i6 



INJURIOUS AND USEFUL INSECTS 



muscles and heart will have been removed with it, unless 

 special pains have been taken to leave them behind. Observe 

 the segmentally arranged muscles and the heart, which looks 



like a transparent streak 

 running along the 

 middle line. 



To examine the de- 

 tails of the heart, it is 

 best to harden it with 

 chromic acid or some 

 other fixing agent, and 

 alcohol. It must then 

 be carefully separated 

 from the cuticle, stained 

 and mounted in balsam. 

 Beginners are not ad- 

 vised to attempt this. 



Pin the insect down 

 in a dissecting dish, and 

 add water until it is 

 just covered. The pins 

 must not stand up ver- 

 tically, lest they should 

 get in the way of the 

 dissecting instruments, 

 but should be inserted 

 nearly horizontally. 

 Free the turns of the 

 alimentary canal, and 

 lay it out on one side 

 of the dissection (out- 

 side the body), but 

 without cutting it 

 through at any point. 



Notice the compon- 

 ent parts of the ali- 

 mentary canal. These 

 are, in order from before backwards: (i) The oesophagus or 

 gullet, which issues from the head, and soon dilates into 

 (2) the crop, a large pear-shaped, thin-walled sac, widest 

 behind ; (3) the gizzard or proventriculus, a firm and thick- 



Fig. 15.— Alimentarycanal of cockroach. X z. 



