THE SILKWORM ITS INTERNAL STRUCTURE. 45 



the body. Of course closure of these tubes would be a serious 

 matter for the insect, the aeration of whose blood depends 

 entirely upon their being kept open and in free communication 

 throughout with the atmosphere ; hence the spiral-thread 

 arrangement, a device which we men have imitated in our non- 

 collapsible india-rubber gas-tubing, which carries a wire coiled up 

 inside. A very pretty sight do these tracheae present under the 

 microscope, and they have the advantage of being very easily 

 prepared for inspection. All that is necessary is to snip oft a 

 little piece of one of the tubes, to place it with a drop of water on 

 a glass slide, and then to cover it with a thin glass coverslip, and 

 it may then be put under the microscope and viewed most 

 successfully. 



The muscles are extremely numerous, and are easily seen as sets 

 of little, straight, whitish bands, arranged in various directions. 

 Some go straight down the sides, and by their contraction serve 

 to shorten the length of the body many of these are shown in 

 Fig. 13 ; others pass in various directions obliquely across the 

 segments, and by their united actions serve to alter the diameter 

 or the body. Others move the legs and claspers, and again others 

 the jaws, but these are too small to be dissected out by any but 

 an expert. This well-developed muscular system gives the body 

 of the caterpillar extreme flexibility, and enables it to be bent 

 about in all manner of directions. 



One more organ yet remains to complete our account of the 

 anatomy of the silkworm larva ; it is that delicate apparatus which 

 we necessarily destroyed on commencing our dissection, viz., the 

 dorsal vessel, or heart. To see this, we pin out another caterpillar, 

 wrong side upwards, i.e., on its back ; now cutting a slit in the 

 skin the whole length of the body, we pin it out, and then remove 

 the nervous and digestive organs, and the silk glands. By this 

 means we lay bare the organ in question. It runs along the 

 middle of the back, just under the skin, where it will be seen as 

 a filmy whitish band. Its structure is rather difficult to make out. 

 It consists of a flattened tube, closed at the hinder end, but 

 passing in front into a narrower tube which is the aorta, or main 

 artery of the body. This cannot, however, be traced very far 

 forward. The sides of the dorsal vessel have some perforations, 

 which serve as entrances for the blood. The whole apparatus is 

 overlaid as we see it in the dissection, i.e., really ?/#*&rlaid, by 

 a series of pairs of fan-shaped muscles, called the alary muscles, 

 whose function is not very certainly known. The blood of the 

 silkworm is not red like ours, but almost colourless. It is not 

 contained in blood vessels, for, with the exception of the rudi- 



