18 INVERTEBRATE ZOOLOGY 



The circulatory system. This system is very simple in insects, 

 owing to the great complexity of the respiratory system. In- 

 stead of the blood being carried to the respiratory organs to 

 be aerated, as is the case in vertebrates, rendering necessary 

 a complicated system of blood-tubes connecting the remotest 

 parts of the body with the respiratory organs, the respiratory 

 organs are themselves a system of tubes which introduce air to 

 every part of the body. The insect has a blood fluid which 

 lies in the body-cavity. The only circulatory vessel present is 

 the tubular heart. This organ, whose position has already been 

 noted, has a closed hinder end and segmental valvular openings 

 along its sides. By its contractions the blood is sent into the 

 forward portions of the body, whence it flows back into the 

 hinder portions, and enters the heart again through the valvu- 

 lar openings. To observe the heart of an insect is not always 

 easy, owing to its position so near the dorsal body-wall 

 and its great delicacy of structure. An easy method is to 

 mount a live, transparent, aquatic insect larva, such as that 

 of the mosquito, on a slide in water and observe it under a 

 compound microscope. The heart and its action may be easily 

 studied. 



The nervous system. Cut off the alimentary tract at its forward 

 end, taking care not to injure the two nerve connectives which 

 pass to the brain, and remove all the viscera from the body. 

 The nerve cord will be seen lying on the ventral body-wall of the 

 abdomen, in the median line, slightly concealed by fat. It will 

 be seen to be double and to contain, in the abdomen, five 

 enlargements, the ganglia, from each of which fine nerves radiate. 

 Trace the nerve cord from the abdomen into the thorax. It is 

 here protected by hard projections of the body-wall, which 

 must be carefully removed. Four large ganglia will be found 

 here, the three posterior ones of which are the thoracic ganglia. 

 The one in the forward portion of the prothorax really belongs 

 to the head and is called the subcesophageal ganglion. From it 



