INSECTS AND ARACHNID A. 251 



lie across these, forming its concentric circles, or rather polygons, are 

 studded at intervals with viscid globules (B), which appear to give to 

 these threads their peculiarly adhesive character; and it does not seem 

 by any means unlikely that each kind of thread should be produced by 

 its own pair of spinnerets. It was observed by Mr. R. Beck, that these 

 viscid threads are of uniform thickness when first spun; but that undu- 

 lations soon appear in them, and that the viscid matter then accumulates 

 in globules at regular intervals . The total number of spinning-tubes va- 

 ries greatly, according to the species of the Spider, and the sex and age of 



FIG. 439. 



Ordinary thread (A), and viscid thread (B), of the common Spider. 



the individual; being more than 1000 in some cases, and less than 100 in 

 others. The size and complexity of the secreting glandules vary in like 

 manner: Thus in the Spiders which are most remarkable for the large 

 dimensions and regular construction of their webs, they occupy a large 

 portion of the abdominal cavity, and are composed of slender branch- 

 ing tubes whose length is increased by numerous convolutions; whilst 

 in those which have only occasional use for their threads, the secreting 

 organs are either short and simple follicles, or undivided tubes of moder- 

 ate length. 



