PHYLUM ARTHROPOD A 121 



sinus; this is evidenced by the complete lack of mesenteries. The coelome is 

 in fact practically entirely wanting in insects. A yellowish material, the fat 

 body, representing stored food, will be found attached to the viscera and body 

 wall. Note also the slender bandlike muscles originating on the body wall and 

 attached to the movable appendages. 



Place the specimen in a wax-bottomed dissecting pan, pin it down by pins 

 through the wall, and fill the pan with water. 



a) The respiratory system: There is perhaps no other system to be found in 

 the animal kingdom which so excites the admiration of the zoologist as the respira- 

 tory system of the insects. Unfortunately it is impossible to study it satis- 

 factorily in preserved material. It consists of a series of tubes, symmetrically 

 and segmentally arranged, called the tracheal tubes, or tracheae. These open 

 to the exterior through the spiracles, or stigmata, which have already been noted 

 on the surface of the body. To see the tracheal tubes, push the viscera of the 

 grasshopper gently to one side and look on the inside of the body wall opposite 

 the points where the spiracles are situated. A white tube will be seen extending 

 inward from each spiracle. With a little practice one will soon be able to identify 

 similar tubes throughout the body. Note also particularly in the thorax the 

 large white air sacs, which are connected with the tracheae and serve to increase 

 the capacity of the tracheal system. The tracheal system of the grasshopper 

 consists of three pairs of longitudinal trunks, connected by segmental branches, 

 giving off branches to all parts of the body and provided in certain places with 

 air sacs. Recall the tracheal system seen in the fly larvae (Section II, F, 3), and 

 examine Hegner's Figs. 135 (p. 243) and 137 (p. 244). 



Remove a piece of a tracheal tube, mount in a drop of water on a slide, and 

 examine with the microscope. Observe the spiral thread on the inside of the 

 tracheal tube. What is its function? Draw. 



The tracheae are tubes produced by ingrowth of the ectoderm and lined 

 therefore by the same chitinous layer which covers the body, in which the spiral 

 threads arise by local thickenings. The tracheae arise therefore in a manner 

 opposite to that of gills, which are outgrowths of the body wall. Both serve the 

 same purpose, an increase of surface. But while gills contain blood which 

 obtains oxygen through their thin walls, the tracheal tubes are filled with air. 

 The finest ramifications of the tracheal tubes are in contact with single cells; 

 and further, the blood in the large blood sinuses is in contact with the walls of 

 the tracheae. Thus oxygen is brought directly in contact with all parts of the 

 body, even the smallest; and a respiratory system which is regarded as the most 

 efficient among animals results. The air in the tracheal system is changed 

 through respiratory movements of the body. 



b) The circulatory system: Associated with the development of a remarkably 

 efficient respiratory system, there is a correspondingly poor differentiation of the 

 circulatory system. There is a long segmented heart situated in the median 



