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finger, and will stretch, when the finger is withdrawn, to 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 Dictynidce and Uloboridce 
have spinning organs differing from those of all other 
Fig. 20.—Viscid silk Fig. 21.—Spinnerets of Fig. 22.—Last two segments 
from an orb web. a Dictynid spider. of hind leg of spider, show- 
The middle pair of ing calamistrum. 
spinnerets are con¬ 
cealed by the first 
pair, r, cribellum. 
spiders. They have in front of the usual spinnerets an 
additional organ, which is named the cribellum (cri : berium) 
(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. 
