94 INSECTS CHAP. 



on its under-side ; this arises from extension of some part of the 

 epicranium, as shown in Fig. 49, B, where h, the cavity of 

 insertion of the antenna, appears to be ' situate on the under- 

 surface of the epicranium, the appearance being due to an 

 infolding of an angle of the part. 



There is always a gap in the back' of the head for the passage 

 of the alimentary canal and other organs into the thorax ; this 

 opening is called the occipital foramen. Various terms, such as 

 irons, vertex, occiput, temples, and cheeks, have been used for 

 designating areas of the head. The only one of these which is of 

 importance is the gena, and even this can only be defined as the 

 anterior part of the lateral portion of the head -capsule. An 

 extended study of the comparative anatomy of the head-capsule 

 is still a desideratum in entomology. The appendages of the head 

 that are engaged in the operations of feeding are frequently 

 spoken of collectively as the trophi, a term which includes the 

 labrum as well as the true buccal appendages. 



The appendages forming the parts of the mouth are paired, 

 and consist of the mandibles, the maxillae, and the labium, the 

 pair in this latter part being combined to form a single body. 

 The buccal appendages are frequently spoken of as gnathites. 

 The gnathites are some, if not all, of them composed of apparently 

 numerous parts, some of these being distinct sclerites, others 

 membranous structures which may be either bare or pubescent 

 that is, covered with delicate short hair. In Insects the mouth 

 functions in two quite different ways, by biting or by sucking. 

 The Insects that bite are called Mandibulata, and those that suck 

 Haustellata. In the mandibulate Insects the composition of the 

 gnathites is readily comprehensible, so that in nearly the whole 

 of the vast number of species of that type the corresponding 

 parts can be recognised with something like certainty. This, 

 however, is not the case with the sucking Insects ; in them the 

 parts of the mouth are very different indeed, so that in some 

 cases moxphologists are not agreed as to what parts really 

 correspond with some of the structures of the Mandibulata. At 

 present it will be sufficient for us to consider only the mandibulate 

 mouth, leaving the various forms of sucking mouth to be 

 discussed when we treat of the Orders of Haustellata in detail. 



The upper or anterior pair of gnathites is the mandibles, 

 (Fig. 50, y). There is no part of the body that varies more than 



