CONSTRLX'TION OF AX OIIBWEB. 



75 



Fig. 71. Putting in a 

 radius. 



lines which net the enclosed space have such an entan- 

 glement with the guidon ball as to brace it sufficiently 

 from beneath, the spider instead of dropping down may 

 first climlj up the radius a, carrying a free thread in 

 one of her hind feet which is held out quite well from 

 the radius for this purpose as shown at Fig. 71. When 

 a desired point of attachment, as K, has been reached, 

 the deported line x is drawn taut, fastened, and the radius 

 e is now formed, which in this case will be the second radius. Next the 

 spider returns to the guidon H by e, or more likely by a, and thence 

 drops to the line, mn (Fig. 69), forming a third radius, c. The radii are 

 all inserted in the above manner, and not consecutively, but alternately on 

 the opposite sides of the included space, by single lines or successive coup- 

 lets. The behavior of different individuals, of the same species or of the 

 individuals of different species may show variations more or less decided, 

 but the above action is fairly typical. 



Blackwall says^ that the radii are formed by the spider " without 

 observing any regularity in the order of her progression." On the con- 

 trary there seems to be at least so much order in this act that a 

 Alternate ^^j.^ q£ alternation as to the orientation of the lines is observed, 

 ,. „ which I have called the alternate apposition of the radii, A 

 Radii. purpose to maintain a balance in the radial framework during 

 its construction is thus suggested, although, certainly, an absolute 

 regularity of alternate progression cannot be asserted. 



The order in which the radii are spun into the frame of the orb was 

 quite fully shown in the work of an Epeira sclopetaria observed at Alex- 

 andria Bay, New York. When the observation began the foundation lines 



were already laid, and also the original radii 

 marked A, B, C, D, Fig. 72. These cords were 

 united at the centre by a tuft of silk, and 

 braced by a few concentric lines, which form- 

 ed the basis of the hub. I counted seventeen 

 radii before the spider ceased. Their alter- 

 nate apposition can easily be seen by tracing 

 them in the order of the numerals in Fig. 72, 

 which are arranged in the order of construc- 

 tion. That is, radius 1 was formed by car- 

 rying the line along D to 1 and tightening it. 

 Thence the spider went to tlie centre, ran 

 along B, which had previously been inserted, 

 thence down to 2, where the radius 2 was 

 Fiu. 72. Alternate apposition of radii. formed. From 2 agaiu slie went to the centre, 



^ Op. Cit., page 182. 



