( 318 ) 



anteriorly, the upper surface being somewhat depressed at the sides and behind. 

 The anterior margin is sinuate at each side, and the central portion depressed so 

 as to fit into the concave occipital part of the head. There is no sharp edge 

 either on the occiput or at the base of the pronotnm, nor are head and pronotum 

 closely applied to one another. In Ilemimerus, on the other hand, the hind edge of 

 the head projects backwards, overlapping to a slight extent the pronotnm. This 

 overlapping, which is not often observed among insects, is best known of some 

 Hemiptera parasitic on bats and of the fleas. In the beaver parasite, Plati/j)s>/llus 

 castoris, the head and pronotnm fit well together, and there is, moreover, a comb of 

 spines extending from the edge of the head on to the thorax, bridging over the 

 gap which might be formed when the head is bent down. The overlapping we 

 find in these parasites, which live in the fur of mammals, renders the surface of the 

 head and pronotum more uniform and hence more suitable for gliding through the 

 fur, and is certainly a secondary development whicli has taken place independently 

 in these not nearly related insects. 



In certain lights there appears a thin pale line along the centre of the pro- and 

 mesonotum, which is also found in Hemimerus. 



The mesonotum is similarly rounded as the pronotum, but still more strongly 

 at the sides. The metanotnm, however, though its sides are also strongly rounded, 

 resembles in outline more the abdominal segments, inasmuch as its hindmargin is 

 slightly incurved instead of rounded. The metanotnm, moreover, is a little broader 

 than the pro- and mesonotum. The three nota jiroject much less sideways than in 

 Hemimerus, and do not even quite conceal in a dorsal view the pleural sclerites and 

 trochanters (PI. XVI. fig. 1). 



As the head is longer above than below, the membrane connecting it with the 

 thorax is more extended beneath than above (PI. XVII. figs. 1 and 8). Ventrally 

 this membrane expands between the submentum (sni) and a small sclerite which 

 lies in front of the sternum of the prothorax and undoubtedly belongs to the thorax 

 and not to the head. Hansen considered this transverse sclerite in Hemimerus to be 

 the sternum of the labium, i.e. a part of the liead, and drew it as lying close to the 

 snbmentnm and well separated from tlie presternum. Our siiecimens oi Hemimerus, 

 however, prove that the sclerite iu question has the same position in that insect as 

 in Forficula and Arixenia. VerhoeflF,* with customary acumen, recognised it from 

 Hansen's figures as being part of the thorax, and identified it as the sternal plate of 

 the " microsternnm." t The sclerite, I think, is homologous with what I termed 

 " mesoclidium " in the mesosternite of insects, the sclerites extending from this 

 central plate upwards corresponding to the peri- and parasterna.J The mesoclidium 

 is not developed in the mesosternite of Arixenia, but is quite distinct in tlie 

 meso.sternite of some otlier Orthoptera — for instance, Acridi/im. 



The prosternnm is nearly as long as it is broad, and overlaps the mesosternum, 

 which latter projects over the metasternnm. The sternal parts of the thorax are 

 ranch less densely hairy than the nota (the hairs are not indicated in our figs. 1 

 and 8 of PI. XVIL). The meso- and metasterna are much broader than long. 

 The metasternnm is the widest of the sterna, and bears on each side a small 

 groove, from which extends inward a narrow, rod-like, pointed endoskeleton. The 

 co.xal cavities are large and lateral, and are situated at the hind edge of the 



• In Zool. Anzeig. xxv. p. 204 (1902). 



t Kor the morphology of the thorax oE insects, see Btlruer, in Zool. .Inzeig. p. 'I'M (11)03). 



} In Verh. V. Tiitrrit. Ziwl. CuiujreM, BirVui, p. 820(1902). 



