It is, however, only in the males that these odours can be detected, and, though palpable enough to human nostrils, their power of diffusion is apparently extremely limited. They would seem to serve as aphrodisiacs for the stimulation of the female, and, as a consequence, there is no need that they should start into activity until the male has arrived at the immediate neighbourhood of his prospective mate. [Illustration: Plate 31. BRIGHT COLOURS WHICH CANNOT UK ATTRIBUTED TO “SEXUAL SELECTION.” 1. Eyed Hawk-moth, under the influence of excitement. 2, A Butterfly, Zeuxidia horsfieldi, Feld, showing tufts of scent-diffusing scales on the hind-wings. Face page 200.] With the females of the Moths, however, matters are otherwise. For the most part Moths are nocturnal, and hence could not distinguish one another when on the search for mates, and in many species the females are wingless, and consequently are unable to move from the immediate neighbourhood in which they emerged from the pupal stage. In either case some means of informing the males of the presence of females is an imperative necessity for the continuation of the race. This is provided by means of a subtle odour exhaled by the females which, though imperceptible to human nostrils, must possess an extraordinarily penetrating power. Weismann gives an instance of this in the case of the nocturnal Eyed Hawk-moth (_Smerinthus ocellatus_). He placed some females, without any special intention, in a covered vessel near an open window. “The very next morning several males had gathered, and were sitting on the window-sill, or on the wall of the room close to the vessel, and by continuing the experiment I caught, in the course of nine nights, no fewer than forty-two males of this species, which I had never believed to be so numerous in the gardens of the town....” To this power of exhaling odours we may attribute the wingless condition of many Moths, for otherwise the loss of flight would have brought about extinction long before any perceptible reduction in the wings had taken place. The odour which such prisoners emit seems to possess an irresistible attractiveness, and this fact is commonly taken advantage of by entomologists. The Common Vapourer Moth (_Orgyia antiqua_) affords a good illustration of this. The female is wingless, and little more than a pouch for eggs, but in certain seasons it is very abundant, even in the midst of London. That experienced entomologist Prof. Selwyn Image, in a letter to my friend Mr. John Cooke, remarks, on this theme, that the Caterpillars may be seen crawling by hundreds in and around the squares, while the males may be seen flying up and down New Oxford Street or Tottenham Court Road. If a virgin female be put in a box placed outside the window, within a very short space of time, often not more than a few minutes, several males will be fluttering round her. This device for attracting males is commonly known as “assembling.”