182 



HAEDWICKE'S SCIENCE-GOSSIP. 



The first and second spinnerets always produce 

 plain or non-adhesive threads : if the spider be of a 

 species that spins viscid threads, these are always 

 emitted by the third pair. There is one family of 

 British spiders which has an extra and very remark- 

 able pair of spinnerets in the lower set, which pro- 

 duce threads of a peculiar character: they are de- 

 scribed furtlier on. 



As may be supposed, I selected the commonest 

 spiders for observation, and house-spiders happened 

 to come handiest. The web of a Tegenaria, and I 

 believe of every spider, contains three sorts of 

 threads, not two only, as usually stated. Two of 

 these are plain, and stretched taut from point to 

 point (a and b, fig. 118), and they differ in nothing but 

 size, being spun by the first and second spinnerets, 

 of which in all spiders the first is larger than the 

 second, although in some instances it has a fewer 

 number of silk-tubes. The third thread ((?,fig.llS)is 

 exceedingly elastic, and studded with viscid globules, 

 or, if these be absent (as in the web selected for illus- 

 tration), it is slack, irregular, and sometimes much 

 curled. The existence of viscid threads in a 

 Tegenaria's web seems to me doubtful, for with a 

 Smith & Beck's best quarter-inch objective (the 

 highest power within my reach) no globules are 

 visible, though several contributors to this journal 

 have stated that they may be found. The thick 

 threads which form the beams or foundation of the 

 ■web, are spun by the first pair of spinnerets and are 

 often doubled or trebled. This is especially the 

 case at the edges of the web, where greater strength 

 is necessary. 



The appariitus by means of which a spider forms 

 its silk is a series of glands within the abdomen, 

 near and attached to the spinnerets, and immediately 

 beneath the liver and intestinal canal. The glands 

 of the upper and lower sets of spinnerets differ 

 somewhat in character and shape, as is noted below. 

 Fig. 119 is a drawing of one of the third spinnerets of 

 Tegenaria domestica, with its glands, of which only 

 a few are shown. These communicate with the silk- 

 tubes by ducts {b). They vary in size in different 

 individuals, but in a large Tegenaria Tsoth of an inch 

 is an average length. Each gland has its own duct 

 and silk-tube. On the first pair of spinnerets there 

 are about CO silk-tubes ; on the second pair, although 

 the spinnerets are smaller, about 80. The silk-tubes 

 on these two pairs are alike ; but they differ in shape 

 from those of the third pair and are much larger 

 (see fig. 120, a and i^). There are nearly 220 tubes on 

 the third pair, thus making altogctlicr about 3G0 on 

 the six spinnerets. 



The glands likewise, ■which are proper to the first 

 and second pairs of spinnerets, differ from those 

 belonging to the third. Fig. 121 represents one of 

 them with its duct and silk-tube, drawn to the same 

 scale as figure 119, for the sake of comparison. It is 

 a simple sac, slosed at one end, and terminating at 



the other in the duct, which carries the secretion to 

 the silk-tube. On the surface of the gland is a coat- 

 ing of cells, probably epithelial, which are surrounded 

 by a very delicate membrane. The points of differ- 

 ence in the silk-glands of the third spinnerets are 

 these : they are smaller (about one quarter the 

 length), of a different shape, and chiefly, they are 

 enveloped by a bag or case interposing between the 

 actual gland and the epithelium (see A, fig. 122, b 

 and c) (which bag is wanting in the other glands), 

 while the epithelium is apparently without the mem. 

 branous covering by which in them it is always sur- 

 rounded. This case continued as a tube surrounds 

 the duct for some distance, in all probability as far 

 as the silk-tubes, but I have not been able to trace 

 it so far. 



These distinctions in form and construction of the 

 upper and under sets of spinnerets with their glands, 

 prove, I think, the common idea that each of the 

 threads of a spider's web is the joint production of 

 the six spinnerets (from which it necessarily follows 

 that their being viscid or plain depends merely on 

 the will of the animal) to be quite eiToneous ; and 

 on the contrary, they prove that each of the three 

 sorts of threads which go to compose a web is the 

 separate formation of one pair of spinnerets specially 

 adapted for producing that one thread. 



It has been argued that the drops of liquid silk 

 coalesce as they emerge from the spinnerets, and so 

 for.m a simple homogeneous thread, but various ob- 

 servations which have before been related in Science- 

 Gossip, have convinced me that such is not the case. 

 The following also tends to contradict this theory, 

 namely — when a garden spider has caught a fly, as 

 every one knows, she very expeditiously binds it in 

 a covering of silk. Until I saw the exact process, 

 I often wondered how she could manage to accom- 

 plish this so quickly. She places the tips of her six 

 spinnerets almost in a line, at the same time seem- 

 ing to erect each separate silk-tube, and thus puts 

 forth, not a single thread, but a broad band of many 

 detached threads, which is rapidly wound round the 

 unfortunate fly. The examination of the web of a 

 house-spider under a high magnifying power, will 

 show that many of its main threads are frayed, like 

 a rope worn by use ; this could not occur if they 

 were homogeneous. 



MICROSCOPY. 



How TO Cut Leaf-sections. — Mr. C. Stewart 

 describes his mode of obtaining sections of fresh 

 leaves as follows : — He inserts a piece of the leaf 

 in a notch cut in a carrot, and then cuts slices 

 through both carrot and leaf. The sections are 

 afterwards soaked in water in a watch-glass under 

 an exhausted receiver, stained with bajmatoxylon 

 and transferred through water, absolute alchobol. 



