1 1 6 Invertebrata. 



surface. Scarcely a plant exists but it harbours some 

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

 supply food for other species. Insects are usually of 

 small size and have the six foremost segments united 

 to form a head. The three succeeding segments form 

 a thorav, 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 seven segments not bearing 

 any limbs, and followed by one, two or three abdom- 

 inal rings, to which are appended the sting or its 

 equivalent, the ovipositor. A black beetle, a blue- 

 bottle 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. 



The head of an insect also bears one pair of 

 antennce 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 taste, and also of 

 communication 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 obtains its food. In beet- 

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

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



