EXTERNAL STRUCTURE OF INSECTS. 27 



eyes, with upward and downward but little lateral scope, 

 have an irregular flight, usually directed up, down, or for- 

 ward, but very little from side to side. The field of 

 vision of the grasshoppers, &c., is still more limited, and 

 their flight is short and hesitating. In the earwigs, which 

 fly little and crawl much, the eyes are small, and are 

 placed on the top of their flat heads. 



It has also been observed that in the eyes of some 

 insects (as the dragonfly) the eyelets are by no means 

 uniform in size, the surfaces of the upper being con- 

 siderably larger than those of the lower eyelets. The 

 inference from this appears to be, that there is some 

 variation in the length of the sight the eyelets turned 

 earthwards being probably shorter sighted than those 

 which are on the look out for birds of prey and other 

 aerial enemies.* 



Besides these compound eyes most insects have also a set 

 of semi-globular simple eyes, or " ttemmata" or " ocelli." 

 These are usually three in number and placed in a triangle 

 upon the forehead (see fig. 6). They maybe easily observed 

 in the bee, wasp, and dragonfly. It is supposed that they 

 are intended for the perception of near objects only. 



What has been written applies only to perfect insects. 

 The larva of insects (i.e., insects in an early stage, as cater- 

 pillars, &c.) never possess compound eyes, but (with the 

 exception of such as from their subterranean or other habits 

 require no eyes, and therefore have none) have always 

 one or more pairs of simple eyes, resembling the stem- 

 rnata already mentioned. Where several of these exist, 



* For much more on the subject of insects' eyes the .reader is referred 

 to some interesting papers by Mr. Parson "On the Discoveries of Miiller 

 and others," in the "Magazine of Nat. Hist." for 1831 ; and to "The 

 Honey Bee, its Natural History, Habits, &c," by James Samuelson and 

 Dr. J. B. Hicks : Van Voorst. 



