The Internal Anatomy of Spiders 



These glands He directly on the ventral side of the body cavity; 

 they are long, cylindrical, of uniform diameter, and convoluted 

 (Fig. 176). They resemble in structure the ampullate glands. 



The function of these glands is the production of the silk 

 of which the egg-sac is made. Spiders of the two families in 

 which they are wanting do not spin true egg-sacs. This is the 

 only case of a sexual difference in the silk glands of spiders. 



The aggregate or treeform glands. — These are found only in 

 the orb-weaving spiders and in the Linyphiidae and Theridiidae. 

 They are a symmetrical in form, being irregularly branched and 

 lobed. (Fig. 177.) The duct bears on its middle part many knobs 

 which are filled with cells. There are six of these glands, four large 

 and two small. Three open near together on the inner surface of 

 each hind spinneret, through spigots the tip of which is pointed 

 (Fig. 1 72 e). The term aggregate was suggested by the lobed form 

 of the glands, which gives them a compound appearance. 



It is believed that the function of these glands is to secrete 

 the viscid drops of the viscid and elastic thread. As these glands 

 are found in the Theridiidae, as well as in the orb-weavers, it is 

 important to determine whether viscid threads occur in the 

 webs of these spiders. It is certain that they are uncommon or 

 at least are inconspicuous, but they have been observed in the 

 webs of Linyphia, Steatoda, and Theridion. 



The lobed glands. — These occur only in the Theridiidae and are 

 either two or four in number. They open on the hind spinnerets 

 through spigots without a tip (Fig. 178). The lobed glands are very 

 irregular in form, consisting of a mass of irregular lobes (Fig. 179). 



The silk of which the swathing film of the Theridiidae is made 

 is secreted by the lobed glands. Correlated with their presence 

 is the development of only a small number of aciniform glands. 



The cribellum glands. — These are found only in those fami- 

 lies in which a cribellum and calamistrum are present. They 

 open on the cribellum through exceedingly small pores (Fig. 141, 

 p. 135). The glands are exceedingly numerous; they are spherical 

 and are often grouped; that is several many be enclosed in a 

 common sheath of peritoneal membrane. 



It is probable that the function of the cribellum glands is 

 to secrete the woof of the hackled band. 



Figure 180 represents the relative position of the silk glands 

 in a spider. 



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