VISION. 483 



mode in which these diminutive beings exercise 

 the sense of vision. Four descriptions of visual 

 organs are met with in the class of Articulated 

 animals ; the first are the simple eyes, or stem- 

 mata, as they are termed, which appear as lucid 

 spots, resembling those we have noticed in the 

 higher orders of Annelida ; the second, are the 

 conglomerate eyes, which consist of clusters or 

 aggregations of simple eyes ; the third, are the 

 compound eyes, which are formed of a vast 

 assemblage of small tubes, each having its re-^ 

 spective apparatus of humours and of retina, and 

 terminating externally in a separate cornea, 

 slightly elevated above the general surface of 

 the organ : the fourth kind of eyes, which have 

 not yet been distinguished by any particular 

 appellation, are constituted by a number of 

 separate lenses, and subjacent retinae, but the 

 whole covered by a single cornea common to 

 them all. 



Few insects are wholly destitute of visual 

 organs, either in their larva or perfect states.* 

 The larvae of those insects which undergo a com- 

 plete metamorphosis have only stemmata ; but 

 those which are subjected only to a partial 

 change of form, as the Orthoptera, the Hemip- 



• This is the case, however, with the genus Claviger, among 

 the Coleoptera ; Braula (Nitzch) among Diptera, and also some 

 of the species of Pupipara, Nycterihia^ and Melophagus, which 

 are all parasitic insects : there are also five species of ants, whose 

 neuters have no eyes. (Muller, Annales des Sc. Nat. xvii. 366.)' 



