1 68 ELEMENTARY ZOOLOGY 



gas are not carried by the blood but by special air-tubes. 

 The respiratory system of insects is very different from 

 that of other animals. 



Mount a bit of trachea in glycerine on a glass slide and 

 examine under the microscope. Note the fine spiral line 

 (looking like transverse annular striations) which is a 

 thickening of the chitinous inner wall of the tube and 

 which by its elasticity keeps the tracheal tubes open. 



The heart, already noted, is composed of a longitudinal 

 series of very thin-walled chambers, each with a pair of 

 lateral openings into the body-cavity and with terminal 

 openings into the adjacent chambers. The blood, which 

 is colorless or greenish or yellowish, is sent forward 

 through the successive heart chambers by regular contrac- 

 tions until it finally pours from the most anterior chamber 

 freely into the body-cavity. Here it bathes the body- 

 tissues, flowing perhaps in regular paths, giving up food 

 to the tissues and taking up food from the alimentary 

 canal, until it finds its way through the lateral openings 

 into the heart chamber again. There are no arteries or 

 veins. 



Note the large mass of muscles in the metathorax. 

 Note, by attempting to remove it, that the anterior part 

 of the muscle mass is attached to a chitinous partition-wall 

 between the meso- and meta -thorax. Remove this parti- 

 tion-wall (and one between the metathorax and abdomen) 

 and note that certain muscles run deeply down into the 

 body. By pulling on the bits of chitin to which the 

 muscles are attached, the muscles (if they have not been 

 cut) can be stretched to the length of three-quarters of an 

 inch. When released they will contract. (This stretch- 

 ing and contracting takes place only in fresh specimens.) 

 What are these large and numerous muscles of the thorax 

 for? 



Remove the thin membrane stretching over the abdomen 



