370 J.ECTURE XVI. 



supports, in the narrow interspace on tlie vertex of the head, three 

 simple eyes, called ocelli and stemmata. 



In all insects the eyes are sessile, or, if supported, as in a few rare 

 instances {Diopsis, e. g.), on prolongations of the head, such peduncles 

 are not moveable like those which support the compound eyes of the 

 higher Crustacea. The eyes are either "simple" or "compound:" 

 the first are found in the larva, the second in the imago of hexapod 

 insects ; in some of the latter both kinds occur. The blind exceptions 

 in the class are few ; such are certain species of Ptiliurn that live 

 under the bark of trees, the Claviger, which dwells in ants' nests, and 

 the Anophthalmus, peculiar to dark caverns. The larvae of Hyme- 

 noptera and Diptera, and most of the apodal larvae of Coleoptera, 

 are blind. 



A simple eye (ocellus, stemma) is composed of a cornea, behind 

 which is a spherical or cylindrical lens, lodged in a kind of calyx 

 formed by an expansion of the optic nerve, and which is surrounded 

 by a layer of diversely coloured pigment. 



There is only one ocellus on each side of the head in the lice 

 (PediculidcB, Nirmidce), cochineal insect (Coccidce), and the larvee of 

 the Phryganidce. The ocelli are in groups of four to eight in the 

 skiptails {Poduridce)^ the hexapod larvae of the Strepsiptera and 

 Coleoptera, the larvae of Lepidoptera, Hemerohidce, MyrmelionidcB, 

 and RaphididcB. 



The Centipede has many simple eyes, arranged in a cluster on 

 each side of the head, and requiring only a little closer approximation 

 to form a compound eye. The required approximation takes place in 

 the lulus, but the optic nerve, instead of swelling into a ganglionic 

 mass, separates into a pencil of nerves at the base of the cluster, one 

 for each ocellus. The transition to the large compound eye of the 

 hexapod is made by the lulus and Scutigera ; but the interval is very 

 wide between the Myriapods and Anellids in regard to both the 

 number and structure of the organs of vision. 



The lateral compound eyes of winged insects are generally cir- 

 cular, sometimes oval, or reniform ; they occupy the sides of the head, 

 and sometimes encroach upon the upper part so as to meet there. 

 In some Capricorn beetles, as Tetraopes, the antennae project from 

 the middle of the ovate eyes and divide them into an upper and lower 

 half: the compound eyes of certain beetles of the genera Ateuchus 

 and Geotriipes are almost or quite divided into two on each side by 

 the encroachment of the canthus ; some Ephemercc and the Gyrimdce 

 have two pairs of compound eyes : in the latter they are situated one 

 on the upper, the other on the lower surface of the head, and must 



