SHEET-BUILDING SPIDERS 161 



of this until I saw the same occur in the broiHng heat 

 of the Euphrates valley. There some little spiders 

 used to spin their sheets of web close to the river. 

 In the mornings they were often spangled with dew. 

 Then the sun would rise ; its rays would grow intense 

 the moment it appeared ; the temperature would 

 slowly creep up to ioo° F. ; the sands would burn 

 to the touch and the air quiver with shimmering heat, 

 but the glittering drops would still remain suspended 

 from the sheet of silk. Of such protection is this 

 webbing that, even at midday in a broiling sun, I 

 have seen the drops still pendent on the sheet. No 

 doubt this property is of value to the spider in retaining 

 for its use a plentiful supply of moisture. 



Another physical property of the spider's snare 

 which adds still further to its own intrinsic beauty is 

 its power of separating the white sunlight into its 

 primary constituent rays. We see this best in the 

 circular snare when it is stretched between a pair of 

 pines high above us in the forest. The snare is sus- 

 pended in the vertical line, the sun is approaching the 

 zenith, and we look from below at a steep angle into 

 the snare so as to see the sun's rays streaming down 

 through it from above. We look up at it through the 

 dark trees ; we can scarcely see it against the clear 

 sky, when suddenly it becomes suffused with a lovely 

 glow and a rich stream of coloured light illuminates its 

 silken lines. Every filament has become a prism ; the 

 sun's white light is broken into many parts and the 

 whole circle of the silken fabric trleams with a rainbow 

 light. It is a vision of transient beauty amidst the 

 conifers when all around is the silent forest wrappc^d 

 in a gloomy shade. 



M 



