THE ORGANS OF INSECTS 9 



importance in the scheme of classification of the 

 Trichoptera. 



Packard writes: "Of the eyes of insects The Eyes, 

 there are two kinds, the simple and the com- 

 pound. Of the former there are usually three, 

 arranged in a triangle near the top of the head 

 between the compound eyes." 



The simple eye, known as the ocellus, consists 

 of a single lens. The compound eye is the 

 oculus or ommateum, and is composed of a large 

 number of ocelli lying in close juxtaposition. 



In the EPHEMERID^, the oculi of the male are 

 invariably larger than those of the female, and 

 are formed after a somewhat different model. 



It should be noted that the presence or 

 absence of ocelli is of importance in the correct 

 determination of genera in the TRICHOPTERA. 



The thorax is made up of three distinct The Thorax, 

 segments, each of which carries a pair of legs, the 

 wings being attached, the fore-wings or wing- 

 covers to the second, and the hind-wing, when 

 present, to the third segment. The first seg- 

 ment is termed the prothorax; the second, the 

 mesothorax ; and the third the metathorax. 



The fore-wing is known entomologically as 

 the anterior wing, and the hind-wing as the 

 posterior wing. 



Wings are composed of double chitinous mem- The Wings, 

 branes, strengthened by an elaborate system 

 of nervures and cross nervures, of much import- 

 ance in furnishing characters for sub-classifica- 

 tion. I give here drawings of the anterior and 

 posterior wings of one of the TRICHOPTERA 



