Wings of Insects.—These exhibit variety of form and structure, as well as of beauty of colouring. At an early period the orders of insects were mainly founded upon these interesting appendages. The Orthoptera were the straight wings; the Neuroptera the nerved; the Trichoptera the hairy wings; the Coleoptera the cased or sheathed wings; the Diptera the two wings; the Hymenoptera the married wings; and the Lepidoptera the scaled wings. A number of wings are small and membranous, and may be mounted dry for examination under the microscope. Others are better seen mounted in benzol-balsam. The elytra, iridescent wing cases of the diamond, and other beetles, as well as the wings of the more highly coloured butterflies, make pretty objects mounted dry for opaque illumination by the Lieberkühn or reflector. The thicker horny cases of other members of the beetle tribe require long soaking, as described in a former chapter.
The wings of moths and butterflies are covered with scales or feathers, carefully overlapping each other, as tiles are made to cover the tops of houses. The iridescent variety of colouring on insects’ wings arises from the peculiar wavy arrangement of the scales. Figs. 404 and 405 are magnified representations of a few of them. No. 1, a scale of the Morpho menelaus, taken from the side of the wing, is of a pale-blue colour; it measures about 1⁄120th of an inch in length, and exhibits a series of longitudinal striæ or lines, between which are disposed cross-lines or other striæ, giving it very much an appearance of brick-work (better seen in [Fig. 405], No. 1).
Fig. 405.—Portions of Scales, magnified 500 diameters.
1. Portion of scale of Morpho menelaus; 2. Portion of large scale of Podura plumbea; 3. Scale from the wing of Gnat, its two layers being represented; 4. Portion of a large scale of Lepisma Saccharina; 5. Body scale of Gnat, magnified 650 diameters.
Polyommatus argiolus, azure-blue ([Fig. 404], Nos. 2 and 6), are large and small scales taken from the under-side of the wing of this beautiful blue butterfly; the small scale is covered with a series of spots, and exhibits both longitudinal and transverse striæ, these should be clearly defined, and the spots separated by a quarter-inch object-glass. No. 3, Hipparchia janira, is a scale from the meadow-brown butterfly: on this brown spots, having an irregular shape with longitudinal striæ, are seen. No. 4, Pontia brassica, cabbage butterfly, was at one time taken to be an excellent criterion of the penetration and definition of an object glass. It is seen to have a free extremity or brush-like appendage. With a fairly good power, the longitudinal markings appear like rows of small beads. Chevalier selected for his test object the scale of the Pontia brassica. Mohl and Schacht extolled Hipparchia janira as a good test of penetration in an objective of moderate angular aperture. Amici’s test object is Navicula rhomboides, the display of the lines forming the test.
Fig. 406.—Podura villosa, male and female, highly magnified.
The Tinea vestianella, clothes-moth, is furnished with unique scales. Small and destructive as this moth is, it suffers much from a parasitic mite, and from which it is unable to free itself.
The Podura scale (Fig. 405), with its delicate transparent membrane and curiously inserted “notes of admiration,” as they were called, was long believed to be an excellent test object for the highest powers of the microscope, but I believe it is no longer regarded in that light: indeed, most insect scales have declined in the value and estimation of the skilled microscopist. This is in part due to the improvements made in the objective. The high-angled glasses have cleared up obscure points in the structural characters of the minuter forms of life, and the scales of insects are no longer found to be difficult test objects for the modern objective of a Zeiss or a Powell to resolve. Nevertheless, the scale of the Podura belonging to the order Thysanura, a curious little insect commonly known by the name of springtail, usually found living in most obscure places, and too small to attract attention, is not likely to be entirely thrust aside. The springtails (Collembola) are furnished on the under-side of the first abdominal segment with a curious tube or sucker, from the orifice of which glandular process a secreted viscid matter is protruded; they are remarkable also from the fact that in most of them no trace of a tracheal system has yet been discovered. The eyes when present are in the form of simple or grouped ocelli, the antennæ number six joints, and the abdomen has but six segments, often only three. The forked tail is a curious process turned forward and attached to one of the tender segments and held in position under the body; when released it springs back and bounds up to a very considerable height. [Fig. 406] represents Podura villosa. There are several species, one of which (P. aquatica) is found floating in patches on pools of water on bright summer days.