( Ixxviii ) 



are here and there plainly visible, and there is not rarely a conspicnons 

 homogeneity preserved in more than one gronp of species or of genera, and in 

 more than one particular structure, as we shall see by reviewing shortly the 

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



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

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

 o-enerally directed frontad, not anad, and are occasionally so close together that 

 the iiroximal outline of the tergite cannot be ascertained without removing the 

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

 divided mesially or simple. The symmetrically divided tergite represents the 

 generalised state of the segment. If the two halves are divided down to the 

 junction with the nintii segment, they are a little movalde against each other ; 

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

 move together and only vertically (PI. XXVI. f. 28. 33 ; PL XLII. f. 1-25). 

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

 Ambiilicinae, and the Sesiinae. It appears 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 which is found only 

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



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

 portion correspondingly longer, resulting in a narrow sinuate tergite and then in 

 a non-sinuate one. This is the ordinary type of a simple tentli 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 the figures will show (Pis. XXV., 

 XXVI., XXVII., XLIV., etc.). 



The tenth sternite is a belt running from the base of tiie tergite vontrad, 

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

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

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

 and produced into one or two processes or lobes of various shapes and sizes. 

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

 distinctly separate plate (PI. XXIV. f. 16 ; 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 the tergite. However, in some Sesiinae 

 (PI. XLII. f. 11. 25; PI. XLIII. f 1. 4. 5) 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 

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

 side of the lateral i)art of the sternite we find, especially often in Ambidicinae, 

 the membrane swollen to a Ijump beset with sensory hairs (PI. XXVI. f. 5. 9). 

 An asymmetrical development of tlie sternite will be referred to below. 



