A type of chordotonal organ, observed in the body-segments of the larvæ of several insects by Leydig, Weismann, Graber, Grobben, and Bolles Lee, is to be seen in the transparent larva of Corethra (Fig. 296), where the auditory organ extends to the skin. It contains at the point cs two or three auditory rods. In the opposite direction a fine ligament (cl) passes from cg to the skin; in this way the auditory organ is suspended in a certain state of tension, and is favorably situated to receive even very fine vibrations. A similar apparatus has been detected in the larva of Ptychoptera.
Antennal auditory hairs.—It is not at all improbable that the antennæ of different insects contain auditory as well as olfactory structures. Lubbock has suggested that the singular organs which have only been found in the antennæ of ants and certain bees, and to which he gives the name of “Hicks’ bottles” (Fig. 281), may act as microscopic stethoscopes, while Leydig also regards them as chordotonal organs.
That, however, some of the antennal hairs of the mosquito, as first suggested by Johnson and afterwards proved experimentally by Mayer, are auditory, seems well established. Fastening a male mosquito down on a glass slide, Mayer then sounded a series of tuning-forks. With an Ut4 fork of 512 vibrations per second, some of the hairs were seen to vibrate vigorously, while others remained comparatively at rest. The lower (Ut3) and higher (Ut5) harmonics of Ut4 also caused more vibration than any intermediate notes. These hairs, then, are specially tuned so as to respond to vibrations numbering 512 per second. Other hairs vibrated to other notes, extending through the middle and next higher octave of the piano.
Mayer then made large wooden models of these hairs, the one corresponding to the Ut3 hair being about a metre in length, and on counting the number of vibrations they made when they were clamped at one end and then drawn on one side, he found that it “coincided with the ratio existing between the numbers of vibrations of the forks to which covibrated the fibrils,” or hairs. It should be observed that the song of the female mosquito corresponds nearly to this note, and would consequently set the hairs in vibration. Mayer observed that the song of the female vibrates the hairs of one of the antennæ more forcibly than those of the other. Those auditory hairs are most affected which are at right angles to the direction from which the sound comes. Hence from the position of the antennæ and the hairs a sound will be loudest or most intense if it is directly in front of the head. If, then, the song of the female affects one antenna more than another, the male turns his head until the two antennæ are equally affected, and is thus able to fly straight towards the female. From his experiments Mayer found that the male can thus guide himself to within 5° of the direction of the female. Hence he concludes that “these insects must have the faculty of the perception of the direction of sound more highly developed than in any other class of animals.” (Also see Child’s work.)
Special sense-organs in the wings and halteres.—Organs of a special sense, which Hicks supposed to be those of smell, were found by him near or at the base of the wings of Diptera, Coleoptera, and less perfect ones in Lepidoptera, Neuroptera, and Orthoptera, with a trace of them in Hemiptera; but these were considered by Leydig to be auditory organs, since he found the nerves to end in club-shaped rods, like those of Orthoptera.
Hicks found, as to the halteres and their sense-organs, that the nerve in the halter is the largest in the insect, except the optic nerve; and that at the base of the halteres is a number of vesicles arranged in four groups, to each of which the nerve sends a branch. Afterwards Bolles Lee discovered that the vesicles, undoubtedly perforated, contain a minute hair, those of the upper groups being protected by hoods of chitin. He regarded them as olfactory organs, while Lubbock seems inclined to consider them as auditory structures. Graber also regards the vesicles of Hicks as chordotonal organs.
In his elaborate account of the balancers, Weinland concludes that the organs of sense of varying structure occurring at the base of these appendages allow the perception of movements which the halteres perform and which enable the fly to steer or direct its course. The halteres can thus cause differences in the direction of the flight of a fly in the vertical plane. If the balancers act unequally, there is a change in direction.
e. The sounds of insects
Insects have no true voice; but sounds of different intensity, shrill cries, and other noises are produced mechanically by insects, either being love-songs to attract the sexes, to give signals, to communicate intelligence, or perhaps to express the emotions. The loud, shrill cry of the Cicada, or chirp of the cricket, is evidently a love-call, and results in the mating of individuals of separate broods more or less widely scattered, thus preventing too close interbreeding.
The simplest means of making a noise is that of the death-watch (Anobium), which strikes or taps on the wall with its head or abdomen. Longicorn beetles make a sharp sound by the friction of the mesoscutellum against the edge of the prothoracic cavity, the head being alternately raised and lowered, Burying-beetles (Necrophorus) rub the abdomen against the hinder edges of the elytra. Weevils make a loud noise by rapidly rubbing the tips of the abdomen on the ends of the elytra.