12 INSECTS 



and activity is much the same among insects as it is 

 in the higher animals, although the method is some- 

 what different. The blood is equally the agent for 

 transporting nutritious material to whatever point it 

 may be needed, and the oxygen to burn out waste 

 products is as essential. 



Under the microscope, insect muscle is not so very 

 different from that of man: it has similar transverse 

 striations, and contracts and extends in a similar 

 manner, but, nevertheless, there is no chance of con- 

 fusing insect with vertebrate muscle. This muscular 

 system is under the control of a nervous system which 

 is very highly developed in detail, but consists essen- 

 tially of a double cord extending from one end of the 

 body to the other, lying just above the under or ventral 

 surface and furnished with a series of enlargements or 

 ganglia, of which that lying in the head and termed the 

 brain is the largest, although it exercises no such dominant 

 influence as does that organ in the higher vertebrates. 



There is quite a difference, of course, among insects, 

 in the amount of specialization in this nervous system. 

 In some and especially in larval forms all the ganglia 

 are similar in size and appearance, and there is one for 

 every segment; in others the tendency to centraHza- 

 tion is marked, all the thoracic ganglia being united into 

 one, while in the abdomen two or three of the posterior 

 gangha join in the control of the reproductive system 

 and the various accessory parts connected with it. The 

 thoracic centre controls the organs of locomotion and a 

 paralysis that is practical death results immediately 

 when this ganglion is cut, whereas we can cut off the 

 head and abdomen, of a house-fly for instance, without 

 interfering with its power to use legs or wings. This 

 fact was known to the digger wasps long before the 

 entomologist knew it, for when such a digger wishes to 



