10 FOREST ENTOMOLOGY. 



made up of a large number of six-sided cells, each of which is termed 

 a corneal facet (fig. 11). The ocelli or simple eyes are placed between 

 the compound eyes, or just in the middle of the forehead. 



The antennae, or horns, rise from the head. They are composed of 

 several joints, and form good points for the discrimination of species. 

 Any text-book of general entomology, or monograph of some particular 

 group, will show that specific points very largely depend on the 

 antennae. This, for instance, is the case with the Aphididse or plant- 

 lice. The joints of the antennae have to be counted and compared. 

 By a joint is meant not so much the division line between the pieces 



Fig. 11. Compound eye of insect as seen under the microscope. (Flatters.) 



as the actual portions themselves if separated at these lines. In 

 counting joints we must begin from the head outwards, making sure 

 the horn is properly and clearly separated from the head. As a rule, 

 the antenna is stouter at the junction with the head, and gradually 

 tapers towards the other extremity : thus we say in a general way, 

 from base to apex. The antennae receive various designations, accord- 

 ing to the structure, as, for example, in fig. 12. The physiological 

 functions of the antennae are not fully understood, but they are sup- 

 posed to be, to some extent, sense-organs. 



The thorax or middle segment is theoretically composed of several 



