NO. 7 THE INSECT CRANIUM SNODGRASS 3I 



that a Y-shaped suture cannot be discovered on the head of a 

 Chry so pa-larva." 



Mecoptera. — The line of the exuvial cleft on the larval head of 

 Mecoptera (fig. ii A, B, C) is identical with that of the Megaloptera 

 in that the frontal arms go between the anterior tentorial pits and 

 extend through the clypeus to points just mesad of the anterior 

 mandibular articulations. The same structure is shown by Steiner 

 (1930) and by Bierbrodt (1942) in the larval head of Panarpa com- 

 munis L. (fig. II C), and by Applegarth (1939) and Cook (1944) 

 in that of Apterohittaciis apteriis (MacL.) (B). 



The area embraced by the arms of the cleavage line on the larval 

 head of Panarpa conmmnis is shown by Steiner and by Bierbrodt to 

 be itself divided into three parts by a Y-shaped groove (fig. 11 C, 

 FR), the stem of which is continuous with that of the line of cleavage 

 (CL). Bierbrodt contends that the arms of this inner Y are the true 

 "frontal sutures," while those of the outer Y (CL) are secondary 

 lines of splitting at ecdysis. The latter, however, clearly are to be 

 identified with the frontal lines of cleavage in other insects, which 

 are commonly called the "frontal sutures." The lines of the inner Y, 

 therefore, are more reasonably interpreted by Steiner (1930) as 

 secondary grooves forming internal ridges correlated with muscle 

 attachments ; the labral muscles take their origins in the angle between 

 the arms. In the larva shown at A of figiu'e 11 the frontal area 

 between the arms of the cleavage line is marked only by a shallow 

 pigmented groove in the form of an inverted V, and apparently some- 

 thing similar is present in Apterohittacus as illustrated by Applegarth 

 (1939) ; in Harpohittaciis and Boreus the frons shows no correspond- 

 ing structure. 



The frontal and the clypeal areas of the mecopteron larvae are sepa- 

 rated by a broad depression or a groove (fig. 11 A, B, C). The labral 

 and the cibarial muscles are shown both by Bierbrodt (1942) and by 

 Cook (1944) to arise respectively on the areas above and below this 

 groove (B), but Cook regards the entire region between the arms of 

 the cleavage line as the "clypeus," an interpretation entirely incon- 

 sistent with his own showing of the muscle attachments. 



The line of exuvial cleavage so conspicuous on the larval head is 

 greatly reduced or obliterated in the adult mecopteron. No trace of 

 it is shown by Ferris and Rees (1939) in Panorpa nuptialis Gerst., 

 by Evans (1942) in Nannochorista inaculipennis Tillyard, or by 

 Crampton (1932) in Merope. In Panorpa connminis L,, Heddergott 

 (1938) describes and figures a small V-shaped groove over the 

 median ocellus, which he regards as a remnant of the "frontal 



