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324 nine. In the females it frequently terminates in a tubular ovipositor, the joints of which are retractile within each other. The legs are generally long and slender, the articulations of the tarsi always five in number. The terminal joint, or that which bears the claws, is often provided with two or three membranous lobes, by the aid of which the fly is enabled to walk on glass and other smooth surfaces against gravity. This it was long supposed to do by the pressure of the atmosphere, the lobes in question acting as suckers and forming a vacuum. It has been recently conjectured, however, in opposition to this view, that it is accomplished by means of a glutinous secretion. The larvæ of dipterous insects are in some respects even more peculiar than the mature fly. They are generally of a conical shape, the head being the narrowest part, and in all cases destitute of feet. The head is small, retractile, and variable in form even in the same individual,—that is to say, it is composed of a comparative soft fleshy substance which the insect can modify in shape at pleasure, to answer its various purposes. The colour is generally pale, but sometimes it is dark, and even bright red. The stigmata, in the species not aquatic, are most commonly placed in a cavity in the hinder segment of the body, which is capable of closing over them, so as to preserve them from being closed up by the fluid and putrid substances among which the larvæ often live. The breathing apparatus of the aquatic larvæ is often very singular, consisting of appendages of various