(All are wingless when young, and wingless adult forms occur in all the orders: order APTERA lacks wing-bearing thoracic structures.) A single pair of wings is characteristic of the order DIPTERA. A jointed beak, that is sheath-like, inclosing the other mouth parts, is characteristic of the order HEMIPTERA. A coiled sucking proboscis and a wing covering of dust-like microscopic scales are characteristic of the order LEPIDOPTERA. Horny sheath-like fore wings, covering the hind wings and meeting in a straight line down the middle of the back, will distinguish the order COLEOPTERA. Hind wings folded like a fan beneath the thickened and overlapping fore wings, will distinguish most members of the order ORTHOPTERA. The possession of a sting (in females) and of two pairs of thin membranous wings—the small hind wing hooked to the rear margin of the fore wing—will distinguish the common HYMENOPTERA. Besides these, there remain a number of groups most of which have in the past been included under the order NEUROPTERA, among which the Mayflies will be readily recognized by the lack of mouth parts and by the possession of two or three long tails; the dragon flies by the two pairs of large wings, enormous eyes, and minute bristle-like antennæ; the scorpion flies, by the possession of a rigid beak, with the mouth parts at its tip; the caddis flies, by their hairy wings and lack of jaws; the lace wings, by the exquisite regularity of the series of cross veins about the margin of their wings, etc. [Illustration: FIG. 139.—MAY FLY. What order (see table)? ] [Illustration: FIG. 140.—SILVER SCALE. (Order?) ] =Exercise in the Use of the Table or Key.=—Write the name of the order after each of the following names of insects:— Wasp (Fig. 122) Weevil (Fig. 163) Squash bug (Fig. 184) Ant lion (Fig. 170) Dragon fly (Fig. 177) Ichneumon fly (Fig. 159) House fly (Fig. 172) Flea (Fig. 173) Silver scale or earwig (Fig. 140) Codling moth (Fig. 141) Botfly (Fig. 138) =Moths and Butterflies.=—Order ____? Why ____ (p. 82)? The presence of scales on the wings is a never-failing test of a moth or a butterfly. The wings do not fold at all. They are so large and the legs so weak and delicate that the butterfly keeps its balance with difficulty when walking in the wind.