46 SILKWORMS. 



mentary aorta mentioned above, no such organs are discoverable. 

 It simply fills all parts of the body not occupied by the different 

 organs, but, nevertheless, it does not remain stagnant, but is 

 caused to circulate through the length and breadth of the body. 

 The mechanism which produces this result is the dorsal vessel. 

 If we watch carefully the back of a living silkworm, we shall see 

 down the middle a mark which appears to be just underneath the 

 skin, and is like a bluish or greyish band. It is continually 

 changing its diameter ; at one time it contracts till it almost 

 disappears, and then again expands to its full width ; but, as the 

 narrowing does not take place throughout its whole length at one 

 time, but progresses gradually from behind forwards, there is 

 afforded the curious appearance of a series of waves coursing 

 towards the head. This is the beating of the heart, and the 

 bluish band is none other than the dorsal vessel itself. From the 

 general cavity of the body the blood enters the heart by the slits 

 in its sides, and then, by the contraction of its walls, it is pushed 

 forward through that organ, and out at the aorta into the body 

 cavity again. On the expansion of the walls of the heart, more 

 blood rushes in from the body, to be again propelled towards the 

 head as before. 



The rate of the pulsations depends, of course, upon the health 

 of the insect, which again is affected by the quality of its food. 

 When it is fed on its natural diet of mulberry leaves, the heart has 

 been observed to beat from forty to forty-five times per minute, 

 but when fed on lettuce, which is sometimes used by amateurs 

 as a substitute for the true food, this rate, which indicates the 

 natural and healthy condition of the animal, is reduced to from 

 twenty to twenty-five, showing a considerable falling off in vitality 

 and energy. 



The reproductive organs do not exist in any other than a 

 rudimentary condition till the insect becomes a moth. The 

 caterpillar is, functionally, neither male nor female, and has no 

 power of reproducing its kind. On the other hand, the silk 

 glands which are very highly developed in the silkworm, are con- 

 fined to the caterpillar stage, and in the perfect insect no trace 

 of such a thing can be found. This is, of course, no more than 

 was to be expected. The chief object of the glands is to provide 

 silk for the formation of the cocoon, and almost the only other 

 occasions on which they are brought into play are the moulting 

 periods, just before each of which, as has been already mentioned, 

 they are called upon to provide the few threads required for 

 anchoring the old skin. By the time the cocoon is finished, they 

 are, as a rule, emptied of their contents, and as no more food 



