THE HEAD CAPSULE. 123 



continuous with tlie sclerosed ridp;es between the paracephala 

 and the median region of the head capsule. The ventral arch 

 is a narrow plate, which probably represents the gula. The 

 dorsal arch forms the basal edge of the two epi-occipital plates. 



The epi-occipital plates are triangular; they are united in the 

 median line by a strong ridge-like suture. Each exhibits a 

 slight hollow near its base, covered by fine setae. 



I have been unable to determine whether these plates are 

 part of the metacephalic ring, or whether they belong to the 

 non-segmental region of the head capsule. 



A fossa, the cotyloid cavity, in the external surface of the late- 

 ral part of the metacephalic ring, articulates with the condyles. 



The ventral arch is at first distinct, but is subsequently fused 

 with the pars basilaris. The structure of the metacephalic 

 ring has a striking analogy with that of the occipital bone in 

 vertebrates, as the ventral arch is obviously comparable with 

 the basi-occipital, the lateral parts articulating with the neck, 

 with the ex-occipitals, and the epi-occipital plates with the 

 epi-occipital. 



The Internal Skeleton of the Head is exceedingly rudimentary, 

 if the scleral ring, to be described with the compound eye and 

 the cephalo-pharynx which belongs properly to the proboscis, 

 be excepted. It consists of a jugum and of a pair of slender 

 rod-like sclerites, entocephala, which I think probably represent 

 the lateral bars already described in the head capsule of the 

 Earwig (page 117). 



The Jugum is formed by the union of two stout curved 

 processes, one arising from the upper and inner angle of each 

 lateral part of the occipital ring. When seen from behind, the 

 jugum apparently divides the occipital foramen into an upper 

 and a lower opening, but it really lies in a plane a little anterior 

 to the foramen. It assists in supporting the tentorial mem- 

 brane, an incomplete septum formed chiefly by a network of 

 tortuous tracheal vessels, which extends from the jugum to the 

 entocephala and the lower margins of the great eyes. 



The Entocephalon is a slender rod- like sclerite on each side, 

 which is not easily demonstrated, as it usually falls out when 



