SPIDERS: THEIR STRUCTURE AND HABITS. 123 



specimen of this genus that weighed only about 20 grains. This 

 is precisely what we might have expected, — that nature has been 

 careful to proportion the supply to the demand, here as elsewhere. 



(2) — In the same class of spiders the spinning-tubes are very 

 unequally distributed among the three pairs of spinnerets, — being 

 far more numerous, and at the same time more minute, on the 

 lower pair than on the upper and intermediate ones. 



(3) — The lower pair in all spiders have two spinning-tubes 

 much larger than the rest ; and in the Epeiridce the upper and 

 intermediate pairs also have each two or three that are larger, and 

 of different shape from the others. 



(4) — In all cases the silk-glands are larger or smaller according 

 to the size of their respective tubes. 



From the unequal distribution of the spinning-tubes we might 

 conjecture that all the pairs of spinnerets have not the same 

 office ; and when we observe that the Epeiridce and others, which 

 spin three varieties of thread, have three pairs of spinnerets, and 

 that the CiniflofiidcE (to be noticed presently), which spin four 

 varieties of thread, have four pairs, we seem naturally led to the 

 conclusion that each of the different sorts of thread which con- 

 tribute to the composition of a web, is the separate formation of 

 one pair of spinnerets, specially adapted for that one thread. 



Connecting facts 3 and 4 with the large supply of viscid 

 material requisite for the wants of an Epeira, there appears to be 

 ground for assuming that the five large glands and spinning-tubes 

 attached to the upper and intermediate pairs of spinnerets 

 furnish the adhesive liquid, and apply it as a varnish to the 

 elastic threads drawn out from the lower pair. It would then 

 run into dots or globules, like moisture on a hair, by the attraction 

 of cohesion. The liquid gum used for soldering purposes may 

 likewise be the special product of the two large glands and 

 tubes, always present in the lower pair of spinnerets. 



The feet of the Retiary Spiders are beautifully adapted to their 

 office of rope-walking and rope-making. They need no scopula, 

 but are provided with three principal claws at the extremity of 

 the tarsus^ and several secondary ones on its under side, all being 

 pectinated (Plate 12, Fig. 7). In some species of Epeira, as 

 many as five of these secondary claws may be counted, and their 

 office is obviously to guide the threads drawn out in traversing 

 their complicated webs, so as to prevent entanglement. Many of 

 the Epeiridce have also a strong, movable spine inserted near the 

 end of the tarsus of each hind leg, on the under side, which 

 bends abruptly upwards at its extremity towards the claws. This 

 serves the office of a thumb, and with the claws gives the foot a 

 firm grip of that thread by which the creature suspends itself, 



