NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS — SNODGRASS 73 



proved to have any developmental relation to the compound eyes. 

 Those of most primitive structure occur in the larvse of the Dipteran 

 families Chironomidse and Culicidce, while the ocelli of the Ephe- 

 meridae are perhaps the most highly evolved type. These facts and 

 others indicate that the structure and comparative complexity of 

 the ocelli have no phylogenetic significance, and suggest that ocelli 

 are not the primitive visual organs of insects, but are secondarily 

 acquired organs supplemental to the compound eyes, or substituting 

 for them where the latter have been suppressed. 



The ocelli are characterized by their individuality, but they cannot 

 be defined on the basis of a type structure, because they differ so 

 much among one another and have in general no feature that may 

 not be found in ommatidia of compound eyes. They may occur 

 in groups, though a group does not function as a single organ, but 

 a compound eye, as already noted, may degenerate into detached 

 ommatidia. The following six types of simple eyes are to be dis- 

 tinguished : 



1. The lateral ocelli of Chironomid and Culicid larvae. 



2. The median ocellus of Collembola. 



3. The lateral ocelli of larvse of insects having a pupal stage, ex- 

 cept the eyed larvse of Diptera and Hymenoptera. 



4. The frontal or dorsal ocelli of adult insects and of the young 

 of insects having no pupal stage, except Collembola and Ephemerida, 

 and the lateral eyes of adult fleas. 



5. The frontal ocelli of Ephemerida. 



6. The lateral ocelli of Tenthredinoidea. 



The ocelli of Chironomid and Culicid larvse are the simplest of 

 all insect eyes and have a striking similarity of structure to the eyes of 

 the Turbellarian worm, Planaria. There are two on each side of the 

 head, each eye consisting of a few simple sense cells lying beneath 

 a clear area of the cuticula, with their distal ends in a pigmented 

 cup of the hypodermis. The pigment probably serves to limit the 

 direction of the light rays that may fall upon the sense cells. 



A median frontal ocellus similar in structure to that of Dipteran 

 larvse has been noted in a few species of Collembola. That of Orche- 

 sclla is somewhat more highly evolved than the Dipteran larval eye, 

 and its sense cells have rhabdomere borders. These two groups of 

 insects are so widely separated, however, that it does not seem 

 likely there can be any genetic relationship between the eyes of one 

 and those of the other. 



The lateral ocelli of Neuropteran, Coleopteran, and Lepidopteran 

 larvse consist of an invagination of the hypodermis beneath a lenti- 



