INSECTS AND THEIR NEAR RELATIVES. 



23 



the greater part of the web be touched, it will adhere to the 

 finger, and will stretch, when the finger is withdrawn, to 

 several times the original length. But if one of the radiat- 

 ing lines or a portion of the outer framework be touched, it 

 will neither adhere to the finger nor be stretched. If the 

 spiral line be examined with a lens, it will be found to bear 

 numerous bead-like masses of viscid matter (Fig. 20); this 

 explains its adhesiveness. 



It is supposed that the two kinds of silk are spun from 

 different spinnerets, and that the viscid silk comes from the 

 front pair. When this silk is first spun the viscid matter 

 forms a continuous layer of liquid on the outside of it. But 

 very soon this layer breaks up into the bead-like masses — in 

 a way similar to that in which the moisture on a clothes-line 

 in a foggy day collects into drops. 



Spiders of the two families Dictynidte and Uloboridce 

 have spinning organs differing from those of all other 



Fig. 20. — Viscid silk 

 from an orbweb. 



Fig. 21. — Spinnerets of 

 a Dictynid spider. 

 The middle pair of 

 spinnerets are con- 

 cealed by the first 

 pair, c, cribellum. 



Fig. 22. — Last two segments 

 of hind leg of spider, show- 

 ing calamistrum. 



spiders. They have in front of the usual spinnerets an 

 additional organ, which is named the cribellum (cri-bel 'lum) 

 (Fig. 21). This bears spinning-tubes like the other spinner- 

 ets, but these tubes are much finer. These spiders have 

 also on the metatarsus of the hind legs one or two rows of 

 curved spines : this organ is the calamistrum (cal-a-mis'trum) 

 (Fig. 22). By means of the calamistrum these spiders comb 

 from the cribellum a band of loose threads, which forms 

 a part of their webs. 



