NO. 7 THE INSECT CRANIUM SNODGRASS 35 



coronal and frontal "sutures." A review of recent papers on the head 

 of larval Diptera will show, moreover, that the structure is essen- 

 tially the same in all dipterous families in which the larva has a free 

 head capsule. Anthon (1943) has described and figured the larval 

 head in representatives of the Rhyphiceridae, Ptychopteridae, Tri- 

 choceridae, and Psychodidae; Cook (1944b) gives a full description 

 of the head structure and musculature in a species of Chironomus; 

 Ross and Roberts, in their Mosquito Atlas (1943), illustrate the 

 larval head structure in 33 species of Anopheles, besides a species 

 each of Aedes and Culex, and Cook (1944a) describes species of 

 Theohaldia, Anopheles, Liitsia, and Armigeres, giving both the 

 structure and the musculature of the larval head and mouth parts. 



In all the forms illustrated by the writers just mentioned, the head 

 of the larva presents a prominent Y-line having usually a short coronal 

 stem and widely divergent frontal arms that go mesad of the 

 antennae and embrace most of a large facial area that continues 

 downward without interruption to the base of the labrum (fig. 12 A-D, 

 F). As shown here in Aedes (E) and Anopheles (G), the ecdysial 

 splits in the head exuviae extend to points just mesad of the antennal 

 bases. On the large shield-shaped facial area of the apotome thus 

 separated from the parietal lobes are attached in the intact head, as 

 shown by Cook (1944a), the muscles normal to the frons and the 

 clypeus (D). There would hence appear to be no reason for regarding 

 this area as other than that of the f rontoclypeal region of the cranium. 

 However, Anthon (1943) designates it the "frons," and Cook 

 (1944a) calls it the "clypeus." The first assumes that a downward 

 extension of the frons has crowded out the clypeus, or reduced it to 

 a pair of lateral lobes at the base of the labrum, and presumably that 

 the frons has taken over the clypeal muscles ; the second assumes the 

 opposite, namely, that the clypeus has invaded the region of the frons 

 and has appropriated the frontal muscles. If either assumption is 

 necessary, one would seem to be as good as the other, but neither is 

 substantiated by evidence to show that a shifting or transposition of 

 any part of the cranial surface has taken place. The muscle attach- 

 ments maintain the same relative positions that they have in insects 

 in which the frontal and clypeal areas are separated by an epistomal 

 groove, or in other insects in which this groove is absent. Since an 

 epistomal sulcus is not usually present in holometabolous larva, and 

 is variable in its occurrence even in adult insects, and since we have 

 no evidence in dipterous larvae of either the frons or the clypeus 

 having invaded the territory of the other, there is no reason for 

 regarding the facial region between the arms of the ecdysial line and 



