THE PROCESS OF SPINNING 343 



How the thread is drawn out. Having seen, says Blanc, how the 

 two masses of silk (brins), in passing through the spinning appara- 

 tus (or press), join each other, constituting the frothy silken fluid, 

 thus becoming modified in form, it remains to examine the way in 

 which the thread is drawn out of the spinneret. If we examine a 

 caterpillar while spinning, it will be seen that in moving its head it 

 draws on the frothy mass of silk fixed to the web of the cocoon. 

 This traction certainly aids very much the exit of the thread, but it 

 is not the only cause. 



The silk, Blanc affirms, is pushed out by a force a tergo, developed 

 by different agents, such as the pressure of the distended cuticle or 

 the silky mass contained in the reservoir, as seen in the section of a 

 worm which has spun its cocoon. But if we consider a caterpillar 

 before it has begun to spin, it is difficult to explain the mechanism 

 of spinning. As Blanc has often observed, in making sections of 

 the heads of silkworms, two cases arise. Sometimes the worm has 

 already spun a little, and a certain length of the frothy silk (bave) 

 issues from the orifice of the spinneret, where it forms a small 

 twisted bundle. At other times the worm has not spun since its 

 last moult or the frothy mass of silk has broken within the head, 

 and we find the end in the common tube. In the first of these two 

 cases, the worm, dilating its press, is able by a general contraction 

 to discharge a little of the gritty material (gr&s) which lines the ball 

 of silk hanging at the end of the spinneret. It can also reject a 

 certain quantity of the secretion of Filippi's glands and thus soften 

 the gritty substance. The little plug of silk can then adhere to the 

 body with which it conies in contact. 



In the same case it is necessary that the two bits or portions of 

 silk traverse the press, and this normally has a calibre less than 

 their diameter. The worm should then distend the spinning tube 

 as much as is practicable, so as to make the openings as large as 

 possible. It has been stated that the press is, in this condition, at 

 least as large as the mass of frothy silk. This Blanc believes 

 (although Gilson thinks otherwise) is pushed by a force a tergo, and 

 reaches the funnel of the spinning canal ; its two bits of silk (brins) 

 unite there, penetrate into the canal itself, and, owing to successive 

 impulses produced by the general contractions of the worm, press 

 through and pass out of the spinneret. 



While the silkworm is engaged in spinning its cocoon, the spin- 

 neret and press execute very varied movements, determined by the 

 elevator, depressor, retractor, and protractor muscles of the labium, 

 as well as those of the press. These movements, originally very 



