ITS OUTER SKELETON. 



39 



occipital foramen. In front it gives off two long crura, or 

 props, which pass to the ginglymus, and are reflected thence 

 upon the inner surface of the clypeus, ascending as high as the 

 antennary socket, round which they form a kind of rim. Each 

 crus is twisted, so that the front surface becomes first internal 



* 



and then posterior as it passes towards the ctypeus. The form 

 of the tentorium is in other respects readily understood from 



Fig. 17- Fore-half of Head, with tentorium, seen from behind, x 12. 



the figure (fig. 17). Its lower surface is strengthened by a 

 median keel which gives attachment to muscles. The oesophagus 

 passes upwards between its anterior crura, the long flexor of the 

 mandible lies on each side of the central plate ; the supra- 

 cesophageal ganglion rests on the plate above, and the sub- 

 cesophageal ganglion lies below it, the nerve-cords which unite 

 the two passing through the circular aperture. A similar 

 internal chitinous skeleton occurs in the heads of other Orthop- 

 tera, as well as in Neuroptera and Lepidoptera. Palmen* thinks 

 that it represents a pair of stigmata or spiracles, which have 

 thus become modified for muscular attachment, their respiratory 

 function being wholly lost. In Ephemera he finds that the 

 tentorium breaks across the middle when the skin is changed, 

 and each half is drawn out from the head like the chitinous 

 lining of a tracheal tube. 



* Morphologic des Tracheen-systems, p. 103. 



