When sweetest.
(Tennyson.)
It is now pretty well accepted that the auditory organs of the mosquito are situated in the antennae. Sixty years ago Johnston of Baltimore was investigating the hearing-apparatus of a gnat, and came to the conclusion that—
The animal may judge of the intensity or distance of the source of sound by the quantity of the impression; of the pitch, or quality, by the consonance of particular whorls of stiff hairs, according to their lengths; and of the direction in which the modulations travel, by the manner in which they strike upon the antennae, or may be made to meet either antenna, in consequence of an opposite movement of that part. That the male should be endowed with superior acuteness of the sense of hearing appears from the fact that he must seek the female for sexual union either in the dim twilight or in the dark night, when nothing save her sharp humming noise can serve him as a guide.
Fig. 17.—A, Anopheles maculipennis, male, showing large, feathered antennae. B, Head of female, showing antennae with feathering little developed. (From Nuttall and Shipley.)
Johnston also notes that the male mosquito is the more difficult to catch. The bushy, complicated antennae of the male show that of the two sexes, with the mosquito, as with man, the male is primarily the hearer, the one who has to listen.
Another American, Mayer, twenty years later made some interesting experiments confirming the views held by Johnston. He managed to cement with shellac a species of Culex on to a glass slide, and, placing it beneath a low-powered microscope, watched the response of the antennae to tuning-forks of varying strengths. He found that under the influence of a fork producing 512 vibrations per second certain hairs of the antennae vigorously vibrated, whilst others were left unmoved. He measured the amplitudes of the vibrations of these hairs under the influence of the sound emitted by various tuning-forks. Different hairs were seen to vibrate to different notes. Mayer also observed that when the sound came from a direction in line with the long axis of the antennary hair vibrations ceased altogether. Hence he argued that the antennae could register the direction whence the sound came. Observing the antennae under the microscope, he confirmed the view that the vibrations ceased when the hairs pointed towards the source of sound, and on drawing a line in the direction in which the hair pointed, he found that it always cut within 5° of the position of the source of sound. He concludes:—
The song of the female vibrates the fibrillae of one of the antennae more forcibly than those of the other. The insect spreads the angle between his antennae, and thus, as I have observed, brings the fibrillae, situated within the angle formed by the antennae, in a direction approximately parallel to the axis of the body. The mosquito now turns his body in the direction of that antenna whose fibrils are most affected, and thus gives greater intensity to the vibrations of the fibrils of the other antenna. When he has thus brought the vibrations of the antennae to equality of intensity he has placed his body in the direction of the radiation of the sound, and he directs his flight accordingly, and from my experiments it would appear that he can thus guide himself to within 5° of the direction of the female.
There has always been some divergence of opinion as to how the buzzing sound to which the male so readily reacts is produced. Howard once thought that it was due to vibrations of certain chitinous processes in the large tracheae. Our experiments showed, however, that when the wing was cut off closer and closer to its origin the sound decreased in volume, but the note progressively rose. Unlike human beings, the male at all times emits a higher pitched note than the female, and in both sexes the note rises after feeding. ‘The greater the meal, the higher the note.’ This is, however, by no means confined to mosquitos. It is a matter which any one must have noticed when assisting at a public dinner or when dining in a college hall.