The twilight-flyers afford a distinct class to the night-flyers, and have several well-marked characteristics. These are termed hawk-moths, and have long, sharp, scythe-like wings. The death's-head moth, the largest and most interesting British species, belongs to this group. It seldom comes abroad before darkness has fallen, and is always conspicuous in its nocturnal flight. Linnæus, following his habitual system of nomenclature, placed this insect in the "sphinx" family on account of the form of its magnificent caterpillar, and gave it the specific name of _Atropos_, in allusion to the popular superstition. Atropos being, according to Hesiod, the one of the fates whose office it was to cut the thread of human life, spun by her sisters, Clotho and Lachesis. Modern entomologists have preserved the idea of Linnæus, giving to the new genus the name of _acherontia_--pertaining to Acheron, one of the streams which, in the Greek mythology, have to be passed before entering the infernal regions. A low, wailing sound which this insect emits has greatly added to the terror which its appearance inspires among ignorant rustics. The death's-head moth is a really splendid insect. Its stretched wings cover four and a half inches, and it is the largest of the British Lepidoptera. As is well known, it has its popular name from a marvellously good representation of a skull and crossbones upon the upper part of the thorax--a mark which has caused it to be an object of dread in every country which it inhabits. Fluttering at the window in the darkness, or entering the house by the open door, just after the close of twilight, it is considered a certain omen of death. Like the hoarse croak of the raven, and the "boding" hoot of the owl, the appearance of this moth is said to be followed by disease and death. The power possessed by the death's-head insect of emitting a shrill, creaking sound, is thought to be unique among the British Lepidoptera, and each time the strange sound is emitted, the whole body gives a convulsive sort of start. The insect can be induced to utter this strange note by being irritated.