49G EXTERNAL ANATOMY OF INSECTS. 



cius should call these eyes simple a . In some insects, how- 

 ever, as in the Slrcpsiptera Kirby, the lenses are not nu- 

 merous : in Xe?ws they do not exceed fifty, and are di- 

 stinctly visible to the naked eye b . These lenses vary in 

 magnitude, not only in different, but sometimes in the 

 same eyes. This is the case hi those of male horse-flies 

 and flies, those of the upper part of the eye being much 

 larger than those of the lower c . The partitions that 

 separate the lenses, or rather bezels, in which they are set, 

 are very visible in the eyes just mentioned, and those of 

 Xenos ; but in many insects they are only discernible at 

 the intersecting lines of separation between the lenses. 

 In hairy eyes, such as those of the hive-bee, the hairs 

 emerge from these septa. Every single lens of a com- 

 pound eye may be considered as a cornea, or a crystal- 

 line humour, it being convex without and concave within, 

 but thicker in the middle than at the margin : it is the 

 only transparent part to be found in these most remark- 

 able eyes. Immediately under the cornea is an opaque 

 varnish, varying according to the species, which pro- 

 duces sometimes in one and the same eye spots or bands 

 of different colours. These spots and bands form a di- 

 stinguishing ornament of many of the Tabani and other 

 flies. And to this varnish the lace-winged flies (Hemc- 

 robius, &c.) are indebted for the beautiful metallic hues 

 that often adorn them. When insects are dead, this 

 varnish frequently loses its colour, and the eye turns 

 white : hence many species are described as having white 

 eyes which when alive had black ones. The consistence 



a Philos. Entomolog. 19. ^ p LATE XXVI. Fig. 38. 



e Hooke Microgr. schem. xxiv. 



