13 



In many instances, the compound eyes occupy by far 

 the greater part of the entire surface of the head, and the 

 number of elements or facets which go to make up one of 

 these compound eyes often runs to many thousands, although 

 sometimes it is small. 



It is not likely that insects have power of vision over 

 long range, or that they see distinctly. They are all conscious 

 of light and darkness, and some of them distinguish certain 

 colours; many of them discriminate moving bodies, and 

 others perhaps, recognize at greater or less distances those 

 insects on which they prey, or insects or birds to which they 

 are likely to fall a prey. 



Circulation. 



The arrangement of the circulatory, digestive and 

 nervous systems in insects is a typical characteristic. The 

 alimentary canal is central in position ; the organ of circulation 

 is dorsal, and on account of its position, has received the 

 name of dorsal vessel ; the nervous system is ventral. The 

 accompanying diagram (Fig. 1 1) represents a median longi- 

 tudinal section through the body of a caterpillar. The dorsal 

 vessel, the digestive system and the nervous system are 

 shown in their relative positions, and are referred to by 

 letters in the legend lines. The blood of insects is not con- 

 fined in veins, arteries and capillaries, as is the case in the 



Fig", ii. Diagrammatic vertical section lengthwise through 

 body of insect to illustrate relative positions of some of the 

 more important organs. 



(s), ser/meufs of ho ly wall: (m), muscle layer: (h) heart or da ml vesst I : 



(a), alimentary canal : (). nervous system : (), reproductive organs. 



(Redrawn from Coms'ock's Manna I for the Study of Insects.) 



higher animals ; it fills the body-cavity and bathes all the 

 organs, even penetrating into the legs and wings. It is 

 usually colourless, but is sometimes tinted yellowish, green, 

 and even red, but does not derive its red colour from red 

 corpuscles, and is not red blood in the same way as is the 

 blood of the higher animals. 



