Fig. 103. Last two segments of hind leg of spider, showing calamistrum.
It is supposed that the two kinds of silk spun by the orb weavers 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 bead-like masses—in a way similar to that in which the moisture on a clothes line on a foggy day collects into drops ([Fig. 101]).
There are two families of spiders that have spinning organs differing from those of all other spiders. They have in front of the usual spinnerets an additional organ, which is named the cribellum ([Fig. 102], c). This bears spinning tubes like the other spinnerets, but these tubes are much finer. These spiders have also on the next-to-the-last segment of the hind legs one or two rows of curved spines; this organ is the calamistrum ([Fig. 103]). By means of the calamistrum these spiders comb from the cribellum a band of loose threads which form a part of their webs.
The Curled-thread Weavers.
The spiders possessing a cribellum and a calamistrum represent two families, one of which makes irregular webs; the other, those which are of definite form.
Fig. 104. Web of a curled-thread weaver.
Fig. 105. Fragment of a curled-thread weaver's web, enlarged.