Thus, the genus _Macroglossa_ is characterized by a straight anal horn, a spherical head, and by a marking composed of longitudinal stripes, these characters not occurring elsewhere in this combination. The nearly allied genus _Pterogon_, on the other hand, cannot be based on the larvæ only, since not only is the marking of the adult larva very distinct in the different species, but the anal horn is present in two species, whilst in a third (_P. Œnotheræ_) it is replaced by a knob-like eye-spot. The genus _Sphinx_ (_sensû strictiori_) is distinguished by the simple, curved caudal horn, the smooth, egg-shaped head and smooth skin, and by a marking mainly composed of seven oblique stripes. The genus _Deilephila_ is distinguished from the preceding by a dorsal plate, situated on the prothorax and interrupting the marking, as well as by the pattern, which here consists of a subdorsal line with ring-spots more or less numerous and developed; the skin also is rough, “shagreened,” although it must be admitted that there are exceptions (_Vespertilio_). The genus _Chærocampa_ admits also of being based on the form-relationship of its caterpillars, although this is certainly only possible by disregarding the marking and taking alone into consideration the peculiar pig-like form of the larvæ. The genus _Acherontia_, so nearly related to _Sphinx_, possesses in the doubly curved caudal horn a character common to the genus (three species known[189]). Finally may be mentioned the genus _Smerinthus_, of which the larvæ, by their anteriorly tapering form, their shagreened skin and almost triangular head with the apex upwards, their simply curved anal horn, and by their seven oblique stripes on each side, constitute a genus as sharply defined as that formed by the moths. Although in all the systematic divisions hitherto treated of there are cases where the form-relationship of the larva does not completely coincide with that of the imago, such incongruences are of far more frequent occurrence in the smallest systematic group, viz. species. The larvæ of two species have very frequently a much nearer form-relationship than their imagines. Thus, the caterpillars of _Smerinthus Ocellatus_ and _S. Populi_ are closely allied in structure, marking, and colouring, whilst the moths in these two last characters and in the form of the wings are widely separated.[190] Judging from the larvæ we should expect to obtain two very similar moths, but in fact both _Populi_ and _Ocellatus_ have many near allies, and these closely related species sometimes possess larvæ which differ more widely than those of more distantly related species of imagines.