1 1 6 Invertebrate. 



surface. Scarcely a plant exists but it harbours some 

 species of the tribe, and many animals, living or dead, 

 supply food for other species. Insects are usually of 

 small size, and have the four, five or six foremost seg- 

 ments united to form a head. The three succeeding 

 segments form a thorax, which alone bears the legs, 

 one pair on each of its rings, and when wings are 

 present they are borne by the middle and hindmost 

 of these thoracic rings. The abdomen consists of 

 eight segments not bearing any limbs, and followed by 

 one, two or three post-abdominal rings, continuous 

 with and not easily distinguished from the hindmost 

 abdominal segments. A cockroach, a bluebottle fly, 

 and a butterfly may be taken as types of the class. 



Organs of Sense. The head of an insect bears a 

 pair of compound eyes, and often several simple eyes 

 in a cluster. The former have a cornea or transparent 

 surface divided into many facets, each of the nerve rods 

 having its own pigment mass and its own cornea. In 

 the common house-fly there are 2000 such facets in each 

 eye, and in the dragon-fly there are 28,000 (fig. 56). 



The head of an insect also bears one pair of 

 antenna or feelers, jointed organs which vary much in 

 shape and structure, being sometimes simple, filiform, 

 comb-like, or lamellar. These are organs of touch 

 and hearing, possibly of smell, and also of communi- 

 cation between one insect and its fellow. 



Month. The mouth is on the fore and under part 

 of the head, and varies in shape according to the 

 method whereby the insect is accustomed to feed. In 

 beetles, dragon-flies, &c. the mouth is armed with chew- 

 ing jaws. There are two lips, an upper or labrum (fig. 



