THE ANATOMY OF INSECTS. g 



nium, and clypeus) pertain to the dorsal surface ; and the fourth 

 (gula) to the ventral surface. 



Frequently the sutures between some of these sclerites arc obso- 

 lete ; but by studying a series of insects each of these parts can be 

 distinguished. 



The Compound Eyes. — In many insects the most striking in appear- 

 ance of the fixed parts of the head are the eyes (3).* These are 

 situated one on each lateral aspect of the 

 head. They are usually nearly hemispherical 

 and of considerable size. When examined with 

 a microscope, they present the honey-comb- 

 like appearance shown in Fig. 16. Each of 

 the hexagonal divisions of the eye is a cornea 

 of a distinct eve. These lame eyes are there- Fi G .i6.-Part of compound eye, 



J n J greatly enlarged. 



fore compound. Each of the small eyes of 



which they are composed is termed an ocellus. Compound eyes are 



not found in larvae. 



The Simple Eyes. — In addition to the compound eyes, many insects 

 possess simple eyes (4). These are situated in adult insects on the 

 dorsal aspect of the head between the compound eyes, and in larvae 

 on the sides of the head. They vary in number in the adult from 

 one to four. The most common number is three; see Fig. 15. 

 Each simple eye resembles an ocellus of a compound eye. The 

 simple eyes are usually termed ocelli ; sometimes, stemmata. 



When the term ocelli is used in descriptive works, if there is noth- 

 ing in the context to indicate the contrary, it is almost invariably 

 applied to the simple eyes, and not to the elements of the compound 

 eyes. In the same way the term eye usually refers to the compound 

 eyes, unless otherwise indicated by the context. 



The Epicranium. — The epicranium (21 is usually the largest of the three 

 sclerites which pertain to the dorsal aspect of the head. It is that sclerite in 

 which the simple eyes are situated, and which surrounds the compound eyes. 

 It occupies an intermediate position in the dorsal aspect of the head, being 

 bounded caudadt by the occiput, and cephalad by the clypeus. 



From the fact that the epicranium occupies so large a part of the head, it 

 frequently becomes necessary to speak of particular regions of it in making 

 detailed descriptions of insects. Consequently names have been given to cer- 

 tain parts; although those parts are very rarely distinct. These names are 

 front, vertex, and gentz. It is difficult to define definitely the regions of the 

 epicranium to which these terms have been applied. Roughly speaking, the 



* See Tabular Review at end of discussion of External Anatomy. 



f For definitions of the terms denoting position and direction of parts, see Glossary. 



