Nervus vagus and sympathetic system of an Insect: a a, optic nerves.

Fig. 165. Nervus vagus and sympathetic system of an Insect: a a, optic nerves; d d, su-pra-oesophageal ganglion; I, oesophagus; b b, origins of the recurrent nerve ifk; g g, nerves surrounding the oesophagus, and communicating between the supra-cesophageal ganglion, d d, and the first pair of ventral ganglia, h; c c,l I, sympathetic ganglia.

* Biblia Naturae.

(860). The sympathetic system properly so called consists of four small ganglia (fig. 165, cc, l l), the two anterior of which communicate with the brain and with each other by means of connecting filaments. These ganglia are closely applied to the commencement of the oesophagus, and supply it with minute nerves.

(861). Various are the conjectures entertained by different authors concerning the senses possessed by the members of the insect world, and the organs subservient thereunto. The possession of certain sources of perception has been alternately granted and denied; the nature of their sensations has been a fruitful subject of inquiry; and some physiologists have even gone so far as to deny the correspondence of the impressions derived by insects through the medium of their senses with those which we ourselves receive. It would lead us far out of our course did we even advert to the multiplicity of opinions and conjectures promulgated from various sources relative to these inquiries, and, perhaps, with little addition to our real knowledge. It is true that we cannot deny the possibility of the existence of other modes of sensation than those familiar to us; but it is likewise evident that, as we can never have the most remote conceptions concerning their nature, speculations respecting them are not at all calculated to lead to satisfactory conclusions.

We must from necessity take our own senses as the standard of comparison, limiting our inquiries to examining how far insects possess means of intercourse with the external world similar to those which we enjoy, and, when we find certain faculties to exist, investigating the structure of the organs by which they are exercised.

(862). The sense of touch is indubitably bestowed upon all insects; and, to judge from the perfection of the edifices which they build, and the precision of their usual operations, this must be extremely delicate. It is sufficient, however, to look at the external construction of the skeletons of Articttlata to perceive that the hard and insensible integument spread over the entire surface of their bodies is but little calculated to receive tactile impressions. The antennae, or feelers as they are popularly called, have been very generally regarded as being peculiarly instruments of touch; and whoever watches the proceedings of an insect in which these appendages are largely developed will, we apprehend, easily convince himself that they are employed to investigate surrounding objects by contact. Straus-Durckheim regards the feet as being specially appropriated to the sense of feeling; but this opinion seems quite inadmissible. Burmeister places the exercise of touch exclusively in the palpi attached to the maxillae and labium, and observes that, in the larger insects, such as the predatory Beetles, the Grasshoppers, Humble-bees, and many others, the apex of the palpus is dilated into a white transparent and distended bladder, which, after the death of the insect, dries up and is no longer visible.

This bladder he looks upon as the true seat of the sense in question, and remarks that the main nerve of the maxillae and of the tongue spreads to it, and distributes itself upon its superior surface in minute ramifications.

(863). Whether taste exists in insects as a distinct sense may admit of dispute: the tongue, already described, seems but little adapted to appreciate savours; and, seeing this, it is obvious that all opinions assigning the function of tasting to other parts are purely conjectural.

(864). Many insects are certainly capable of perceiving odours; of this we have continual proof in the Flesh-fly and other species, that are evidently guided to their food, or select the position in which to deposit their eggs, by smell; but where the olfactory apparatus is lodged is still a matter of doubt. The antennae and the palpi have each had the power of smelling assigned to them, but without much plausibility. The respiratory stigmata have been pointed out as performing the office of examining the air admitted for the purpose of breathing; yet other authors, with equal probability, look upon the ultimate ramifications of the tracheae as forming one extensive nose. The interior of the mouth has been indicated by Treviranus*; while Kirby and Spence find, in the Necrophori and other insects remarkable for acuteness of smell, an organ in close connexion with the mouth, to which they attribute the perception of odoriferous particles: this is a cavity situated in the upper lip, containing a pair of circular pulpy cushions covered by a membrane transversely striated or gathered into delicate folds.

(865). We are scarcely better informed concerning the organs of hearing; but that insects are capable of perceiving sounds is proved by the fact of many tribes being capable of producing audible noises, by which they communicate. There seems, indeed, to be little doubt that the auditory apparatus is in some way or other connected with the antennae. Some have supposed that these slender and jointed organs, supplied, as they are, with large nerves, are themselves capable of appreciating sonorous vibrations. Burmeister 1 thinks that, as in crabs and lobsters, it is at the base of the antenna that the ear is situated, and observes that if we examine the insertion of these appendages, we shall detect there a soft articulating membrane, which lies exposed, and is rendered tense by the movements of the antenna: this he looks upon as representing the drum of the ear, and conceives that it is so placed as to receive impressions of sound, increased by the vibratory movements communicated to the antennae by the sonorous undulations of the atmosphere.