42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, IO7 



Symphyla. — The symphylid head becomes of interest in connection 

 with a study of the cranial "sutures" of insects, because the Symphyla 

 have been much exploited as possible ancestors of the hexapods. 

 Ferris (1942), in fact, claims to find the basic structure and even the 

 segmentation of the insect head fully revealed in the adult head of 

 Scutigerella immaculata ( Newp. ) . 



The dorsal surface of the head of Scutigerella immaculata (fig. 

 15 A) presents a prominent Y-shaped median line (a), and a pair of 

 lateral lines (b, b) diverging from near the base of the stem of the 

 median line. The Y-line is the external mark of a strong internal 

 ridge, the divergent arms of which extend toward the antennae and 

 unite with the thickened rims of the antennal fossae opposite the 

 pivotal processes that support the antennae. This Y-shaped ridge of 

 Scutigerella must serve to strengthen the otherwise thin wall of the 

 head. The arms of the ridge are regarded by Ferris (1942, p. 29) 

 as representing the "clypeofrontal suture" of insects. Of the external 

 groove of the median stem, Ferris says, "it evidently is the suture, 

 common to so many arthropods, which breaks at the time of ecdysis, 

 and there is no reason to regard it as anything different from the 

 'coronal' suture of insects." A very good reason to the contrary, how- 

 ever, may be drawn from the description by Williams (1907) of 

 ecdysis in Scutigerella immaculata, or the same as recounted by 

 Henriksen (1932), from which it would appear that this symphylid 

 sheds its cuticle at ecdysis in the manner of other myriapods, namely, 

 in most cases, by a transverse rent behind the head. "The most com- 

 mon method," Williams says, "is to disarticulate the cast at the junc- 

 tion of head and first segment and to creep out forward." But the 

 animal may escape from the moulted skin also "by tearing between 

 the legs or just above the attachment of the legs on either side or by 

 tearing the dorsal covering longitudinally or by disarticulating the 

 cast somewhere on the dorsal side and extricating the body." The 

 Y-shaped "suture" on the head of Scutigerella, therefore, Henriksen 

 points out, "does not play any role in moulting." A similar Y-shaped 

 ridge is present on the head of some diplopods (fig. 15 B), but the 

 arms in this case go to the eyes instead of the antennae. Finally, it 

 should be noted that a Y-line of exuvial cleavage on the head is a 

 feature peculiar to insects, and is not "common to so many arthro- 

 pods" as Ferris asserts. 



The two lines that form the V on the head of Scutigerella (fig. 15 A, 

 b, b) are of quite a different nature from the Y-line, being merely 

 clear linear tracts of the cuticle devoid of the minute setae that thickly 

 cover the rest of the head surface. They begin as apparent clefts in 



