440 THE SENSORIAL FUNCTIONS. 



Little is known of the respective functions of 

 these two kinds of eyes, the simple and the com- 

 pound ; both of which are generally possessed by 

 the higher orders of winged insects. From the 

 circumstance that the compound eyes are not de- 

 veloped before the insect acquires the power of 

 flight, it has been inferred that they are more par- 

 ticularly adapted to the vision of distant objects ; 

 but it must be confessed that the experiments made 

 on this subject have not, hitherto, led to any con- 

 clusive results. Duges found, in his trials, that 

 after the stemmata had been covered, vision re- 

 mained apparently as perfect as before : while, on 

 the other hand, when insects were deprived of the use 

 of the compound eyes, and saw only with the stem- 

 mata, they seemed to be capable of distinguishing 

 nothing but the mere presence or absence of light. 

 Reaumur reports, that if the stemmata be covered 

 with an opaque varnish, the insect loses the power 

 of guiding its flight, and strikes against walls or 

 other obstacles ; whereas if the compound eyes be 

 covered while the stemmata remain free, the insect, 

 if thrown into the air, flies perpendicularly up- 

 wards, and continues its vertical ascent as long as 

 it can be followed by the observer. If all the eyes 

 of an insect be covered, it will seldom make any 

 attempt whatsoever to fly.* 



The eyes of insects, whether simple or compound, 

 are immoveably fixed in their situations ; but the 

 compound eyes of the higher orders of the class 

 Crustacea, are placed at the ends of moveable 

 pedicles ; so as to admit of being turned at pleasure 



* Memoires pour servir a I'Histoire ties Insectes, v. 287. 



