6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO7 



claims, results from the dorsal union of the cephalic lobes (forming 

 the coronal stem), and the inclusion within their anterior angle of the 

 procephalic lobe (forming the divergent frontal arms). Other 

 students of insect embryology have given the same explanation of 

 the origin of the cleavage line on the head in orthopteroid insects, and 

 it has been invoked by Steiner (1930) to explain the line in Sialis, 

 though without any substantiating evidence. If the Y-line does thus 

 originate, it may be regarded as a true suture. 



The ecdysial cleavage line on the head, however, as it actually 

 occurs in postembryonic stages, is much more variable than might 

 be expected from a structure of fundamental significance. It varies 

 with different insects in the point of forking on the face, the course 

 taken by the arms, and the points at which the arms terminate. Thus, 

 in some insects the clefts along the arms of the cleavage line at ecdysis 

 cut into or through the compound eyes (fig. i C), in others they 

 end between the eyes and the antennae (D), in still others they go to 

 the antennal fossae (E), while again they run mesad of the antennae 

 (F), and finally they may extend entirely through the clypeus (G). 

 Also the coronal stem of the Y varies in length, and in some insects 

 it is entirely absent (H), in which case the frontal clefts may proceed 

 separately from the occipital margin of the cranium. On the other 

 hand, the frontal arms in some insects are obsolete, and at ecdysis 

 the coronal split extends to the clypeus, or to the labrum ( I ) , cutting 

 the head cuticle through the midline of the face. 



Furthermore, the Y-shaped line of cuticular cleavage on the head 

 is a feature peculiar to insects. If it represents so important a thing 

 as the line of dorsal closure and union of the head components, it 

 should be present also in other arthropods, but it is not — other 

 arthropods have different ways of ecdysis. It seems probable, there- 

 fore, that at least the frontal arms of the cleavage Y are secondarily 

 developed lines of weakness in the head cuticle, and hence are free 

 to follow different courses in different insects. In any case, the 

 embryonic dorsal closure of the head lobes, and that of the thorax as 

 well, is purely an embryonic device for growing around the yolk, and 

 cannot possibly represent any phylogenetic event in the evolution of 

 the free-living ancestors of the insects, which presumably shed their 

 skins as modern insects do. 



Whatever may be the origin of the cleavage line on the insect head, 

 however, and regardless of where the arms branch from the coronal 

 stem or of the course they follow on the face, the arms are consistent 

 in one respect, which is, that they invariably lie between two distinct 

 sets of head muscles ; at ecdysis the frontal clefts always separate the 



