44 



SILKWORMS. 



it is situated. There is a separate pair of ganglia for each seg- 

 ment, so that altogether we can count thirteen of them. If we 

 cut the nerves which proceed from their sides, as far away from the 

 ganglia themselves as possible, we may then raise the whole nervous 

 chain from its bed, and by slipping a strip of black paper under it, 

 float it away, and at the same time throw its form into great 

 distinctness by means of the dark background of the paper. 



There will now remain only two sets of apparatus in our cater- 

 pillar's body the breathing tubes or trachea, and the muscles. 

 We will take the former first. By the time all the preceding 

 dissection has been accomplished, the tracheae will have been 

 considerably damaged, and it will be better to open another 

 caterpillar if we wish to get a good view of the respiratory system 



as a whole. Opening it in the 

 same way as before, and simply 

 pinning out the skin without re- 

 moving any of the organs, we can, 

 if we turn the silk glands aside, 

 trace without much difficulty the 

 course of its main branches. At 

 the position of each spiracle there 

 will be seen a great branching in 

 all directions of tubes of varying 

 diameter (see Fig. 13), and run- 

 ning from each of these to its 

 neighbour above and below a 

 large, straight, unbranched tube. 

 Thus, down each side of the body, 

 in the line of the spiracles, there 

 is a long tube, with tufts of 

 branches at intervals, along its 

 course. Then, as has been pointed out before, numbers *of 

 minute tubes pass in branching tufts up to the walls of the diges- 

 tive tract and hold it in position by spreading over its surface. 

 The extreme ends of the tubes will be too fine to be followed 

 without a microscope ; they penetrate to the remotest corners of 

 the body, and carry on their beneficial work in all parts of the 



Fig. 15. Breathing tubes of insect. 



organism at once. 



The structure of the trachece is extremely interesting. Under 

 the microscope each appears as a transparent tube, in the interior 

 of which can be traced a spiral line with its coils very close to- 

 gether (Fig. 15). This is a stiff elastic thread which serves to keep 

 the tubes open, and prevents them from collapsing under any 

 pressure the surrounding parts may cause by the movements of 



