58 AMERICAN SPIDERS 



of each posterior spinneret through a spigot. They produce the 

 silk for the egg sac. 



j. The aggregate, or tree-form glands. There are six of these 

 irregularly branched, compound glands, opening on the inner sur- 

 face of each posterior spinneret through spigots. From these glands, 

 which are found only in the Argiopidae, Linyphiidae, and Theri- 

 diidae, are produced the viscid drops for the viscid threads of 

 the web. 



6. The lobed glands. Found only in the Theridiidae, these are 

 irregular in shape and lobed, opening on the posterior spinnerets 

 through spigots. The swathing film of the family is produced in 

 these glands, which are developed largely at the expense of the 

 aciniform glands. 



7. The cribellum glands. These numerous, spherical glands open 

 on the cribellum through many tiny pores. They occur only in the 

 cribellate spiders, and secrete the woof of the hackled band. 



As is to be expected, those spiders that use many types of silk 

 have the greatest number and volume of glands. The abdomen of 

 the sedentary orb weavers is largely filled by glands; whereas the 

 vagrants are less bountifully supplied. In some males the cylindrical 

 glands are missing, and in many males the other glands are less well 

 developed than in females. Inasmuch as the male's need for some 

 types of silk virtually ceases when he becomes adult, the lack of 

 specific glands is of no great importance. 



The spider has at its command these various types of silk glands 

 and can call upon them for its many needs. Flexible fingers are the 

 spinnerets: they can be extended, withdrawn, compressed, and 

 manipulated like human hands. The filaments produced are some- 

 times simple threads in multiples of two, but frequently they are 

 composite lines and are drawn from different glands. The viscid 

 spiral of the orb-weaver snare, for example, is composed of a double 

 ground line, possibly coming from the aciniform glands, on which 

 is superimposed a thin coating of viscid silk from the aggregate 

 glands. Only when this line is spun in a particular way does it take 

 on the characteristic form of a beaded necklace. The spiral is spun 

 rather slowly, and the spider pulls out the coated line and lets it 

 go with a jerk. As a result, the fluid is arranged in globules, spaced 

 along the line and far more sticky than a thin, uniform covering. 

 The rate of pull and the degree of the tension determine the finished 

 product. The spider spins leisurely or swiftly, according to its need. 



