INTRODUCTION 7 



out from the front end of the hind-gut and lie freely in the 

 great blood-space that surrounds, as we have seen, the 

 digestive system. The epithelium that forms the thin 

 walls of these " Malpighian " tubes — so named in honour of 

 Marcello Malpighi, the great seventeenth- century pioneer 

 of insect anatomy — separates from the surrounding blood 

 the waste matter in a state of solution, so that along the 

 tubes it may pass into the intestine and out of the body. 



The working of the various systems thus briefly reviewed 

 is co-ordinated by means of the nervous system, as ever the 

 seat of general control of an animal's activities. An insect 

 is essentially a segmented animal, and in each segment is 

 present a pair of closely apposed nerve-centres or gangHa, 

 usually situated just within the ventral body-wall. The 

 ganglia of the successive segments are linked up by a pair 

 of longitudinal nerve- trunks, and from each ganglion nerves 

 pass to the muscles and other structures of its segment 

 (Figs. 10, 12), so that each ganglion, while it controls the 

 action of organs in its own segment, is capable of receiving 

 nerve-impulses from or sending them to other segmental 

 ganglia. But in insects there may often be observed a 

 tendency for the ganglia of two or more successive segments 

 to become fused together, resulting in an integration of the 

 nerve-centres and a consequent centralisation of nervous 

 control. Such integration is notably illustrated by the 

 coalescence, in all insects, of the anterior three or four 

 ganglia of the head to form a brain (Figs. 10, 6 ; 12, op), 

 situated above or in front of the mouth, and linked to the 

 ganglia behind it by paired nerve trunks passing one on 

 either side of the gullet. From the brain the general 

 activities of an insect are clearly directed, yet mutilation of 

 the creature shows that considerable power of control 

 resides in local centres, for the hinder region of the body 

 may, after separation from the head, continue to move in a 

 manner seemingly purposeful. 



In any animal a large number of its habitual motions 

 may be shown to occur as definite responses to external 

 stimulations of various kinds. Some irritable nerve-ending 



