THE SENSES OF INSECTS. XIH 



sand, for example, as c6mpared with walking on marble. This 

 is easy to measure. When a fish strikes the water with its tail 

 to propel itself forward, it performs a double task; one part 

 consists in pushing backwards a certain mass of water with a 

 certain swiftness, and the other in pushing on the body in spite 

 of the resistance of the surrounding fluid. This last portion 

 of the task only is utilized. It would be greater if the tail of 

 the fish encountered a solid object. Almost all the propelling 

 agencies employed in navigation undergo this loss of labor, 

 which depends on the mobility of the point d' appui. The bird is 

 placed among conditions especially unfavorable. 



The Senses of Insects. The eyes of insects ar>e sometimes 

 so large as to envelop the head like an Elizabethan ruffle, and 

 the creature's head, as in the common house fly, seems all eyes. 

 And this is almost literally the case, as the two great staring 

 eyes that almost meet on the top of the head to form one, are 

 made up of myriads of simple eyes. Each facet or simple eye 

 is provided with a nerve filament which branches off from the 

 main optic nerve, so that but one impression of the object per- 

 ceived is conveyed to the brain ; though it is taught by some 

 that objects appear not only double but a thousand times mul- 

 tiplied. But we should remember that with our two eyes we 

 see double only when the brain is diseased. Besides the large 

 ordinary compound eyes, many insects possess small, simple 

 eyes, like those of the spider. The great German anatomist, 

 Johannes Muller, believed that the compound eyes were adapted 

 for the perception of distant objects, while those nearer are 

 seen by the simple eyes. But it may be objected to this view 

 that the spiders, which have only simple eyes, apparently see 

 both near and remote objects as well as insects. 



The sense of touch is diffused all over the body. As in the 

 hairs of the head and face of man, those of insects are delicate 

 tactile organs; and on the antennae and legs (insects depend- 

 ing on this sense rather than that of sight) these appendages 



