THE ARMATURE OF ORBWEBS : VISCID SPIRALS. 85 



ill onliT, in cases where the spiral .scaffold is not easily accessible.' I have 

 occasionally observed this action, Init cannot certainly continn tiie observa- 

 tion as habitual. At all events, the diagram will show this alternate move- 

 ment as it may be made between the radius 5 and radius (j, or again be- 

 tween radius G and radius 7. 



From this point onward, while crossing laterally the lower portion of 



the orb, and again while a.scending, the sti'idiug movements which have 



been described and illustrated by the previous figures become necessary. 



Usually, however, upon the downward course, she drops from one 



. radius to another, and thus proves herself, like any human la- 



>^3iVirig'. , . . Till I'll' 



borer m mechanics ami architecture, both able and willing to 



make the best use of her time and strength by varying her ordinary habit, 

 and availing herself of natural conditions. I have described what is the 

 habitual course, as it may be seen by any careful observer. But there occur 

 exceptions, and I have sometimes had occasion to note tlie fact that some 

 spiders allow themselves to be so swayed by habit that they persist in 

 crawling around the sides of all the sections, instead of relieving them- 

 selves by the direct drop after the manner of their congeners. I suppose 

 tliat even in Spiderdom there is room for the ultra conservative consti- 

 tution. 



Mr. Blackwall thus describes the mode of spinning the spirals, which 

 correctly summarizes the detailed account which I have given. From the 

 circumference of the orb the spider j)asses along a radius to the outer 

 line of the spiral scaffolding (having fixed a viscid thread to the end of 

 the radius) — along which she goes to the adjoining radius, drawing out 

 the tliread, in her transit, with tlie claws of her hind leg nearest tlie 

 circumference. She then transfers the thread to the claws of the other 

 hind leg, and passing down the radius at which she has just arrived to the 

 circumference, she places the foot of her hind leg j^reviously employed in 

 drawing out the threads on the point in the radius to which 

 Black- ],(^j. fiianient is to be attached, and bringing her spinners to the 



. ,. 'spot there makes it secure. The precise place in each radius at 

 scription. ^ , 



which to fix the thread is always ascertained by the situation 



of the foot of the hind leg, and this is determined by touching with the 

 feet of those legs nearest the circumference, the marginal line, or when 

 the structure of the net is further advanced, the last formed circumvolu- 

 tion of the viscid spiral line.^ 



The spider advances, as we have seen, by a zigzag movement, partly 

 striding, partly swinging, catching alternately upon the opposing founda- 

 tion spirals and the next radius. Every radius crossed, at every crossing, 

 is touched as above described and the spiral caused to adhere. In some 

 orbs whose radii number forty or fifty, and the spirals as high as sixty, 

 the number of attachments is very great — 2500 or 3000, and they are 



1 "Nets of Geometric Spiders," page 183. 



