104 A BOOK OF INSECTS 



eyes ; but in some instances these eyes are divided, so as 

 to give the appearance of a pair on each side of the head. 

 This is the case with the " whirligig " beetles of the family 

 Gyrinidcv, which swim upon the surface of the water. 

 One isolated half of each eye is directed upwards, the 

 other down, so that the insect is able to search for its 

 aquatic prey, and at the same time to watch for any 

 danger which may threaten from above. Still more re- 

 markable are the eyes of certain male may-flies, in which 

 one part of the eye forms a pillar faceted at its summit, 

 while the other part occupies a more normal position at 

 the side of the head. Male two-winged flies of the genus 

 Bibio also have divided eyes ; while in other Dipter.a — 

 none of which are found in Europe — both the eyes and 

 antennae are carried on long, horn-like projections from 

 the sides of the head. 



In addition to their conspicuous compound eyes, many 

 insects possess ocelli, or simple eyes, commonly three in 

 number. Each ocellus is a small polished lens, beneath 

 which a cup-shaped mass of pigment cells forms a retina, 

 which is connected by nerves with the brain. Certain 

 larvae have a little group of similar eyes on each side of 

 the head ; but others, especially such as are surrounded 

 by an abundance of food and live in darkness, have no 

 eyes at all. 



We have already seen that in three families of the 

 Orthoptera specialised ears are situated either on the front 

 leg below the knee, or at the base of the abdomen ; while 

 somewhat similar structures are found on the fore-legs of 

 stone-flies, termites, and ants ; also on the tarsi, or feet, 

 of some beetles. The external openings of the ears in 

 long- horn grasshoppers and crickets are usually apparent 

 as two curved slits in each tibia at a point where this joint 

 is somewhat swollen. Each slit opens into a chamber, 



