THE ALLURING OR SCENT-GLANDS
It is difficult to draw the line between repelling and alluring glands. Attention was first definitely called to the alluring odors of Lepidoptera by Fritz Müller, who showed that the males of certain butterflies are rendered attractive to the other sex by secreting odorous oils of the ether series. He pointed out that the seat of the odor is the androconia (see p. 199), while either repellent or pleasant odors are exhaled from abdominal glands.
Those of Pieris napi yield a scent like that of citrons, Didonis biblis gives off three different odors from different parts of the body, besides having a distinctly odorous spot on the hind wings. Both sexes have a sac between the fourth and fifth abdominal segments which exhales a very unpleasant (protective) odor, while the males have on the succeeding segment a pair of glands from which proceeds an agreeable odor like that of the heliotrope. Callidryas argante throws off a musky odor. In Prepona laertes the odor is like that of a bat, in Dircenna xantho it is vanilla-like, the androconia being situated on the front edge of the hind wings. In Papilio grayi the odor is said to be as agreeable and intense as in flowers. Certain sphingids are known to exhale a distinct odor, which Müller has traced to a tuft of hair-like scales at the base of the abdomen, and which fits into a groove in the first segment, so as to be ordinarily invisible.
Fig. 367.—Scent-tufts: 1, of Leucarctia acræa; 2, of Pyrrarctia isabella.—After Smith.
In the noctuid genus, Patula, the costal half of the hind wing is modified to form a large scent-gland, and in consequence the venation has been modified. The still greater distortion of the veins in the allied genus, Argida, was attributed by the author to its once having possessed a similar scent-gland, now become rudimentary by disuse. (Hampson.)
Peculiar white or orange-colored, hairy, thread-like processes have been found protruding from narrow openings near the tip of the abdomen of Arctian moths (Fig. 367), which throw off, according to J. B. Smith, “an intense odor, somewhat like the smell of laudanum.” We have perceived the same unpleasant odor emanating from the males of Spilosoma virginica and Arctia virgo, as well as Leucarctia acræa.
We are informed by C. Dury that similar but longer hairy appendages are thrust out by the male of Haploa clymene. Many glaucopid moths protrude similar glandular processes. Thus Müller tells us that on seizing a glaucopid female by the wings, nearly the whole body became enveloped in a large cloud of snow-white wool which came out of a sort of pouch on the ventral side of the abdomen.
The male of a glaucopid was seen to dart out a pair of long hollow hairy retractile filaments which in some species exceed the whole body in length. The apparatus secretes a peculiar odor, probably serving to allure the female (Nature), and certain Zygænidæ have on the inner side of the paranal lobes (Afterklappen) glands filled with a sweetly scented fluid. Smith has detected a peculiar brush of hair-like scales in a groove between the dorsal and ventral parts of the basal two segments of the abdomen of Schinia marginata (family Noctuidæ), and when removed it exhaled a laudanum-like smell.
The pupa of Citheronia regalis gives out from the end of the abdomen a scent reminding us of laudanum.
Fig. 368.—Scent-tufts on middle legs of Catocala concumbens.—After Bailey.
Another mode of disseminating pleasant, alluring odors is that of the males of certain moths, which bear pencils and tufts on their fore or hind legs, and in the case of an Indian butterfly on the greatly elongated palpi. Those on the legs are ordinarily concealed in cavities or furrows in the leg, and may be thrust out and expanded so as to widely diffuse their odor. Such are those of the males of Catocala (Fig. 368), which resemble an artist’s fitch brush. In Hepialus hecta, where the arrangements for protecting the tufts are quite abnormal, Bertkau has detected the cells which secrete the odorous fluid. In the male of another Hepialus (H. humuli) a peculiar scent proceeds from the curiously aborted and altered hind tibiæ. (Barrett.) In one case, that of a geometrid moth (Bapata dichroa of New Guinea), these pencils occur on all the legs. (Haase). In many species a distinct odor is perceptible when the leg bearing the pencil or tuft is crushed.
These eversible scent-glands have been supposed to be mostly restricted to the Lepidoptera, and to a single known case in the Trichoptera, but similar alluring male glands also occur in the Orthoptera (Locustidæ). H. Garman has described and figured in the cave cricket (Hadenœcus subterraneus) “a pair of white fleshy appendages protruding from slits between the terga of the 9th and 10th abdominal somites, the nature of which is not clear,” adding, “the slits through which the organs appear are situated one on each side anterior to and a little within the cerci. When fully protruded, the glands are white, cylindrical, a little tapering, and are about one-eighth of an inch long.” He believes that they are protruded during the period of sexual excitement, and suggests that “the sense of smell is certainly the one best calculated to bring the sexes together in the darkness of caves.” We had previously noticed these organs in alcoholic specimens, but supposed that they were fungous growths. On dissecting and making microscopic sections of them, the gland is, when extended (Fig. 369), seen to be a long, ensiform, sharp, band-like process, with numerous retractor muscular fibres. When at rest each gland is folded about five times, forming a bundle lying on each side of the end of the intestine. The walls are formed of a single layer of epithelium, as seen in Fig. 369, B.
Fig. 369.—Eversible scent-glands (a) of Hadenœcus, nat. size: Kingsley, del.; A, a gland outstretched, with the retractor muscular fibres; t, part of the tergite. B, section of the gland, showing the single layer of epithelial cells, and the muscular fibres (m).—Author del.
In the male of the common wingless cricket, Ceuthophilus maculatus, we have discovered what appears to be a pair of scent-glands lying directly over the last abdominal ganglion. They form two large white sacs situated close together, with a short common duct which passes back and opens externally upwards by a transverse slit on the under side of the last segment of the body.