True Tales of the Insects
Anatomy.
In dealing with a family so active as a rule, so famous for the extraordinary movements of some of its members, one turns naturally to investigate their instincts and general intelligence. But it is hard for us to appreciate the intelligence of insects. It depends in them, of course, largely on the development of the organs of special sense; and the study of sensations, if one of the most fascinating, is at the same time one of the most difficult of the departments of entomology. Recent research has resulted in much definite knowledofe of the structure of the sense-organs, but there is a great lack of
experimental basis for conclusions as to the functions of the various organs described. However, there is a great variation in the degree of perfection of the different senses in different insects.
The sense of sight must be well developed in the Acridiidse, for they are furnished with two large well-developed compound eyes, and three simple ones (ocelli), supplied with nerves of special sense. The compound eyes are situate at the sides of the head; in front are the ocelli, one on each side between the eye and the insertion of the antenna; the third ocellus being in the middle, immediately in front of the base of the antennas. This family is one of the large groups of insects in which the coexistence of the two kinds of eyes is most constant. The ocelli, however, vary much in their development, being in some cases prominent and easily perceived, while in others they are very imperfect, difficult to detect, apparently functionally useless. In what way the insects specially need two sorts of eyes, is not clear. Later on we shall see that a similar condition in regard to sensitiveness to sound is believed to exist in this family. The antennae are organs of touch; and the palpi not only serve the same function, but probably are endowed with the sense of taste.
It need hardly be said an insect breathes by means of a complicated system of air-tubes or trachece, ramifying through every part of its structure, and communicating
with the air by a row of spiracles, or air-holes, or breathing-holes (stigmata), in the sides of the body. Acridiidae are remarkable amongst the Orthoptera for the possession of air-sacs or vesicular dilatations in the interior of the body in connection with the tracheae. Many winged insects possess such vesicles. In bees, wasps, moths and butterflies, flies, and in some beetles, and some bugs, they are found well developed, being most numerous and capacious in volant insects which sustain the longest and most powerful flight. But in the larval or immature forms of these they do not exist, nor do they occur in truly apterous insects. In the flying locusts they are as numerous and as large as in any group, more numerous perhaps than in the bee in proportion to the body, while certainly there is a greater number of large sacs, and both sexes are equally well provided. Packard describes the distribution of these remarkable elastic sacs in the Rocky Mountain Locust. There is a thoracic set, consisting of a pair of very large size, with which are connected some smaller ones in the head; and an abdominal set. The last are very extraordinary, being of such large size that they may be regarded as only becoming fully expanded at a time when the body being comparatively empty, food being wanting, there is no normal distention of the alimentary canal, and the other contents of the body are as yet undev^eloped. Although according to one observer they
are present even in apterous forms of AcridiidDe, distinct vesicles are absent in the neighbouring groups of Orthoptera. Those Orthoptera which do not take long flights have no need of air-vessels.
No doubt they are connected with the power of flight; doubtless they assist the insect in its aerial movements. The body of a large grasshopper or locust is naturally of considerable weiofht, and it seems certain true flight can only be effected when the sacs are dilated and filled with air. That by filling and partially emptying them during the process of breathing the insect is enabled to enlarge its bulk and alter its specific gravity at pleasure, so as to render itself capable of rising and supporting itself on the wing with little muscular eftbrt, was first assumed by Sir John Hunter; and inference and observation confirm the view that the use of the vesicles is to lighten the body, to float up the insect in the air. The precise mode in which they are dilated is not understood. With its sacs in full play, plainly a locust becomes an aeronaut, a sort of balloon.
To this fact are largely due the enormous powers of flight possessed by these insects; the intimate association of the complex arrangement of air-tubes and air-sacs with the powers of flight is very evident. It will be seen that, once having risen from the ground, the insect can sail for hours in the air, constantly filling and re-filling its internal buoys or balloons, and thus be wafted
straight on its course for miles by favouring; winds. Meanwhile, scarcely as much muscular force is spent through the day as is exerted during a few vigorous hops. Towards evening, and in damp and cloudy weather, the powers of flight are lessened, owing to the diminished power of respiration. In the possession and use of these air-sacs, locusts may be compared with
birds. When in addition to the sacs we find many expanded or dilatable tracheae, chiefly in parts of the body where there is not room for the sacs themselves, w^e can duly estimate the wondrous powers of the locust as an aeronaut. Intellectually the Acridiidce appear to be the equals of most other insects, while many are inferiorly endowed. Those that excel them in this respect are the ants and bees or wasps, the social hymenoptera, which have a brain constructed on a higher, more complicated, plan than in the other winged insects. It must be understood that the word " brain " so applied is a mere courtesy term, as the brain of our insects does not correspond to the brain of a vertebrate animal. It consists of a double ganglion, placed in the upper part of the head, the first and largest of the two ganglia therein. Moreover, it Is a much more complicated organ than any of the others, having parts which are wanting in them, hence it Is par excellence nearer to our Idea
of a brain than any of the other gangha. The other ganglia are eight, three thoracic and five abdominal. This series of nerve-centres, connected by nervous cords, together constitute the nervous system.
The Gift of Song,
If the Acridiida; are remarkable amongst the Orthoptera for their air-sacs, they are no less interesting on account of another wondrous possession, which they
giftshare in common with the other Saltatoria—their of song. Singers in the true sense they are not; they should rather be called minstrels; for, strictly speaking, owing to their peculiar mode of breathing, they have nothing that corresponds to our voice, and it may be accepted, as Aristotle expressed long ago, " that no living creature hath any voice but such only as are furnished with lungs and windpipes." While less famed as instrumentalists than the neighbouring Saltatoria, they are not without their resources in musical powers, and there is evidence that they are of great importance to the creatures, though it is hard to state definitely in what way. In some of the aberrant forms of Acridiidse, some parts of the structures of the body are clearly subservient to the musical organs, and have every appearance of being specially directed to securing their
efficiency. The situation and structure of these sound-producincr instruments we will pass over for the present.
The stridulation, or " song," is mainly accomplished by rubbing" together the inner surface of the hind legs and the outer surface of the tegmina or wing-covers. These latter parts have considerably elevated or projecting veins, one of which is slightly more thickened, and has a sharp rasp-like edge; the inner face of the hind thighs or femora carries a series of small bead-like prominences; by scraping these upon the rasp-like surface of the veins of the winQfs the winQf is thrown into a state of vibration, and a musical, monotonous, nearly uniform sound is produced. A Stenobothrus, when about to stridulate, plants itself in a nearly horizontal position, with the head a little elevated; the shanks of the hind legs he is apt to draw close within a groove beneath the thigh, evidently made to receive it. The legs are now raised and lowered with a more or less reo'ular and continued motion, the thio-hs beino- prated against the firm edge of the tegmina. The wings are the responsive instrument, the viol, to which the leg performs the office of bow in this musical performance. Every movement of the fiddle-bow produces a note. the notes varying in rapidity, number, and duration in different species. Few are aware that every kind of grasshopper has its distinctive note; a practised ear can distinguish the song of even closely allied species.
Scudder, who has given much attention to the subject of Orthopteran music, says in North America " the uniformity with which each species of Stenobothrus plays its own song is quite remarkable." One species, S. ciirtipemiis, makes about six notes per second, and continues them for one and a half to two and a half seconds; another, kS. melanopleurus, produces from nine to twelve notes in about three seconds. In both cases the notes follow each other uniformly, and the movements are less rapid in the shade than in the sun. Scudder has even reduced the notes of several species to a written music. This stridulation of grasshoppers is specially characteristic of the male. Yet it is not always an attribute with them of the male only. It was for long supposed that the males alone sang, that they alone were endowed with the musical apparatus. Females were indeed perceived rubbing their thighs and wing-covers together, but as they appeared to be destitute of instruments, and as no sound resulted from their efforts, it was con-cluded that these were merely imitative. It is, however, discovered that musical orgrans do exist in the females of various species of Stenobothrus, Doubtless they are rudimentary as compared with those of the males, but they are believed to be really phonetic, although the appropriate movement produces no sound perceptible to our ears.
The musical knobs of Acridiidse would seem to be modified hairs, and Graber mentions the finding in females of the stages intermediate between knob and hair. Much variety exists in the structure of these instruments in different species; in StenohothriLS lincatiis raised folds replace the musical pegs.
Some grasshoppers stridulate during flight, by the friction of the winofs and winQf-covers. Whether this has any real importance remains almost unconsidered. It appears to be under the control of the insects, for they often omit it when alarmed. Some species produce a uniform noise during the whole of their undeviating flight; others make it only during the Intervals of their course, and seem to stridulate more at will.
Acridiau Ears.
If the body of an Acridian insect be carefully examined, there will be found in the majority of the species, on each side of the first abdominal segment, an organ which there is much reason for believing to be of the nature of an ear. It is situated a little over the articulation of the hind leg, close to the spot where the sound is, as above described, produced, and just behind the first abdominal spiracle. These ears vary in form, but consist of a stretched or tense membrane. The membrane may be