In the imago or Moth, Sphinx Ligustri reaches the last stage of its life-history, the sexually mature insect. The dissimilarity between the head, thorax and abdomen, which first appears in the pupa, is now far greater, but the outlines of these three heteronomous regions are much softened and obscured by the thick coat of hair and scales that clothes the body. The small head distinguished by its light colour carries in this stage large sensory organs, - convex, black, pigmented eyes and antennae. As the food consists of the liquid nectar of flowers the organs of the mouth are reduced and modified, the only conspicuous parts being the spiritrompe or antliae, the homologues of the galeae in the biting mouth, which are extended in one of the specimens, and the hairy labial palpi which are, as in all Lepidoptera, turned forwards beneath the head. The thorax is broad and clothed dorsally with black-brown hairs, with a streak of white hairs over the roots of the wings. These organs are composed of a thin membrane supported by thick nervures, the whole surface being covered with thickly set and variously coloured scales, arranged in distinctive patterns and characteristic of Lepidoptera. The fore- and hind-wings on each side are connected by a hook and bristle.
The bristle springs from the fore-margin of the hind-wing near its root and ends in a tuft of stiff black hairs. The hook springs from the under surface near the margin and at the root of the fore-wing. The three pairs of thoracic-limbs are of but secondary importance as organs of locomotion and are used chiefly for support. The tarsi end with strong claws. The abdomen is large and pointed posteriorly. Its first somite is clothed dorsally with black hairs, a few white being intermingled. The remaining somites bear a longitudinal median dorsal grey-brown band with a central dark line; and each somite except the last has to either side of the median band a transverse bar of pink hairs, and a fringe of black hairs to its posterior margin. This black fringe encroaches more and more on the surface of the somites at the expense of the pink band the further back the somite is in the series.
The female differs from the male as follows. The antennae are more slender, and want the setae-like hairs on the brown surfaces of the joints: the thorax is rounder in front and the fore-wings have generally a more curved anterior border and less acute apex: there is no hook developed on the fore-wings for the bristle. There are five pink bands on the abdomen instead of six: the last somite is broad basally, conical and as long at least as the two preceding somites, whereas in the male the abdomen tapers gradually to a point and is terminated by two valves with a vertical slit between them.
The Lepidoptera are often divided into two chief sub-groups - the Rhopa-locera with the antennae ending in a club, and the Heterocera to which Sphinx belongs. The Heterocera have various types of antennal structure. They frequently possess the retinacular apparatus binding the fore- and hind-wings together, and their posterior tibiae have four instead of two spines.
The Sphingidae, the family of which Sphinx is the type, are characterised by the prismatic shape of the antennae and the long bristle-like character of their terminal joint; by a three-jointed very hairy labial palp with a minute terminal and two broad compressed basal joints; by a one-jointed maxillary palp; a robust body and relatively small wings. Their mode of flight is peculiar and sustained. Hence the popular name of Hawk-moths given to these insects. Ocelli are generally stated to be absent, but Cattie affirms their existence in Acherontia Atropos.
To see the form and composition of the regions of the body it is necessary to divest it of hairs and scales by careful brushing. The head has no sutures. The prothorax is ring-like and is hence often termed 'collar.' Its tergum carries at each outer angle a vesicular dilatation clothed with long hairs, the patagium of Kirby and Spence. The prothorax is united to the mesothorax by membrane, but the latter and the metathorax are firmly connected. The mesothorax is very large. Its tergum is broken up into a large scutum and a lozenge-shaped scutellum behind. The fore-wings are attached to it, but their roots are covered by concavo-convex shields, the tegulae or wing-covers1. The metathorax is small. Its scutum is narrowed medianly where the scutellum projects forward. It bears the hind wings. The nervures of these organs become plain when the scales are brushed off. They contain extensions of the tracheae and blood-channels. For their arrangement, as well as the composition of the lateral walls of the thorax, the student must consult the larger works on Entomology. The limbs consist of the same parts as in the Cockroach. The anterior coxae are free; the median and posterior are closely attached to the thorax. The trochanter is small; the femur short.
The anterior tibiae have at their proximal end a peculiar enlarged moveable spine. The median tibiae have a pair, the posterior two pairs, of distal posterior spurs. The tarsi are six-jointed, and not clothed with hairs like the rest of the limb.
1 The terms 'patagium' and 'tegula' are often misapplied. They are defined as in the text above by Kirby and Spence in the Orismology, Vol. iii. of the Introduction to Entomology. Patagia are structures peculiar to Lepidoptera, whereas tegulae are found in various other orders.
The abdomen consists of ten somites. Each consists of a strongly chitinised tergum and sternum united laterally by a soft pleural membrane. The first of the series in both sexes, the ninth and tenth in the male, the seventh to the tenth in the female, require more notice. The first is firmly united to the meta-thorax. It is constricted, and its tergum is divided into a median and two lateral pieces, a division brought about apparently by the attachment of muscles. A slight groove and a difference in the chitin mark its separation from the second tergum. Its sternum is incurved and continuous with the second sternum.