Fig. 168.—Hind leg of Porthetis sp. Transvaal.
In reference to the ocelli, which are shown in Fig. 166, we may remark that the Acridiidae is one of the large groups of Insects in which the coexistence of compound and single eyes is most constant, though in some of the wingless forms the ocelli are very imperfect. We know at present of nothing in the habits of Acridiidae to render two kinds of eyes specially necessary. We shall subsequently see that a similar condition in regard to the function of hearing is believed to exist in this family.
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 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.
Fig. 169.—Ovaries of Oedipoda caerulescens: a, calyx; b, its gut-like appendage; c, sebific gland; d, termination of body. (After Dufour.)
The salivary glands are small. The alimentary canal is capacious but not coiled. It has no gizzard, but the crop has a peculiar structure, apparently as a substitute. There are diverticula connected with the true stomach. The Malpighian tubes are elongate and extremely numerous. The pair of testes is united in a single envelope. The form and arrangement of the ovaries is remarkable (Fig. 169); the egg-tubes are united by the convergence of their terminal threads into a single mass; outside of each ovary there extends a large calyx, into which the tubes open; each calyx is prolonged at its extremity, and forms a long, convoluted tube.
Fig. 170.—Inner face of femur of Stenobothrus, male, showing line, a-a, of musical beads. (After Landois, magnified three times.)