The squamæ.—In the calyptrate Muscidæ, a large scale-like membranous broad orbicular whitish process is situated beneath the base of the wing, above the halter; (Fig. 94, 10 sq.) it is either small or wanting in the acalyptrate muscids. Kirby and Spence state that when the insect is at rest the two divisions of this double lobe are folded over each other, but are extended during flight. Their exact use is unknown. Kolbe, following other German authors, considers the term squama as applicable to the whole structure, restricting the term alula to the other lobe-like division.
More recently (1890 and 1897) Osten-Sacken recommends “squamæ; in the plural, as a designation for both of these organs taken together; squama, in the singular, would mean the posterior squama alone, and antisquama the anterior squama alone;” the strip of membrane running in some cases between them, or connecting the squama with the scutellum, should be called the post-alar membrane. By a mistake Loew, and others following him, used the word tegula for squama, but this term should be restricted to the sclerite of the mesothorax previously so designated (Fig. 90, A, t). The squama or its two subdivisions has also by various authors been termed alula, calypta, squamula, lobulus, axillary lobe, aileron, cuilleron, schuppen, and scale. (Berlin Ent. Zeitschrift, xli, 1896, pp. 285–288, 328, 338.)
The halteres.—In the Diptera the hind wings are modified to form the halteres or balancers, which are present in all the species, even in Nycteribia, but are absent in Braula.
Meinert finds structures in the Lepidoptera which he considers as the homologues of the halteres of Diptera. “In the Noctuidæ,” he remarks, “I find arising from the fourth thoracic segment (segment médiaire), but covered by hair, an organ like the halter of Diptera.” (Ent. Tidskrift., i, 1880, p. 168.) He gives no details.
In the Stylopidæ, on the contrary, the fore wings are reduced to little narrow pads, while the hind wings are of great size.
The thyridium is a whitish spot marking a break in the cubital vein of the fore wing of Trichoptera; these minute thyridia occur in the fore wings of the saw-flies; there is also an intercostal thyridium on the costal part of the wings of Dermaptera.
The fore wings of Orthoptera are thicker than the hinder ones, and serve to protect the hind-body when the wings are folded; they are sometimes called tegmina. It is noteworthy, that, according to Scudder, in all the paleozoic cockroaches the fore wings (tegmina) were as distinctly veined as the hinder pair, “and could not in any sense be called coriaceous.” (Pretertiary Insects of N. A., p. 39.) Scudder also observes that in the paleozoic insects as a rule the fore and hind wings were similar in shape and venation, “heterogeneity making its appearance in mesozoic times.” In the heteropterous Hemiptera, also, the basal half of the fore wings is thick and coriaceous or parchment-like, and also protects the body when they are folded; these wings are called hemelytra. In the Dermaptera the small short fore wings are thickened and elytriform.
The elytra.—This thickening of the fore wings is carried out to its fullest extent in the fore wings of beetles, where they form the sheaths, shards, or elytra, under which the hind wings are folded. The indexed costal edge is called the epipleurum, being wide in the Tenebrionidæ. During flight “the elytra are opened so as to form an angle with the body and admit of the free play of the wings” (Kirby and Spence). In the running beetles (Carabidæ), also in the weevils and in many Ptinidae, the hind wings are wanting, through disuse, and often the elytra are firmly united, forming a single hard shell or case. The firmness of the elytra is due both to the thickness of the chitinous deposit and to the presence of minute chitinous rods or pillars connecting the upper and lower chitinous surfaces.
Fig. 139.—Longitudinal section through the edge of the elytrum of Lina ænea: gl, glands; r, reservoir; fb, fat-body; m, matrix; u, upper,—l, lower, lamella.—After Hoffbauer.