20 INJURIOUS AND USEFUL INSECTS 



salivary glands and Malpighian tubules of insects are bathed 



in the blood, and not injected with blood, as in higher 



animals, probably explains their diffuse form. 



By tearing away the fat-body bit by bit the student will 

 bring into view the two large 

 tracheal tubes which run along the 

 sides of the body. They send out 

 branches to all the viscera, and com- 

 municating branches to one another, 

 besides receiving short branches 

 from the spiracles. Cut out a piece 

 of a tracheal trunk, and examine it 

 with the microscope. Notice the 

 spiral thickening of the lining cuticle ; 

 the silvery or dark appearance of 

 the tubes varies according to the 

 direction in which the light falls 

 upon them, and is due to contained 

 air. 



Clear the nervous system. Note 

 the ganglia and the connectives 

 which unite them. 



Follow the nerve cord into the 

 head. The dissection is too difficult 

 for an unpractised hand. The head 

 must be fixed to a block of paraffin 

 with one side uppermost. See that 

 the head is dry externally ; then 

 melt the paraffin in one place with 

 a piece of hot metal, and lightly 

 press the head against it. Dissect 

 away the side of the head. In front 

 will be seen the brain, which gives 

 off two very short connectives to 

 the sub -oesophageal ganglion. A 

 chitinous plate, the tentorium, is 

 interposed, through which the con- 

 nectives pass. The oesophagus lies 



upon the tentorium, and passes between the connectives. 

 In a female cockroach the ovaries, which lie in the hinder 



part of the abdomen, may be cleared and examined. Each 



Fig. i8. — Nervous system of 

 female cockroach. 



