STRUCTURES USED IN CLASSIFICATION. 15 



therefore be considered in order and as represented in the 

 figure. 



The Head. — The heads of Heteroptera vary much in form 

 and size and are composed of four or more segments or rings, 

 solidly fused together to form a single cavity or hard box of 

 chitin, known as the epicranium. This contains the brain and 

 accessory ganglia, and the mouth cavity. It bears or gives 

 support to the antennae, mouth parts, eyes and ocelli ; also in- 

 ternally to the muscles moving the rostrum or beak. The 

 broad basal portion of the epicranium back of the eyes is 

 known as the occiput, the narrower portion between the eyes, 

 the vertex, while the long, often more or less deflexed frontal 

 portion is the front or face. Below the front and between it 

 and the base of beak there are usually three more or less dis- 

 tinct parts, separated by sutures or grooves. The median one 

 of these is the tylus, sometimes called the dypeus. The parts 

 adjoining this on either side are known as the jugae or upper 

 cheeks. On the outer sides bf the jugae and in front of the 

 eyes are the lorac, these however being often indistinct or even 

 wanting. Below the eyes and between the lorae and gula or 

 throat are the genae or lower cheeks. The lower side of the 

 head is composed of the gula or throat, lying above the base of 

 the beak when the latter is in repose. On the front of the 

 gula and each side of the base of beak are usually two plates 

 known as the bucculae. 



On the upper sides of the head are the two compound eyes. 

 They vary much in shape, size and convexity and are often 

 larger in the males than the females. of the same species. Each 

 is made up of many hundreds of six-sided facets or lenses, in 

 each of which ends a single filament of the optic nerve. Two 

 small simple eyes or ocelli are present in most Heteroptera, but 

 are absent in the largest family, the Miridae, and hence are 

 not shown in figure 1. They are usually situated on the ver- 

 tex between the basal halves of the compound eyes. These 

 ocelli are thought to be inherited from the obscure eyes of the 

 worm-like ancestry of the bug, while the many facetted com- 

 pound eyes of insects and crustaceans have been evolved to 

 satisfy the needs of the more recent existence of these groups. 



The rostrum, or beak as it is called in this work, is to the bug 

 the most necessary external appendage of the head. It is at- 

 tached to the lower front part of the head, close to the tip of 

 the tylus and when not in use is carried flat, usually in a groove, 



