Acridiidae are remarkable amongst the Orthoptera for the possession of air sacs or vesicular dilatations in the interior of the Insect in connexion with the tracheae (Fig. 176). Such vesicles are found in many of the higher winged Insects, but not in larval forms, or in those that are destitute of powers of flight.[211] They, no doubt, assist the Insect in its movements in the air. The body of a large grasshopper or locust is naturally of considerable weight, and it is more than probable that true flight can only be accomplished when these vesicles are dilated and filled with air. The exact mode in which the sacs are dilated is not known; possibly it may be accomplished by the elasticity of the structure of the vesicles coming into action when the other contents of the {283}body are not completely developed, or are temporarily diminished. Although air vessels are absent in the neighbouring groups of Orthoptera, Dufour says they are present even in apterous forms of Acridiidae, but he gives no particulars.[212] Packard has given an account[213] of the arrangement of these remarkable sacs in the Rocky Mountain Locust. He finds that there are two sets: a thoracic group, consisting of a pair of very large size, with which are connected some smaller sacs placed in the head; and an abdominal set, which forms a very remarkable series. The figures we give (Fig. 176, A, B) show that these sacs are of such large size that if fully distended they must interfere with the development of the ovaries, and that they must be themselves greatly diminished, if not obliterated, by the distension of the alimentary canal. We may look on them as only coming into full play when the normal distension of the canal is prevented, and there is only small development of the reproductive organs. Under such circumstances the locust becomes a sort of balloon, and migrates. In addition to the air sacs there are many dilatable tracheae, placed chiefly in parts of the body where there is not space for the large air sacs. These are, for the sake of clearness, omitted from our figure. The ganglia constituting the brain are simpler in Acridiidae than they are in the higher Insects, such as bees and wasps, and have been specially studied by Packard[214] and Viallanes.[215] The other ganglia of the nervous cord are eight in number, three thoracic and five abdominal. [Illustration: FIG. 169.—Ovaries of _Oedipoda caerulescens_: _a_, calyx; _b_, its gut-like appendage; _c_, sebific gland; _d_, termination of body. (After Dufour.)]