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. [Illustration: _Fig. 103. Last two segments of hind leg of spider, showing calamistrum._] 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. [Illustration: _Fig. 104. Web of a curled-thread weaver._] An irregular web of a curled-thread weaver is shown in Fig. 104, from a photograph. In this web the framework is of ordinary silk; and upon this framework is placed a band of curled or tangled threads (Fig. 105). An insect alighting on a net of this kind is likely to get its feet caught in the tangled silk, and to be held fast till the spider can pounce upon it. Nets of this kind are found on bushes and on the sides of buildings. [Illustration: _Fig. 105. Fragment of a curled-thread weaver's web, enlarged._] There are two quite distinct types of regular webs made by spiders possessing a cribellum and a calamistrum. One is a round web which resembles at first sight those of the orb weavers; but it differs from the ordinary orb-web in that the spiral thread is made of curled or hackled silk. These webs are nearly horizontal, and are usually made between stones or in low bushes; they are not common. [Illustration: _Fig. 106. Web of the triangle spider._]