EXTERNAL ANATOMY OP INSECTS. 495 



in every state. — In considering compound eyes, I shall 

 advert to their structure, number, situation, Jigure, cloth- 

 ing, colour, and size. 



As to their structure, — when seen under the microscope 

 they appear to consist usually of an infinite number of con- 

 vex hexagonal pieces. If you examine with a good glass 

 the eye of any fly, you will find it traversed by numberless 

 parallel lines, with others equally numerous cutting them 

 at right angles, so as apparently to form myriads of little 

 squares, with each a lens of the above figure set in it. The 

 same structure, though often not so easily seen, obtains in 

 the eyes of Colcoptera and other insects. When the eye 

 is separated and made clean, these hexagons are as clear 

 as crystal. Reaumur fitted one eye to a lens, and could 

 see through it well, but objects were greatly multiplied 3 . 

 In Coleopterous insects they are of a hard and horny 

 substance ; but in Diptera, &c. more soft and membra- 

 nous. The number of lenses in an eye varies in different 

 insects. Hooke computed those in the eye of a horse- 

 fly to amount to nearly 7,000 b ; Leeuwenhoeck found 

 more than 12,000 in that of a dragon-fly c ; and 17,325 

 have been counted hi that of a butterfly d . But of all in- 

 sects they seem to be most numerous in the beetles of 

 Mr. W. S. MacLeay's genus Dynastes. In the eyes of 

 these the lenses are so small as not to be easily discover- 

 able even under a pocket microscope, except the eye has 

 turned white e : it is not, therefore, wonderful, that Fabri- 



3 Reaum. iv. 245. b Microgr. 176. 



c Epht. Mar. 6. 1717- d Amceri. Academ. vii. 141. 



e I possess a specimen in which the eye is partly black and partly 

 white : the lenses are invisible in the black part, but very visible in 

 the white. 



