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TEXT-BOOK OF ENTOMOLOGY 



that the passage of the threads is facilitated. If the muscles totally 

 contract, the spinning canal is opened wide, the threads pass easily 

 upwards and assume the form of a triangular prism (Fig. ,'>:;<; i. 



If this contraction diminishes, the chitinous wall of the spinneret 

 comes together, owing to its elasticity ; the ceiling of the canal ap- 

 proaches the floor ; the cavity tends to take the form of a semicircu- 

 lar slit, and the threads are compressed, flat- 

 tened. As each mass or thread of silk is much 

 more voluminous than the canal, except when 

 the latter is extremely dilated, it follows that 

 the two threads are always compressed, or 

 squeezed together, and that each of them is 

 compelled to mould itself in the groove it 

 occupies and to take its shape. Hence the 

 variations in the appearance of the two masses 

 or divided portions of silk, which as stated 

 present all grades between the form of an 

 isoceles-triangular prism and that of a nearly 

 flat ribbon; but this last case is quite rare. 

 The use of the spinneret, then, is to compress 

 the thread and to change its form more or 

 less considerably, at the same time as it dimin- 

 ishes its diameter. 



Moreover, this constant compression of the 

 thread as it passes through the press keeps 

 it in a certain state of 

 tension so as to allow the 

 caterpillar while spinning 

 to firmly hold its thread. 



Finally, when the worm 

 suspends the contraction 

 of its spinning muscles, 

 the press flattens, vigor- 

 ously compresses the 

 thread, and arrests its motion, in such a way that if there Avas a 

 strain on the silken fluid (bave), it would break rather than oblige 

 the caterpillar to let go any more of it. 



The press does not act directly on the silken thread, but through 

 the gummy layer (gr^s) which transmits over the Avhole surface of the 

 silken fluid (brin) the pressure exerted on it. After having overcome 

 this difficult passage, the silk thread has acquired its definite form ; 

 it rapidly passes out of the spinneret. 



N 



FIG. 335. Spinning ap 

 paratus, seen from above 

 A, opening of the spinneret 

 , central canal of the spin 

 neret (C) ; />, common canal 

 E, canal of Filippi ; F, excre 

 tory canal of a silk-gland 

 i, oriflce of the canal c 

 Filippi's gland ; I, rachis 

 k, ring of the infundibulum 

 b, c, d, e, f, cavity of the 



FIG. 336. Diagram of 

 the press and its muscle-: 

 (/. lower; b, lateral; <. up- 

 per muscles of the pn>~. 

 After Blanc. 



