DISSECTION OF A COCKROACH 



19 



muscular wall of the gizzard can be contracted, so as to 

 prevent the passage of the food to the stomach until it has 

 become fluid or semi-fluid. Then the gizzard is relaxed, and the 

 more or less fluid food passed into the stomach to be absorbed. 



The cockroach is particularly addicted to starchy food, but 

 will devour bits of flesh, and 

 almost everything which is eaten 

 by man. 



The alimentary canal is partly 

 embedded in a mass of white sub- 

 stance, the fat-body, which is made 

 up of a multitude of small fat- 

 particles, attached together and 

 overspread by fine tracheal tubes. 

 When the alimentary canal is dis- 

 placed, the fat-body is fully ex- 

 posed to view. Pull out some 

 pieces with the forceps, and place 

 them under the microscope to ob- 

 serve the tracheal branches. Add 

 ether to the preparation, arid re- 

 place the loss from evaporation as 

 required ; the fat slowly dissolves. 

 Add osmic acid (one per cent, 

 solution) ; the fat turns brown, and 

 ultimately black. TJiese tests for 

 small quantities of fatty substance 

 are often resorted to by the micro- 

 scopic observer. 



Between the wall of the body 

 and the alimentary canal is an 

 irregular space (the body-cavity) 

 partly occupied by the salivary 

 glands, Malpighian tubules, 

 ovaries, etc. It is of no great 

 extent in the cockroach, but be- 

 comes large in proportion to the 

 body in some insect- larv». 

 Through it the blood courses, after escapmg from the open 

 fore end of the heart, and returns agam to the heart by 

 lateral inlets. The circumstance that such organs as the 



Fig. 17. — Tracheal system of 

 cockroach. The dorsal _ integu- 

 ment removed and the viscera in 

 place. 



