8 THE STUDY OF INSECTS 



tubes on each spinneret. The silk is in a fluid state while it is within 

 the body, but it hardens as soon as it comes in contact with the air. 



Ordinarily the tips of the spinnerets are brought close together, so that 

 all the minute threads that emerge from the numerous spinning tubes 

 mite to form a single thread. This, however, may be so delicate as to be 

 invisible, except in a favorable light. Sometimes a spider will spread its 

 spinnerets apart, and thus spin a broad ribbon-like band. We have 

 observed a spider seize a large grasshopper which was entangled in its 

 web, and, rolling it over two or three times, completely envelop it in a 

 sheet of silk spun from its spread-apart spinnerets. 



In the construction of their webs the orb- weaving spiders make use of 

 two kinds of silk. One of these is dry and inelastic ; the other, viscid and 

 elastic. This fact can be easily seen by examining an orb web. If the 

 spiral line which forms the greater part of the web be touched, it will ad- 

 here to the finger, and will stretch, when the finger is withdrawn, to several 

 times the original length. But if one of the radiating 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. 17); 



~H-~ ~\^ this explains its adhesiveness. 



^-4 — - — — V"~ It is supposed that the two kinds of silk are spun from 



—J. \^ different spinnerets. When this silk is first spun the viscid 



matter forms a continuous layer of liquid on the outside of 



fig. 17.— viscid silk ft g u t verv soon this layer breaks up into 



from an orb web. , , ,-, • • -i 



the bead-like masses — ma way similar to \ 

 that in which the moisture on a clothes-line on a foggy day 

 collects into drops. 



Spiders of the two families Dictynidos and Uloboridce have FlG ™'"_ End of 

 spinning organs differing from those of all other spiders, a b jj°™ e J? showin s cri - 

 Thcy have in front of the usual spinnerets an additional 

 organ, which is named the cribellum (Fig. 18). This bears spinning-tubes 

 like the other spinnerets, but these tubes are much finer. These spiders 

 C have also on the metatarsus of 



-^g^-^^^S^^^^^^^^#^ thc hind le S s one or . two TOW . s 

 ^^^^^^^^^^^^m^ of curved spines: this organ is 



^^^^^^^^^^^^^^^^^^^ the calamistrum (Fig. 19). By 



means of the calamistrum 



Fig. 19. — Metatarsus of hind leg of spider showing calamistrum, c. ,i SoiderS COmb from the 



cribellum a band of loose threads, which forms a part of their webs. 



Spiders make use of silk in the construction of their webs or snares, in 

 the building of tubes or tents within which they live, in the formation of 

 egg-sacs, and in locomotion. 



Figure 20 represents the large egg-sac of one of the orb weavers. This 

 is made in the autumn, and contains at that season a large number of 

 eggs — five hundred or more. These eggs hatch early in the winter; 

 but no spiders emerge from the egg-sac until the following spring. If 

 egg-sacs of this kind be opened at different times during thc winter, the 

 spiders will be found to increase in size but diminish in _ number as the 

 season advances. In fact, a strange tragedy goes on within these egg- 

 sacs: thc stronger spiders calmly devour their weaker brothers, and in 

 the spring those which survive emerge sufficiently nourished to fight their 

 battles in the outside world. 



