NO. 7 THE INSECT CRANIUM SNODGRASS 1 3 



II. EXAMPLES OF THE ECDYSIAL CLEAVAGE LINE OF 

 THE HEAD 



Though the Y-line of the ecdysial cleft on the head is well known 

 under the name of the "epicranial suture," a few examples taken from 

 the principal orders of insects will serve to show its variations, which 

 have not been given much consideration by entomologists. Also, it 

 will be noted that grooves of secondary cranial ridges present in larval 

 or adult stages of some insects have been misinterpreted as the 

 "epicranial suture." 



The names of genera and species used in the following descriptions 

 have been furnished by the entomologists in the United States 

 National Museum ; the names of authors of species not given in the 

 text will be found in the legends of the figures. 



Apterygote insects. — Only a few published records are to be found 

 on the manner of ecdysis in Protura, Collembola, Diplura, and Thy- 

 sanura. Henriksen (1932) has shown that in the machilid Petrobins 

 halticits Stach a rupture takes place along the midline of the back of 

 the thorax and is continued on the head, where it ends behind the eyes 

 in a pair of very short lateral branches. At ecdysis of the silverfish 

 Ctcnolcpisma longicaudata Esch., Lindsay (1940) says, "a deep fur- 

 row develops in the mid-dorsal line of the thorax and along the 

 epicranial stuture." In a personal communication to the writer, 

 Charles L. Remington, of Harvard University, describes the exuvial 

 split observed by him in a specimen of Campodea as extending from 

 the neck upon the head as far as the bases of the antennae. Concerning 

 ecdysis in Collembola, Handschin (1926) records that in Onychiurus 

 armatiis Tulb. the skin is split between the head and the thorax by 

 alternate dorsal and ventral bending of the body ; in some other 

 species, however, he says the exuviae are not shed entire, but break 

 up into fragments which in the course of time are rubbed off. Davis 

 and Harris (1936), in their study of Pseudosinella violenta (Fol- 

 som), an entomobryid, observe merely that the insect moves the 

 thorax up and down until the old skin begins to split along the mid- 

 dorsal region of the thorax. Exuviae of the podurid Achorutes 

 examined by the writer show the skin to have been shed in one piece, 

 and that the insect emerged through a rent on the back of the thorax 

 extended on the head, where it appears to be forked above the bases 

 of the mouth parts. 



From these few records it would appear that ecdysis in most of the 

 apterygote insects takes place generally as in Pterygota through a 

 cleft on the thorax and head, but that the frontal arms of the cleavage 



