ORGANS OF SENSE. 



23 



there is no power of accommodation, and most external objects are 

 out of focus ; to make an image, then, the object must be at 

 one definite distance from the lens, and as the lens is usually 

 strongly convex, this distance must be small." Insects with 

 ocelli only must therefore be very short-sighted, and probably 

 in a great number of cases the ocelli are only serviceable in 

 distinguishing light from darkness and so giving warning of any 

 sudden movement or approaching obstacle by the alteration of 

 the light. 



The mode of vision by facetted eyes is a much more difficult 

 problem. Miiller's so-called mosaic theory is, at present, most 

 generally accepted, although it does not seem altogether satis- 

 factory ; it is as follows : — " An image formed by several thousand 

 separate points, of which each corresponds to a distinct field of 

 vision in the external world, will resemble a piece of mosaic work, 

 and a better idea cannot be conceived of the image of external 

 objects which will be depicted on the retina of beings endowed 

 with such organs of vision, than by comparing it with perfect work 

 of that kind." The use of such an eye is to perceive movements 

 rather than form. As remarked by Packard, most animals seem 

 but little impressed by the form of their enemies or their victims, 

 though their attention is immediately excited by the slightest 

 displacement. Hunters, fishermen, and entomologists have made, 

 in confirmation of this view, numerous and demonstrative obser- 

 vations. Gottsche and others favour the view of a separate and 

 distinct image for every cornea, i. e. for every facet. Lubbock, 

 who favours Miiller's theory, gives a long list of reasons opposed 

 to this view, but his last reason seems sufficient for practical 

 purposes, viz.: "that a combination of many thousand relatively 

 complete eyes seems quite useless and incomprehensible." 



The organs of smell appear to be chiefly found in the antenna?, 

 although some of the structures, evidently connected with this sense, 

 that have been observed in these have been regarded by various 

 writers as organs of hearing. There can, however, be no doubt that 

 many of the structures are really olfactory, as this has been 

 proved by various experiments, especially those made by Hauser 

 (" Physiologische und histologische Untersuchungen fiber das 

 Greruchsorgan der Insecten," Zeitschr. f. wiss. Zooi. xxxiv, 1880). 

 Taking a glass rod dipped in carbolic acid and holding it at some 

 little distance from a specimen of Philonthus ceneus, the beetle 

 raised its head, turned it in different directions, and made lively 

 movements with its antennae. When the rod was placed closer, 

 it started back and ran in the opposite direction ; when the rod 

 was removed it occupied itself for some time in drawing its 

 antennae, with the aid of the fore limbs, through its mouth, 

 although it had not touched the acid. The antennae were then 

 removed, and the day after the experiment was repeated without 

 any effect upon the insect. The same results have been produced 

 by the more humane method of placing the antennae in liquid 

 paraffin wax, and so covering them with a layer of wax and excluding 



