223 



THE BRITISH NATURALIST. [November 



ale of such action on the part of the spinners has never before been 

 observed. I therefore will give what I fancy may be the true key 

 which would explain this curious variation in custom. 



I have repeatedly watched Zilla constructing its web, and I have 

 observed that the filling in, or leavmg vacant, of the above-named 

 sector depends very greatly upon the angle which the "free radius" 

 or trap-line makes with the plane of the vv^eb. Now, in an ordinary 

 way, when Zilla, in laying down the spiral line, arives at the space 

 within which the "free radius" lies, she feels for the next radius, finds 

 instead the thickened trap-line, seems to recognise it as different from 

 an ordinary radius, turns right-about face and retraces her course a 

 short space above her last track. The same process is gone through 

 when she arrives once more at the "free radius" on the opposite side, 

 the result being that the sector in which lies the "free radius" is left 

 vacant, while the "spiral line" resolves itself into a series of 

 concentric circles. But, now suppose that the trap-line lies at so wide 

 an angle with the web that the spider fails to reach it with her fore- 

 legs in her course; she then simply swings over the wider space and 

 lays the "spiral Ime" just as an ordinary Epeira would. 



In beginning to lay the " spiral line," as she does, from the centre, 

 of course the trap-line will be for the first few rounds within easy 

 reach — hence it happens, that in many webs, only a sm^.ll unmeshed 

 aperture is left near the centre, at the apex of the wide and usually 

 vacant sector. 



This is not mere hypothesis trumped up to account for the 

 variation, but there are now before me on the buildings in Carlisle 

 numerous examples of the webs of Zilln x-notata, Clk., presenting 

 every variety of meshed and unmeshed sector. Some quite vacant, 

 others half filled up, while others are completely filled in — but in each 

 case the nearness or remoteness of the "free radius" to the plane of 

 the web is significant. I am convinced — for I have observed all kinds 

 of Zilla-vjehs under process of construction — that the angle which the 

 trap-line makes with the plane of the web is the determining factor 

 in this phenomenon. 



Dr. McCook, an accurate observer, in Vol. K of his " American 

 Spiders and their Spinning work," p. 138, says, " In the number of 

 these (webs) made by adults (August), the proportion of full round 

 orbs was much greater than sectoral orbs. I am not able to account 

 for this remarkable difference, as nothing in the site gave a clue." 



Now I think I can show that my observations w^ill furnish the 

 desired clue to the difference in proportion — which I, too, have noticed, 

 and can thus confirm Dr. McCook's report on that point. 



The older the spiders get the larger the webs become, as a rule; 

 the larger the web, the wider the space needed ^o stretch it in ; the 



