the 

 line of evolution that has produced the structure of the back of the 

 head in the higher forms. The resulting condition has Ijeen correctly 

 observed by Stickney (1923), who says: "In many Hymenoptera 

 the mesal margins of the postgenae are fused between the occipital 

 foramen and the articulation of the labium." A very similar modi- 

 fication of the head has taken place in the caterpillars, as will be shown 

 later, in which the parts constituting the " hypostoma " (fig. 51 A, 

 Hst) correspond with the postgenal bridge of adult Hymenoptera. 

 In either case, an unusual thing has happened in that the labium, after 

 being moved forward to unite with the maxillae, has been separated 

 from its own segment by the intervention of parts of the first maxil- 

 lary segment. If the postgenae are lateral tergal elements of the 

 head wall, their ventral union finds a parallel in the prothorax of the 

 honeybee, which is completely encircled behind the bases of the legs 

 by the prothoracic tergum. 



The modifications in the posterior ventral parts of the head in those 

 orders in which a " gula " is developed are difficult to explain if studied 

 only in the higher phases of their evolution, but they can be understood 

 if traced from forms that show the simpler earlier stages of departure 

 from the normal. 



In the Blattidae. the cranium is much flattened, but the essential 

 head structure has not been altered, its posterior parts retaining the 

 same form as in the less movable head of the grasshoppers. In many 

 insects, especially in the Neuroptera and Coleoptera, however, the 

 flattened head is not only turned upward on the neck, causing the true 

 anterior surface to become dorsal and the mouth parts to be directed 

 forward, but the ventral surface of the head has been elongated to 

 preserve the vertical plane of the foramen magnum. In such insects 

 the bases of the mouth parts become separated from the foramen 

 magnum by a wide space, and in this space there appears a median 

 9 



