Among the Lepidoptera these constitute important secondary sexual characters, the antennæ, among the Moths at any rate, presenting striking differences in male and female. The scent-producing organs are very elusive structures, and so far have been definitely traced, among Butterflies, only in the males, where they are formed by certain peculiarly modified scales known as “androconia.” They may be either irregularly scattered over the wing, or may form complex structures. Sometimes they are arranged in the form of brightly-coloured, bristle-like tufts on the hind-wings, sometimes in a fringe along the edge of the hind-wing. In some of the Moths they are arranged to form a thick, glistening white felt, which fills a folded-over portion of the edge of the hind-wing, and in many cases “the perfume can be retained,” Weismann remarks, “and then, by a sudden turning out of the wing-fold, be allowed to stream forth.” In the Ghost-moth (Hepialus humuli), the hind-legs of the male have become pressed into service and have become transformed into scent-bottles, since they are swollen and filled with glands for the manufacture of odorous matter.
The naturalist Fritz Müller discovered the fact that some of the Butterflies which haunted his Brazilian garden exhaled a flower-like fragrance. Anyone can test this curious trait for himself who will take the trouble to brush his finger over the wing of a newly-caught male Garden-White Butterfly (Pieris napi). The white powder which will adhere to the finger will be found to be made up of the wing-scales, which will exhale a delicate perfume of lemon or balsam! Among the Moths the strong odour of musk is exhaled by the Convolvulus Hawk-moth (Sphinx convolvuli).
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.
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.”