136 



AN INTRODUCTION TO ENTOMOLOGY 



When there are three ocelH, the double nature of the median ocel- 

 lus is shown by the fact that the root of the nerve is double, while that 

 of each of the other two is single. 



In certain generalized insects, as some Plecoptera, (Fig. 150) all of 

 the ocelli are situated in the front; but in most insects, the paired 

 ocelli have either migrated into the suture between the front and the 

 vertex (Fig. 151), or have proceeded farther and are situated in the 

 vertex. 



The structure of primary ocelli is described later. 

 The adaptive ocelli. — Some larvae, as those of the Tenthredinidae, 

 possess a single pair of ocelli, which in their position and in their 

 structure agree with the ocelli of the adult insects ; these are doubtless 

 primary ocelli. But most larvae have lost the primary ocelli; and 

 if they possess ocelli the position of them and their structure differ 

 greatly from the positions and structure of primary ocelli. 



Except in the few cases where primary ocelli 

 have been retained by larvae, the ocelli of 

 larvae are situated in a position corresponding 

 to the position of the compound eyes of the 

 adult (Fig. 152); and there are frequently 

 several of these ocelli on each side of the head. 

 This has led to the belief that they represent 

 a few degenerate ommatidia, which have been 

 retained by the larva, while the development of 

 the greater nimiber of ommatidia has been 

 retarded. For this reason they are termed 



Fig. 150. — Head of 



naiad of Pteronacys; 

 dt, spots in the cnti- 

 cula beneath which 

 the dorsal arms of the adaptive ocelli. 



tached; the three The number of adaptive ocelli varies greatly, 



oceUi are on the front and sometimes is not con- 



(F), between these , , . . ,, 



two spots. stant m a species; thus 



in the larva of Corydalus, 

 there may be either six or seven ocelli on each 

 side of the head. 



There are also great variations in the struct- 

 ure of adaptive ocelli. These variations pro- 

 bably represent different degrees of degeneration 

 or of retardation of development. The extreme 

 of simplicity is found in certain dipterous lar\^as ; 

 according to Hesse ('01) an ocellus of Cerato- 

 pogon consists of only two sense-cells. As examples of com- 

 plicated adaptive ocelli, those of lepidopterous larv^ae can be cited. 



Fig. 151. — Head of 

 cricket. 



