Art. XXXII.—Notes on the Stridulating Organs of the Cicada.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 5, 1872, Page 286

Art. XXXII.—Notes on the Stridulating Organs of the Cicada. By Ll. Powell, M.D. (With Illustrations.) [Read before the Philosophical Institute of Canterbury, 1st May, 1872.] At page 351 of “The Descent of Man” (1871) I find the following statement:—“The Cicadidœ usually sing during the day, whilst the Fulgoridœ appear to be night-songsters. The sound, according to Landois, who has recently studied the subject, is produced by the vibration of the lips of the spiracles which are set into motion by a current of air emitted from the tracheæ. It is increased by a wonderfully complex resounding apparatus,

N. Z. Spiders. Genus Salticus Latr.

consisting of two cavities covered by scales. Hence the sound may truly be called a voice. In the female the musical apparatus is present, but very much less developed than in the male, and is never used for producing sound.” As I have not access to Landois' original paper I am, of course, ignorant of the details of his description, but unless the cicada which he describes differs essentially in the nature of its musical organs from those found in New Zealand, and also from those described more or less correctly by other authors, especially Reaumur (see Kirby and Spence's “Introduction,” p. 501, seventh edition, 1856), he is most certainly in error. The stridulating organs of the cicada (Pl. XVIII.) are constructed on a principle which is, I believe, unique. In no other animal, as far as I am aware, are vibrating membranes made use of for the purpose of producing sound, and in this respect they possess a peculiar interest. In the male cicada on the upper surface of the first ring of the abdomen on either side may be seen a semilunar opening with convexity posterior, and on examining this opening with a magnifying glass it will be seen to lead into a shallow cavity closed in by a plicated horny membrane. If a live insect be caught and these membranes be observed during the act of stridulation they will be seen to be vibrating rapidly, synchronously with the beats of the shrill sound. On examining the under surface of the insect an oval plate will be observed immediately behind each posterior leg, of considerable size, and quite free except anteriorly. On snipping off these plates with a fine-pointed pair of scissors we expose on each side a large triangular opening, the apices opposed to one another, and but slightly separated; each opening leads into a roof-shaped cavity of considerable extent. Anteriorly this cavity is closed in by a fragile but opaque membrane divided into two parts by a chelinous rib, the lower half is pure white and marked with parallel creases, the upper half is yellow and tougher looking; posteriorly the cavity is closed by a large tense beautifully transparent membrane, it is very delicate and shines with iridescent colours; it is marked dr in the illustration. If we now carefully cut the body through anteriorly to the membranes here described, and to the stridulating membranes, by a little careful dissection we shall expose the immediate agent of the production of the sound, and see two thick yellow bundles of muscle inserted below into the parietes of the abdomen at the junction of the cavitary membranes. These muscular bundles diverge like the letter V, a delicate aponeurosis is given off from each muscle, which seems to be lost on the rim of the transparent membrane; the muscle itself ends in a round tendon which is inserted into the under surface of the stridulating membrane. This membrane is highly elastic, and the sound is produced by the contraction of the muscle straightening out the plications of the membrane; this produces a click, and, on the muscle relaxing, the membrane from its elasticity springs back with

another click. That this is really the mode in which the sounds are caused may be proved by exposing the parts immediately after killing the insect; on then allowing the muscles to harden a little by exposure, and on pulling them with the point of a pin, the membranes will be seen to straighten and fly back again, accompanied by the production of the usual sound. Now what part do the large transparent drum-like membranes take in the production of the sound? All writers on the subject have attributed to them reverberating qualities for the intensifying of the sound, but a simple experiment appears to disprove this, for if an insect be taken while stridulating and all four of the membranes be destroyed with a pin the sounds are not materially affected, but if one of the stridulating membranes be destroyed the sounds suffer great diminution, and on destroying the other they cease entirely. I was much surprised the first time I tried the experiment to find that the large drums seemed to take no part in the production of the sound, and the idea occurred to me that they might be hearing organs, but on examining the females, which, most remarkable to relate, are dumb and do not possess the stridulating organs, I found that the drums exist indeed, but are quite rudimentary instead of being large as we should expect to find them were they subservient to the sense of hearing. The question remains then of what use are they? That such highly developed structures must be of some use is clear. The three cicadæ found commonly in Canterbury differ in the sounds produced. The small green cicada utters a sound which may be represented by the repetition of the letter “r” thus “r-r-r-r-r-r,” the voice of the larger green species would be expressed by “crrrk–crrrk–crrrk,” while the small black ones found in the hills say “crrrk-r-r-r-r-r.” The voice of this species is remarkably loud and piercing. In connection with the voice of the cicada I may allude to a circumstance which has been frequently observed, viz., inability of some individuals to perceive very acute sounds. This is very noticeable with the song of the small green cicada. I have found many persons who are totally unable to hear any sound when my ears are being pierced with their shrill voices so as almost to give rise to a feeling of pain. There would seem to be in some cases less a deficiency in the organisation of the ear than in the faculty of perception, which is akin to the difficulty experienced by a landsman in perceiving very distant objects at sea. In some individuals, however, there is an absolute inability to hear very acute sounds, and inasmuch as the entire range of the human ear is, according to Helmholtz, eleven octaves, it has been justly remarked that the air may be filled with shrill insect sounds, which may be perfectly audible to the insects themselves but absolutely inaudible to our grosser sense.