ITS NERVOUS SYSTEM AND SENSE ORGANS. 109 



aperture fitted with ground glass was then arranged in each 

 window. At a distance of four metres from the centre of the 

 space between the windows captive Insects were from time to 

 time liberated. One of the windows was fenced with fine trellis, 

 so as to prevent the passage of the Insect, or otherwise altered 

 in form, but the size of the aperture could be increased at 

 pleasure, so as exactly to make up for any loss of light caused 

 thereby, the brightness of the two openings being compared by 

 a photometer. 



It was found that day-flying Insects require a tolerably good 

 light ; in semi-obscurity they cannot find their way, and often 

 refuse to fly at all. By varnishing one or other set in Insects 

 possessing both simple and compound eyes, it was found that 

 day-flying Insects provided with compound eyes do not use their 

 simple eyes to direct their course. TThen the light from one 

 window was sensibly greater than that from the other, the 

 Insect commonly chose the brightest, but the existence of bars, 

 close enough to prevent or to check its passage, had no per- 

 ceptible effect upon the choice of its direction. Alterations in the 

 shape of one of the panes seemed to be immaterial, provided 

 that the quantity of light passing through remained the same, 

 or nearly the same. Plateau concludes that Insects do not 

 distinguish the forms of objects, or distinguish them very 

 imperfectly. 



It is plain, and Plateau makes this remark himself, that such 

 experiments upon the power of unaided vision in Insects, give a 

 very inadequate notion of the facility with which an Insect 

 flying at large can find its way. There the animal is guided by 

 colour, smell, and the actual or apparent movements of all 

 visible objects. Exner has pointed out how important are the 

 indications o-iven by movement. Even in man, the central 



^j */ 



part of the retina is alone capable of precise perception of 

 form, but a moving object is observed by the peripheral tract. 

 Plateau (from whom this quotation is made) adds that most 

 animals are very slightly impressed by the mere form of 

 their enemies, or of their prey, but the slightest movement 

 attracts their notice. The sportsman, the fisherman, and the 

 entomologist cannot fail to learn this fact by repeated and 

 cogent proofs. 



