I am acquainted with the caterpillars of nine European and one North American species, these differing in marking to such a wonderful extent that they appear to offer at first sight but little hope of being able to trace them to a common form. These ten species can be separated, according to their markings, into five groups, which I will briefly define before entering upon their ontogeny. The first group consists of three species, and comprises the commonest and most widely-ranging of all the European species, _Deilephila Euphorbiæ_, as well as _D. Dahlii_ from Sardinia and Corsica, and _D. Nicæa_, a species of very restricted range, which appears to occur only in one small district on the French coast of the Mediterranean. These three species agree in marking to the extent of their possessing in the adult form two rows of ring-spots on each side, whilst the subdorsal line is completely absent. The second group, consisting also of three species, shows a great resemblance to _Euphorbiæ_, but has only one row of ring-spots. It contains _D. Vespertilio_, _D. Galii_, and the Algerian _D. Mauritanica_. For the third group I only know one representative, _D. Livornica_, Esp., which possesses a single row of ring-spots connected by a subdorsal line. Another group is composed of _D. Zygophylli_, which occurs on the shores of the Caspian Sea, and the North American _D. Lineata_; these species possessing a strongly marked subdorsal line, associated with more or less distinct ring-spots, which I shall designate as “open rings,” because their black border does not intersect the subdorsal line, but has the form of an arch above and below it. In the last group, represented by _D. Hippophaës_, which occurs at the foot of the Alps (Wallis), and southward as far as Andalusia, there is only a broad subdorsal line, generally without any trace of a row of spots. The important differences of marking displayed by these five groups are not in any way accidental, but they represent different stages of phyletic development; or, in other words, the five groups are of different ages, the first (_Euphorbiæ_, &c.) being the youngest, and the last (_Hippophaës_) the oldest of the genus. According to their phyletic age, the groups follow each other in inverse order, the first being _Hippophaës_, the second that of _Zygophylli_, the third that of _Livornica_, the fourth that of _Galii_, and the fifth and youngest that of _Euphorbiæ_. Only in this last am I acquainted with the _complete_ development of one species, for which reason I commence with this group, thus proceeding from the youngest to the oldest forms, instead of taking the more natural course from the simplest and oldest to the youngest and most complicated. DEILEPHILA EUPHORBIÆ, LINN.