I am, &c.
[LETTER XLV.]
SENSES OF INSECTS.
At first one would think that the senses of insects might be described in very few words, and scarcely afford matter for a separate letter; but when we find that physiologists are scarcely yet agreed upon this subject, and that the use of some of their organs, which appear to be organs of sensation, has not yet been satisfactorily ascertained—we shall not wonder that it requires more discussion than at the first blush we were aware of. In treating on this head I shall first say something on the senses in general, and then confine myself to those of insects.
Touch, taste, smell, hearing, and sight, I need not tell you, is the usual enumeration of the senses: but as the term includes every means of communication with the external world, the list perhaps might be increased; and there is ground for thinking that the number seven, so signalized as a sacred number[1025], may also here have place. Dr. Virey, an eminent physiologist, whose sentiments on various subjects I have before noticed with approbation[1026], appears to be of opinion that there are really seven senses; which he divides into those that are altogether physical, and those that are more connected with the intellect. The first of these divisions contains four senses,—touch, love, taste, and smell;—the second three,—hearing, sight, and the internal sense of thought, or the brain[1027]. That he is right in adding love to the list seems to me evident, because it is as distinct from touch, as smelling and taste are. With regard to the other, though it may be expected that there should be a transitive sense connecting the intellect (if I may so speak) with the external organ of sense, and as a medium by which the former can receive the notices of the external world furnished by the latter; yet it seems improper to make the entire brain itself a sense. We know that the agent between the common sensory and the sense is the consciousness or perception of the impression. "Seeing we may see and not perceive, and hearing we may hear and not understand." The picture may be painted upon the retina of the eye, the sound may strike upon the tympanum of the ear; but neither the one nor the other be received by the intellect, unless the internal power or faculty of perception be in action and mediate between them. This is what I mean by the internal sense, which, to use a term of Mr. W. S. MacLeay's[1028], is osculant between intellect and sense, or forms the transit from one group of powers to the other.
Of the ordinary senses, sight holds the first rank: it can dart to the region of the stars, and convey by the perceiving sense, to the sensory, ideas of innumerable objects. Next in rank is hearing, which can receive sounds from a great distance; but the ideas it remits are confined only to one object, the variations of tones. In the other organs the sensitive power is much more confined. There is another difference between the intellectual and physical senses:—the former are the only ones that receive and convey sensations of the beautiful and sublime; of harmony and discord,—the latter, though they minister more to our sensual enjoyments, add little to our intellectual; and therefore too devoted an indulgence in them debases our nature, and levels us with the brutes, which use their eyes and ears only for information, not for pleasure[1029].
In man the ordinary five senses are usually in their greatest perfection, although in some animals particular senses have a greater range. The Vertebrates in general are also gifted with the same number, though there are some exceptions. But in the Invertebrates they are seldom to be met with all together in the same object. The Cephalopods have no smell. Several Gasteropods can neither hear nor see. The animals of bivalve shells have neither eyes, nor ears, nor smell; and the zoophytes and the races below them have, it is affirmed, only the single sense of touch, which in them is so extremely delicate as to be acted upon even by light[1030].
Not so our insects. These, there is good reason to believe, possess all the ordinary senses. That they can see, touch, taste, and smell, no one denies. Linné and Bonnet, however, thought them deprived of hearing[1031]; but numerous observations prove the contrary. That they hear in their larva state, is evident from facts stated by the latter physiologist. He found that the sound of his voice evidently affected some caterpillars; which he attributes, but surely without reason, to the delicacy of their sense of touch: at another time, when some caterpillars of a different species were moving swiftly, he rang a small bell; upon which they instantly stopped and moved the anterior part of their body very briskly[1032]. That they possess this faculty in their imago state is confirmed still more strongly by facts. I once was observing the motions of an Apion under a pocket microscope: on seeing me it receded. Upon my making a slight but distinct noise, its antennæ started: I repeated the noise several times, and invariably with the same effect. A Harpalus, which I was holding in my hand, answered the sound in the same manner repeatedly. Flies, I have observed, at brisk and distinct sounds move all their legs; and spiders will quit their prey and retire to their hiding places. Insects that live in society give notice of intended movements, or assemble their citizens for emigration by a certain hum[1033]. But the most satisfactory proof of the hearing of these animals is to be had from those Orthoptera and Hemiptera whose males are vocal. Brunelli kept and fed several males of Acrida viridissima (a grasshopper with us not uncommon) in a closet, which were very merry, and continued singing all the day; but a rap at the door would stop them instantly. By practice he learned to imitate their chirping: when he did this at the door, at first a few would answer him in a low note, and then the whole party would take up the tune and sing with all their might. He once shut up a male in his garden, and gave the female her liberty; but as soon as she heard the male chirp, she flew to him immediately[1034].