Acridiidae possess structures for the production of sound, together with others that are, no doubt, for hearing. The chirping of grasshoppers is accomplished by rubbing together the outer face of the upper wing and the inner face of the hind femur. This latter part bears a series of small bead-like prominences placed on the upper of the two lower ridges that run along the side that is nearest to the body (Fig. 170); the tegmen or wing-case has projecting veins, one of which is slightly more prominent, and has a sharp edge; by scraping this edge over the beads of the femur the wing is thrown into a state of vibration and a musical sound is produced. The apparatus for producing sound was for long supposed to be confined to the male sex of grasshoppers; it was indeed known that females made the movements appropriate for producing music, but as they appeared to be destitute of instruments, and as no sound was known to follow from their efforts, it was concluded that these were merely imitative. Graber has, however, discovered[[216]] that rudimentary musical organs do exist in the females of various species of Stenobothrus (Fig. 171, B). It is true that in comparison with those of the male (Fig. 171, A) they are minute, but it would appear that they are really phonetic, though we can hear no sounds resulting from their use.
Fig. 171.—A, Some of the knobs projecting from the surface of the femur of Stenobothrus melanopterus, male; B, same of the female. Highly magnified. (After Graber.)
Graber considers that the musical pegs of Acridiidae are modified hairs, and he states that in certain females the stages intermediate between hair and peg can be found. There is apparently much variety in the structure of these instruments in different species, and even in individuals of the same species. In Stenobothrus lineatus, instead of pegs, the instrument consists of raised folds.
In some of the aberrant forms of Acridiidae—certain Eremobiides and Pneumorides—the males are provided with sound-producing instruments different to those we have described, both as regards situation and structure.
Fig. 172.—Middle of body of Pachytylus nigrofasciatus, to show tympanum, e. (After Brunner.)
Fig. 173.—Mecostethus grossus: A, Insect with wings expanded; B, profile of head and prothorax. (After Brunner.)
If the dorsal aspect of the first segment of the hind body of an Acridian Insect be carefully examined there may be seen in the majority of species an organ which has somewhat the appearance of an ear (Fig. 172), and which there is great reason for believing to be really an organ of that nature. It is situate a little over the articulation of the hind leg, very close to the spot where the sound is, as above described, produced. There are three forms of these Acridian ears as described by Brunner:[[217]] (1) a membrane surrounded by a rim; (2) the membrane somewhat depressed, a portion of the segment projecting a little over it; (3) the depression very strongly marked, and the sides projecting over it so much that all that is seen externally is a sort of broad slit with a cavity beneath it. This last is the condition in which the ear exists in the genera Mecostethus (Fig. 173) and Stenobothrus, which are among our few native grasshoppers. On minute examination this ear proves to consist of a tympanum supplied internally with nerve and ganglion in addition to muscles, and tracheal apparatus of a complex nature; it is no doubt delicately sensitive to some forms of vibration. Unlike the stridulating organ, these ears exist in both sexes; they are found in a great majority of the species of Acridiidae. The forms in which the ears are absent are usually at the same time wingless and destitute of organs of stridulation; but, on the other hand, there are species—some of them wingless—that are, so far as is known, incapable of stridulation and yet possess these ears.