Fig. 191.—Development of ovipositor of Locusta viridissima: a, terminal segment; a′, cerci; a″, secondary division of terminal segment; b, penultimate (ninth) segment; b′, primary papillae of this segment; b″, secondary divisions thereof; c, eighth segment; c′, its papillae. (After Dewitz.) A, embryo ready for emergence; B, portion of integument of the ventral plates of eighth and ninth segments; C, the appendages in a condition somewhat more advanced than they are in A.
Fig. 192.—Structure of ovipositor of Locusta viridissima: A, arrangement of parts at base, c′ being separated and turned outwards; B, transverse section. The parts of the appendage bear the same lettering as in Fig. 191. (After Dewitz.)
Near to one another, on the middle of the ventral aspect of the true ninth abdominal segment, are seen the two papillae (b′), which at first are the only visible indications of the future ovipositor. If, however, the integument be taken off and carefully examined, it will be found that there exist on the eighth abdominal plate two spots, where there is a slight thickening and prominence of the integument (Fig. 191, B, c). From these two spots the two lower rods of the ovipositor are produced; these two, together with the two growths from the ninth segment, form the four external rods of the ovipositor. Inside these there exist in the completed structure two other rods (Fig. 192, B, b'′). These are produced by a growth from the inner parts of the two papillae of the ninth segment. The relations of the six rods in their early condition are shown in Fig. 191, C, where the two primary papillae b′ of the ninth segment are seen with their secondary offshoots b″; c′ being the papillae of the eighth segment. The subsequent relations of the pieces are shown in Fig. 192; A exhibiting the base of the organ with the lower rods turned on one side to show the others, the shaded parts indicating muscular attachments; B is a transverse section of the organ. In these figures the different parts of the appendages bear the same lettering as they do in Fig. 191. It will be seen that in the completed structures the parts c′ have become very intimately connected with the parts b′ and b″, which belong to another segment.
The Locustidae resemble the Acridiidae in the possession of specialised ears and sound-producing organs; neither of these is, however, situate in the same part of the body as in Acridiidae. The ears of Locustidae are placed on the front legs, below the knee; a tympanum (Fig. 193, A), or a crack giving entrance to a cavity in which the tympanum is placed (Fig. 193, B), being seen on each side of each of the anterior pair of limbs. In this family, as in the Acridiidae, three kinds of ear are recognised according to the condition of the tympanum, which is either exposed (Fig. 193, A) or closed by an overgrowth of the integument (Fig. 193, B), or in a condition to a certain extent different from either of these. The existence of ears placed on the legs is a curious fact, but it is beyond doubt in the Locustidae, and there is good reason for believing that analogous organs exist in this situation in other Insects that have special means of sound-production, such as the ants and the Termites.
Fig. 193.—Ears of Locustidae: A, portion of front leg of Odontura serricauda, adult; p, prominence of integument; r, rim of ear; T, tympanum; b, thickened area thereof; Fu, remains of groove in which the structure was developed. B, portion of front leg of Thamnotrizon apterus; i, inner margin; a, slit-like external aperture of ear; di, overlapping cover of the ear. (After Graber.)
The structure of these organs in the Locustidae has been investigated by Graber,[[247]] and their acoustic functions placed beyond doubt, though to what special kind of sounds they may be sensitive is not ascertained, this point being surrounded by even greater difficulties than those we have discussed in the case of the Acridiidae. In the Locustidae there is a special structure of a remarkable nature in connexion with the ears. In Acridiidae a stigma is placed close to the ear, and supplies the internal structures of the organ with air. There are no stigmata on the legs of Insects, consequently admission of air to the acoustic apparatus in Locustidae is effected by means of a gaping orifice at the back of the prothorax, just over the base of the front leg (Fig. 101); this communicates with its fellow of the other side, and from them there extend processes along the femora into the tibiae, where they undergo dilatation, so as to form vesicular cavities, one of which is in proximity to each drum of the ear. These leg-tracheae are not connected with the ordinary tracheal system; the prothoracic stigma exists in close proximity to the acoustic orifice we have described, but is much smaller than it. It is not yet clear why the acoustic apparatus should require a supply of air apart from that which could be afforded by the ordinary tracheal system. This special arrangement—to which there is hardly a parallel in Insect anatomy—has still to be accounted for; we do not know whether the necessity for it may be connected with the respiratory system or the acoustic organ.
