Fig. 387.
A. Vertical section of Eye of Melolontha vulgans, Cockchafer; B. A few facets more highly magnified, showing facets and pigment layer.
In the accompanying [Fig. 387], A is a vertical section of the eye of Melolontha vulgans, the fan-like arrangement of the facets, together with the transparent pyramidal gathering of the retinal rods proceeding towards the brain; B is a few of the corneal tubes more highly magnified, the darker portion representing the pigment layer of the corneal tubes. In [Plate VI]., No. 133, the under surface of the head and mouth of the “Tsetse” fly, Glossina morsitans, is shown. The proboscis of this fly is long and prominent, and the antennæ are peculiar, inasmuch as the third segment is long, and produced almost as far as the flagellum, which is furnished with barbed hairs along its outer surface only. Although this fly barely equals the blow-fly in size, it is one of the greatest pests to the domestic cattle of Equatorial Africa. The palpi, although arising from two roots, are seen joined together when the fly is at rest, but when in the act of piercing or sucking they divide and the sheath is thrown directly upwards. The palpi are furnished on their convex sides with long and sharply-pointed dark-brown setæ or hairs, while the inner concave sides, which are brought into contact with the proboscis, are perfectly smooth and fleshy. Three circular openings seem to indicate the tubular nature of what in the house-fly is a fleshy, expanded, and highly-developed muscular proboscis (seen in [Fig. 388], Musca domestica). The proboscis (labium) forms the chief part of the organ, dilates into wonderful muscular lips, and enables the insect to employ the tongue as a prehensile organ. The lips are covered with rows of minute setæ, directed a little backwards and arranged rather closely together.
Fig. 388.—Proboscis of House-fly, Musca domestica. (The small circle indicates the object about the natural size.)
There are very many rows of these minute hairs on each of the lips, and from being arranged in a similar direction are employed by the insect in scraping or tearing delicate surfaces. These hairs are tests for the best of high powers. It is by means of these that it teases human beings in the heat of summer, when it alights on the hand or face, to sip the perspiration as it exudes from the skin. The fluid ascends the proboscis, partly by a sucking action, assisted by the muscles of the lips themselves, which are of a spiral form, arranged around a highly elastic, tendinous, and ligamentous structure, with other retractile additions for rapidity and facility of motion.
Fig. 389.—Spiral structure of Tongue of House-fly, from a micro-photograph made with a Zeiss 16 mm. and apochromatic projection eye-piece × 150.
The beautiful form of the spiral structure of the tongue should be viewed under a high magnifying power, when it will be seen that no continuing spiral structure really exists; each ring, apparently detached, does not extend quite round; their action is that of sucking tubes. Fluids are evidently drawn up through the entire fissure caused by the opening between the ends of the whole series of rings. It may well be pronounced a marvellous structure. The mounting of the tongue must be done with a considerable amount of care to show this structure, imperfectly represented in my woodcut.
These insects are of some service in the economy of nature, by their consumption of decaying animal matter, found about in quantities ordinarily imperceptible to most people, and that would not be removed by ordinary means during hot weather. It was asserted by Linnæus that three flies would consume a dead horse as quickly as a lion. This was, of course, said with reference to the offspring of such three flies; and it is quite possible the assertion may be correct, since the young begin to eat as soon as hatched, and a female blow-fly will produce twenty thousand living larvæ (one of which is represented in [Plate VI]., No. 141). In twenty-four hours, each will have increased in weight two hundred times, in five days it attains to its full size, and changes into the pupa, and then to the perfect insect.