212 LECTUllE XVI. 



manifest without ambiguity, but it is more complicated and relatively 

 larger in insects than in any other class of animals. 



What would be thought of a quadruped whose head, with the ex- 

 ception of the mouth and the place of juncture with the neck, was 

 covered by two enormous convex masses of eyes, numbering upwards 

 of 12,000 in each mass? Yet such is the condition of the organs of 

 vision in the dragon-fly, which, besides the two great compound eyes, 

 supports, in the narrow interspace on the 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, on prolongations of the head, such peduncles are not 

 moveable like those which support the compound eyes of the higher 

 Crustacea. 



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 Anellides 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 antennee 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 Geotrupes are almost or quite divided into two on each side by 

 the encroachment of the canthus ; some EpliemercB and the Gyrinidce 

 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 

 serve the aquatic whirligigs to discern at the same time objects be- 

 neath them in the water, and above them in the air. 



The integument of the head, Avhich passes uninterruptedly over the 

 compound eye, then becomes transparent, and is subdivided into a 

 number of hexagonal corneules, varying in number from 50 in the 

 ant, to 4000 in the house-fly, to above 17,000 in the butterfly, and to 

 more than 25,000 in the Mordella beetle. In general each corneule 

 is thicker than it is broad, and thicker at its middle than at its cir- 

 cumference; a layer of pigment here insinuates itself into the inter- 

 spaces between the corneules. In bees and flies fine hairs project from 

 tliese interspaces, which must defend the eye or warn the insect against 



