( Ixxviii ) 



are liuic suid tliore plainly visil)lo, and thorc is not rarely a consjiicnons 

 hoiuoirencity preserved in more than one gronj) of species or of genera, and in 

 more than one particnlar structure, as we shiill see by reviewing shortly the 

 mass of specialisations described in the systematic part of the Revision. 



The tenth tergite bears stiff hairs, wliich stand either singly or form a 

 more or less dense covering on the njiper and lateral surface. The hairs are 

 generally directed frontad, not anad, and are occasionally so close together that 

 the ])roximal outline of the tergite cannot be ascertained without removing the 

 covering of hairs. There are two principal forms of the tergite, it being either 

 diviiled mesially or simple. The symmetrically divided tergite represents the 

 generalised state of the segment, "^f tlie two halves are divided down to the 

 junction with the ninth segment, they are a little movable against each otlier ; 

 but as a rule tlie two processes are not separated proximally, and therefore 

 move together and only vertically (PI. XXVI. f. 2S. 33; PI. XLII. f. 1— 2;")). 

 We find the divided tergite preserved in some generalised Ackerontiinae, some 

 Ambuliriimc, and the Sesiinae. It aj)pears in many modifications. From the 

 divided tergite the undivided one has developed in two ways : — 



(1) One side of the tergite becomes obliterated, a type whicli is found only 

 among the Sesiitiae, and of which we shall have to speak later on. 



(2) The tergite becomes narrower, the processes shorter, and the mesial 

 ])ortion correspondingly longer, resulting in a narrow sinuate tergite and then in 

 a non-sinuate one. This is the ordinary type of a simple tenth tergite. Here 

 the single, symmetrical process is generally more or less compressed, convex 

 above, the apex curved downwards to a hook and mostly pointed. The variety 

 in this type is very great, as a look at tlie figures will show (Pis. XXV., 

 XXVI., XXVIL, XLIV., etc.). 



The tenth sternite is a belt running from tlie base of tlie tergite ventrad, 

 encircling a membranaceous area, from wiiicii projects the anal cone, the end of 

 the gut (A). The ventral transver.se part of the sternite is in by far the larger 

 proportion of Hawk Moths as strongly chitinised as the vertical side-parts, 

 and j)roduced into one or two processes or lobes of various shapes and sizes. 

 In some cases the transverse portion is very feebly chitinised and does not form a 

 distinctly separate plate (PI. XXIV. f. 10; PI. XXV. f. 12. 27) ; in other species 

 it is a very low ridge without lobe or process (PI. XXV. f. 1. 24 ; PI. XXVI. f. 1). 

 As a rule the sternite is shorter than tlie tergite. However, in some Sesiinae 

 (PI. XLII. f. 11. 25; PI. XLIII. f. 1. 4. 0) the reverse is the case. The sternite 

 is simple or divided ; it may agree in this respect with the tergite (PI. XLIII. 

 f. 1), or it may be divided when the tergite is simple (PI. XXV. f. 20 ; PI. XXVI. 

 f. 30), or simple when the tergite is divided (PI. XXVI. f. 1 — 14). It is never 

 hairy like the tergite, but the apex is often granulose, the granules bearing fine 

 Itristles, most likely of a sensory nature (PI. XXIII. f. 1. 2. 22). At the inner 

 side of the lateral part of the sternite we find, especially often in Ambulicinae, 

 the membrane swollen to a hump beset with sensory hairs (PI. XXVI. f. .O. 9), 

 An asymmetrical develojiment of the sternite will be referred to below. 



