Auditory Apparatus of the Culex Mosquito

It is more than presumable that creatures endowed with the faculty of producing and voluntarily modifying distinct sounds, should also possess organs for the apprehension and appreciation either of rhythmic or irregular sonorous vibrations. The insect tribes are precisely in this category ; the apparatus by which is produced their song, their hum, or their chirp, is extremely varied, and the sounds which emanate from it offer a great diversity, even in the larger individuals, or, in other words, so far as our own auditory organs permit our sense to follow the rising scale. That these sounds are in some way perceived by insects themselves we have abundant evidence in the Cricket (Gryllusf) the Grasshopper (Cicada), and especially in the Bee, which responds to another individual in a particular note.^* Some insects are supposed to be silent ; while the smaller varieties, from the exceeding minuteness of their parts, give rise to vibrations so rapid as to be inappreciable by our ears.

It will readily be admitted that if there be a limit for acute sounds, corresponding with the smallest number of vibrations capable of producing an auditory impression, there must also be a limit to the development of an acoustic apparatus ; and “we cannot,” as Dugès remarks, “conceive of a true microscopic ear.” In the Protozoa, therefore, and possibly in the most diminutive insects, we may abandon the idea of a centralization of the faculty of perceiving vibrations, and feel assured that the sense of touch, generally distributed, stands in the stead of a “sensorial speciality.”

From analogy, pursued downwards, we might expect to discover the localization of the “sensorial speciality,” when it exists, in the head of insects, or else from analogy, pursued upwards, might we sometimes look for its seat elsewhere. In fact we find numerous descriptions of an auditory apparatus situated in the head of certain species, and in parts connected with the thorax of other species ; but many of the observers have failed to convince others than themselves ; and other writers have assigned, in some instances, a different function to the organs spoken of as being concerned in audition.

Treviranus^* describes the “organ, probably of hearing” of the Blatta orientalis, as consisting of an oval opening, situated immediately behind the insertion of the antennæ, and covered with a convex white pellicle, and supposes it possible that the club-like antenna of the diurnal Lepidoptera contains an auditory apparatus.

Ramdohr^† presumes that the vesicles placed at the root of the maxillæ, in bees, have a similar function.

Straus-Durckheim locates the seat of hearing in the foliated antennæ of the May-bug.

Carus^‡ considers it possible “that the membrane, which, in the Locusta viridissima, unites the antenna with the head, and offers a tolerably extended surface, is a sort of membrana tympani, or membrane of a kind of fenestra vestibularis, which the movements of the antennæ may relax or render tense.”

De Blainville,^§ finding certain apertures like stigmata in the posterior part of the head of Grasshoppers, supposes that they lead into a cavity which appears to him an auditory apparatus; and Carus admits the probability of this presumption, “as deriving support from the evidence of analogous facts in the higher classes.” But Dug’es found the “apertures” to be simply “depressions and he denies positively the existence of communicating tracheæ and vesicles, and also of an accessory nervous expansion.

L. W. Clarke ^|| describes at the base of the antennæ of Cardbus nemoralis, an auditory apparatus composed of an auricle, an internal and external auditory canal, a tympanum, and a labyrinth. ^¶

Newport^** believes that the antennæ serve as well for touch as hearing.

Siebold ^†† opposes the opinion of Treviranus concerning the two white convex plates existing at the base of the antennæ of Blatta orientalis, and declares them to be simply rudimentary accessory eyes. The same author gives an account of an auditory apparatus belonging to the Acrididæ, consisting of a tympanum, and a membranous labyrinth supplied with an auditory nerve proceeding from the third thoracic ganglion.

The Locustidæ and Achetidæ have similar organs situated in their anterior legs immediately below the coxo-tibial articulation. These organs are composed of a fossa on each side, or of two, more or less capacious, cavities (auditory capsules) with orifices opening forwards ; and each having on the inner side an elongated oval tympanum ; and the two tympana are in close contact with a dilatation of the large tracheal tube of the leg, whose upper extremity is in connexion with an acoustic nerve which derives its origin from the first thoracic ganglion. A neighbouring portion of the tracheal system he supposes to serve the purpose of a Eustachian tube.

And finally, J. Müller, as quoted by Carus,^* regards as organs of hearing “two depressions or pits, in Gryllus hieroglyphicus, situated, one on each side, of the metathorax, on the dorsal aspect, above the attachment of the last pair of legs upon and closed by a delicate membrane, behind which there exists a vesicle, filled with liquid, which receives a nerve from the third thoracic ganglion.”

While bearing in mind the difference between feeling a noise and perceiving a sonorous vibration, we may safely assume with Carus—for a very great number of insects, at least—that whenever true auditory organs are developed in them, their seat is to be found in the neighbourhood of the aniennee. That these parts themselves are, in some instances, concerned in collecting and transmitting sonorous vibrations, we hold as established by the observations we have made particularly upon Culex mosquito; while, we believe, as Newport has asserted in general terms, that they serve also as tactile organs.

The male mosquito differs considerably, as is well known, from the female; his body being smaller and of a darker colour, and his head furnished with antennee and palpi in a state of greater development. (Plate VI. fig. 1.) Notwithstanding the fitness of his organs for predatory purposes he is timid, seldom entering dwellings or annoying man, but restricts himself to damp and foul places, especially sinks and privies. The female, on the other hand, gives greater extension to her flight, and, attacking our race, is the occasion of no inconsiderable disturbance and vexation during the summer and autumn months.

The head of the male mosquito, about 0·67 mm. wide, is provided with lunate eyes, between which in front superiorly are found two pyriform capsules nearly touching each other, and having implanted into them the very remarkable antennæ.

The capsule, measuring about 0·21 mm., is composed of a horny substance, and is attached posteriorly by its pedicle, while anteriorly it rests upon a horny ring, united with its fellow by a transverse fenestrated band, and to which it is joined by a thin elastic membrane. Externally it has a rounded form, but internally it resembles a certain sort of lamp shade with a constriction near its middle ; and between this inner cup and outer globe there exists a space, except at the bottom or proximal end, where both are united.

The antennae are of nearly equal length in the male and the female.

In the male the antenna is about 1-75 mm. in length, and consists of fourteen joints, twelve short and nearly equal, and two long and equal, terminal ones, the latter measuring (together) 0-70 mm. Each of the shorter joints has a fenestrated skeleton with an external investment, and terminates simply posteriorly, but is encircled anteriorly with about forty papillae, upon which are implanted long and stiff hairs, the proximal sets being about 0 79 mm. and the distal ones 0·70 mm. in length ; and it is beset with minute bristles in front of each whorl.

The two last joints have each a whorl of about twenty short hairs near the base.

In the female the joints are nearly equal, number but thirteen, and have each a whorl of about a dozen small hairs around the base. Here, as well as in the male, the parts of the antennae enjoy a limited motion upon each other, except the basal joint, which, being fixed, moves with the capsule upon which it is implanted.

The space between the inner and outer walls of the capsule, which we term confidently the auditory capsule, is filled with a fluid of moderate consistency, opalescent, and containing minute spherical corpuscles, and which probably bears the same relation to the nerve as does the lymph in the scalae of the cochlea of higher animals. The nerve itself, of the antenna, proceeds from the first or cerebral ganglion, advances towards the pedicle of the capsule in company with the large trachea which sends its ramifications throughout the entire apparatus, and, penetrating the pedicle, its filaments divide into two portions. The central threads continue forwards into the antenna and are lost there ; the peripheral ones, on the contrary, radiate outwards in every direction, enter the capsular space, and are lodged for more than half their length in sulci wrought in the inner wall or cup of the capsule.

In the female the disposition of parts is observed to be nearly the same, excepting that the capsule is smaller, and that the last distal antennal joint is rudimental.

The proboscis does not differ materially in the two sexes ; but the palpi, although consisting in both instances of the same number of pieces, are very unlike. In the female they are extremely short, but in the male attain the length of 2·73inm. ; while the proboscis measures but 2·16 mm. They are curved upwards at the extremity.

If an organ of hearing, similar to that described by Treviranus as belonging to the Blatta orientalis, exist in the head of the Mosquito, the tympanum must be of exquisitely minute proportions, because the head, which has a diameter of only 0·67 mm., is almost entirely occupied by the corneal plaques, the capsules, and the attachments of the neck and of the buccal apparatus. The membrana tympani must therefore be so small as to preclude the idea of its being put in vibration by any sounds other than those infinitely more acute than are produced by the insect itself, and the use of such an organ for the purposes of inter-communication must be highly problematical. But no trace of such a disposition is to be found in the head, nor very certainly, also, in the body ; and we are obliged to look for some organ which may answer the requirements of an effective auditory apparatus.

The position of the capsules strikes us as extremely favourable for the performance of the function which we assign to them ; besides which there present themselves in the same light the anatomical arrangement of the capsules, the disposition and lodgment of the nerves, the fitness of the expanded whorls for receiving, and of the jointed antennæ fixed by the immovable basal joint for transmitting vibrations created by sonorous modulations. The intra-capsular fluid is impressed by the shock, the expanded nerve appreciates the effect of the sound, and 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 the stiff hairs, according to their lengths ; and of the direction in which the modulations travel, by the manner in which they strike upon the antennæ, 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. The necessity for an equal perfection of hearing does not exist in the female ; and, accordingly, we find that the organs of the one attain to a development which the others never reach. In these views we believe ourselves to be borne out by direct experiment, in connexion with which we may allude to the greater difficulty of catching the male Mosquito.

In the course of our observations we have arrived at the conclusion, that the antennæ serve, to a considerable extent, as organs of touch in the female ; for the palpi are extremely short, while the antennæ are very movable, and nearly equal the proboscis in length. In the male, however, the length and perfect development of the palpi would lead us to look for the seat of the tactile sense elsewhere ; and, in fact, we find the two apical antennal joints to be long, movable, and comparatively free from hairs ; and the relative motion of the remaining joints very much more limited.