VISION. 341 



nutive beings exercise the sense of vision. Four descrip- 

 tions of visual organs are met with in the class of Articu- 

 lated Animals; the first are the simple eyes, or siemmata, 

 as they are termed, which appear as lucid spots, resembling 

 those we have noticed in the higher orders of Annelida; the 

 second, are the conglomcrale eyes, which consist of clus- 

 ters or aggregations of simple eyes; the third, are the com- 

 "pound eyes, which are formed of a vast assemblage of small 

 tubes, each having its respective 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 co- 

 vered 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 complete metamorphosis have only stem- 

 mata; but those which are subjected only to a partial change 

 of form, as the Orthoptera, the Hemiptera, and the aquatic 

 Neuroptera, have compound as well as simple eyes. Perfect 

 insects, with the few exceptions above noticed, have always 

 compound eyes, generally two in number, placed on the 

 sides of the head: and they are often accompanied by stem- 

 mata situated between, or behind them, on the upper part of 

 the head. These stemmata, when met with, are generally 

 three in number, and are either placed in a row, or form a 

 triangle. Their structure has been minutely examined by 

 Professor Muller, who found them to contain a hard and 

 spherical crystalline lens, a vitreous hum.our, and a choroid 

 coat, with its accompanying black pigment; the whole being 

 covered externally by a convex cornea. The stemmata of 



* This is the case, however, with the genus Claviger, among the Coleoptera 

 Braula (Nltzch) among Diptera, and also some of the species of Pupipara, 

 Nyderibia, and 31elophagus, which are all parasitic insects: there are also five 

 species of ants, whose neuters have no eyes. (Muller, Annales dcs Sc. Nat. 

 xvii. 366,) 



