A LARGE GRASSHOPPER 13 



aid of a hand lens examine the tracheae in different parts of 

 the body. They may be easily detected by their silvery gleam. 

 Notice the arrangement of the main tracheal trunks, including 

 those which connect with the spiracles, also the arrangement of 

 the air sacs, which are expansions of tracheae. Mount a small 

 portion of the fatty tissue containing tracheae in water or glycerine 

 and examine them with a compound microscope. Notice the 

 spiral threads which line the tracheae. Find the connections of 

 the tracheae with the spiracles. 



Exercise 14, Make a drawing of a trachea seen under a high power of 

 the microscope. 



The Circulatory System. This system is very simple in insects, 

 being correlated with the great complexity of the respiratory sys- 

 tem. Instead 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 por- 

 tions and enters the heart again through the valvular openings. 

 To observe the heart of an insect is not always easy, because of 

 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 sKde 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 for- 

 ward 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, often concealed by fat. It will be 



