THE HEAD. 611 



and the hole left in the head, when it is cut off, as the occipita 

 foramen. Running ventral wards behind the eyes from the 

 epicranium to the base of the mandibles and on its inner surface 

 giving origin to the stout mandibular muscle, is the gena or 

 cheek. When the head is prolonged in an antero-posterior 

 plane the region of the head between the occiput and the base 

 of the fused second maxillae or labium is often distinct and is 

 known as the gula. 



Certain hard parts, strengthening the walls of the head and 

 acting as surfaces for the origin of muscles, occur inside the 

 head. As examples of these the tentorium of the Cockroach and 

 the two hollow cylindrical pillars which pierce the head of the 

 gnats may be cited. 



There are two median processes, one in front, and one behind 

 the mouth, which are known as the epipharynx and the hypo- 

 pharynx respectively and which are not usually regarded as 

 having the value of appendages. The former arises from the 

 inner surface of the labrum and in most of the mandibulate 

 insects exists as a membranous lining to that part and to the 

 clypeus. It bears certain taste organs. In such piercing insects 

 as the gnats and fleas it is prolonged with the labrum and helps 

 to form the channel up which the blood of the prey flows. On 

 the opposite or posterior wall of the mouth, lining the labium, 

 is a median process which, when it remains soft and fleshy, is 

 usually referred to as the tongue or lingua or ligula, but when as 

 in gnats (Fig. 376) it is prolonged and forms a projecting 

 process, it is termed the hypopharynx. On it the salivary ducts 

 open. The hairy grooved organ of bees, which projects from 

 the middle of the labium, is usually called the ligula : there is 

 much uncertainty about its homologies. 



According to Heymons the first segment of the head forms 

 the clypeus and labrum, whilst the epicranium and genae 

 are formed from the tergites, or elements of the tergum, of the 

 three jaw-bearing segments whose fused sternites form the 

 hypopharynx. 



The head is united with the thorax by a short flexible neck 

 whose wall is often strengthened by certain chitinous thicken- 

 ings called the cervical sclerites. 



The thorax is composed of three segments, the pro-, meso- and 

 meta-thorax. These remain fairly distinct in such insects as the 



