( 321 ) 



which was donhth'ss due fo the strong (levehjpraent of a very remarkable chitiiiDiis 

 plate, the tentorium of Klenker,* wliifh conceals the ganglioa from view. The 

 tentorinra of Vorfculn is a horizontal plate (brown like the exoskeleton) wliich lies 

 beneath tlie oesoi)hagns, and extends from near the occipital foramen almost to the 

 centre of the head. The plate is slightly concave on the npperside, and nearly 

 evenly incnrved anteriorly. The anterior and posterior angles are each produced 

 into a slender process. The two anterior processes are curved and join the capsule 

 of the head in front of the antennae, while the posterior processes end at the hind 

 wall of the head, all four being so firmly attached to the head that it requires some 

 force to break them off. Between these two pairs of processes there is another pro- 

 cess on each side, branching off from the anterior process and extending obliquely 

 upwards, being but loosely connected with the upper wall of the liead in the neigli- 

 bonrhood of the eye. The tentorium of Arixenia {t.e, PI. XVII. fig. 10, ventral side ; 

 PI. XVIII. fig. 1, dorsal side) is similar to that of Forfieida, except that its 

 anterior half is much broader. The second process lieing subvertical is drawn 

 shortened in our figures. Hemimerm also has a tentorium of the same type 

 {te, PI. XVIII. fig. 2). This endoskeleton divides the capsule of the head into 

 an upper chamber containing the oesophagus and the brain serving the higher 

 faculties, and a smaller lower chamber which contains the sub-oesophageal ganglion 

 working the mouth-parts. Tlie commissures connecting the infra- with the supra- 

 oesophageal ganglion are in front of the tentorium. 



Alimentary Canal. 



Considering that the nervous and respiratory systems and, in the main, also the 

 external anatomy agree so well with what is observed in Forficuln, we were rather 

 surj)rised to find that the gut, deviates markedly from tlie type known in the 

 earwigs. In fact, the alimentary canal of Ilemiinerus resembles that of Forjicula 

 much more than does the gut of Arixenia. The digestive system of insects is often 

 remarkably different in forms not very distantly related. A difference in the kind 

 of food on which the species of insect subsists appears to be generally accompanied 

 by some distinct difference in the shape or structure of the digestive organs, and 

 this may account for the peculiarities observed in Arixenia. 



The oesophagus of Arixenia {oe, PL XVIII. fig. 1) consists of two divisions. 

 The anterior division, extending from the mouth to the occipital foramen, is 

 very muscular in itself, and numerous muscles are attached to its lateral and 

 upper surfaces. The upper wall unites with the underside oi' the upper lip, the 

 muscles of the latter extending backwards above the oesopliagus. On opening 

 the oeso])hagus from above, and at the same time pressing the under lip from 

 beneath, the lower wail of the oesophagus can be seen ending in the hypopharynx 

 (or endolabium), which is described above (p. 313). Where the oesophagus leaves 

 the head it is constricted, and from this point backwards its wall is thin and very 

 expansible. This wide j)ortion, which extends into the abdomen, where it ends 

 abruj)tly, is the " croji." Upon the oesophagus follows the siiort proventricuhis or 

 gizzard (//r), which has internally six folds lined with a chitinons membrane armed 

 witli minute teeth, which stand rather far apart. The folds project into the stomach 

 as conical jirocesses, which are about three times as long as they are broad at their 

 bases. The teeth on these processes are minute, transverse, and slightly curved 

 ridges, the teeth placed on the apical portions of the processes being produced into 



* IHstert. GottiiKjeii (1883). 



