438 



SCIENTIFIC NEWS. 



[Oct. 26, 1888. 



THE SENSES OF INSECTS. 



II. — Sight. 



Although the question would at first seem almost 

 superfluous, it is somewhat difficult to determine 

 whether or not Sight is the most important of the senses 

 which insects possess. On the one hand it is argued, and 

 with great show of reason, that the eyes are generally 

 very numerous, that they command a wide field of view, 

 and that they are mostly present in two, or even in three 

 different forms — strong evidence, one would think, in 

 favour of an affirmative reply to our query. But it is con- 

 tended, on the other hand, that there are many insects — • 

 notably several of the myrmecophilous beetles — which 

 have no eyes at all, while it has also been asserted that, 

 owing to the convexity of the facets which make up the 

 compound eyes, vision, even when present, can only be 

 found of service at close quarters ; in other words, that 

 insects are short-sighted. 



On this point, therefore, evidence is somewhat con- 

 flicting, and it is not very easy to arrive at any definite 

 conclusion. But it is, at all events, well to point out that 

 the arguments of those who deny the importance of the 

 eyes are open to considerable objection. For, in the first 

 place, those insects which are blind are almost invariably 

 dwellers underground, where eyes can be of no possible 

 service. And it would be as reasonable to argue, because 

 they lack those organs, that therefore eyes are not of the 

 highest importance to insects in general, as it would be to 

 urge the eyeless condition of certain cave-dwelling fish as 

 the base for a similar contention with regard to the piscine 

 race. 



And with respect to the second assertion, viz., that in- 

 sects are short-sighted, little or no proof has been brought 

 forward in its support. And we know, without a doubt, 

 that many insects can see for considerable distances. 

 Moths, for instance, are attracted to a gas-lamp hundreds 

 of yards away ; and though it may be urged that they 

 receive but a blurred and ill-defined impression of the 

 flame until they approach it closely, the fact remains that 

 their vision is at least in some degree serviceable in per- 

 ceiving distant objects. And he would be a bold man who 

 should lay down the limit at which that vision ceases to 

 be accurate. 



On the other hand, it is impossible to over-estimate 

 the importance of the fact that the facets of the eye- 

 masses are exceedingly numerous, more especially since 

 they are so set as to command a view in almost every 

 direction without any necessity for turning the head. 

 And the number of these facets is at least in some degree 

 proportionate to the speed and activity of the individual 

 species ; for the more agile insects would, of course, re- 

 quire the more perfect sight. Thus in the ant, which is 

 comparatively slow in its movements, and in which flight 

 is restricted to the single ascent made by the males and 

 females before pairing, there are no more than fifty dis- 

 tinct facets in the eye. In Blaps mucronata, which is one 

 of the most sluggish of our British beetles, there are 

 perhaps two hundred and fifty. In Meloii, which, al- 

 though slow of foot, occasionally exhibits some approach 

 to activity, there are about twice as many. But in cer- 

 tain dragon-flies, which are the analogues among insects 

 of the hawks among birds, there are twelve thousand or 

 more ; in some swift-winged butterflies there are seven- 

 teen thousand ; and in a certain active beetle (Mordelld) 

 there are more than twenty-five thousand. There are 

 exceptions to the rule, no doubt, but it seems a fairly 



regular one upon the whole ; and its bearing upon the 

 importance of the visual organs cannot be overlooked. 



Besides these compound eyes, there are in most in- 

 sects, though not in all, a very limited number of simple 

 eyes, or ocelli, which are generally situated upon the 

 upper part of the head ; and these bear a distinct resem- 

 blance, as far as the general character of their structure 

 is concerned, to the eyes of the higher animals. In the 

 words of Mr. Rymer Jones, each of these eyes "consists of 

 a minute smooth, convex, transparent cornea, in close con- 

 tact with which is a small globular lens ; behind this lens is 

 placed the representative of the vitreous humour, upon 

 which a nervous filament spreads out, so as to form a 

 retina. The whole is enclosed in a layer of brown, red, 

 or black pigment, which, bending round the anterior 

 surface of the eye, forms a distinct coloured iris and 

 pupillary aperture." 



These simple eyes are sometimes present alone, as in 

 the Thysanura. Sometimes they are wanting, as in the 

 great majority of the Cokoptera. But usually they are 

 present in addition to the compound eyes, and, being 

 directed towards the zenith, command just that quarter 

 which is beyond the ken even of the higher facets of the 

 compound eyes. 



These facets, whatever their number, are always 

 hexagonal, and are agglomerated into two great masses, 

 one of which is situated upon either side of the head. 

 In the Whirligig Beetles (Gyrini), which skim about on 

 the surface of ponds, etc., each of these eye-masses is 

 divided in two by a corneous ridge, so that, practically 

 speaking, the compound eyes are four in number, the 

 upper pair being available for use in the air, while the 

 lower pair, which alone are immersed, are equally 

 available for use in the water. In a'most all cases they 

 project well beyond the level of the cheeks and temples, 

 as perhaps for the sake of analogy we may be permitted 

 to term them, so that the field of view which they com- 

 mand is remarkably extensive, and scarcely any point of 

 the compass is altogether out of sight. 



Each of these numerous facets is provided with its own 

 optic nerve ; and these nerves, radiating, as it were, 

 from a common centre, transmit the impressions which 

 they receive to a common retina. Behind this is a mass 

 of nervous matter, which leads direct to the brain, or 

 rather to the cephalic ganglion which represents that 

 organ ; and this mass communicates with the general 

 retina by what are known as " secondary " nerves, and 

 which are very short and delicate. 



Passing to the eyes themselves, each facet is furnished 

 with, or itself consists of, a double convex lens, so 

 formed as to bring to a focus the rays passing through 

 it. Behind every such lens is a six-sided transparent 

 prism, analogous to the vitreous humour in the eyes of 

 higher animals ; and . to the bases of these prisms are 

 attached the optic nerves. 



It has long been a question with anatomists whether 

 insects do, or do not, see with more facets than one at 

 once. That all should be simultaneously employed, or 

 should carry independent impressions to the brain, is 

 manifestly impossible ; and two theories have been put 

 forward with the view of clearing up the difficulty. 

 One of these is to the effect that, although all the facets 

 simultaneously see, they convey but one common im- 

 pression, just as one impression only is borne to the 

 brain by our own two eyes. But as the facets are set 

 in different directions, this explanation is clearly in- 

 admissible. The second theory suggests that one facet 



