152 INTRODUCTION TO ZOOLOGY. 



of each single thread, and then of the mathematical precision 

 and rapidity with which, in certain cases, the net itself is 

 constructed; and to add to all this, as an example of the 

 wonders which the most common things exhibit when carefully 

 examined, the net of the Garden-spider consists of two dis- 

 tinct kinds of silk. The threads forming the concentric circles 

 are composed of a silk much more elastic than that of the 

 rays; and are studded over with minute globules of a viscid 

 gum, sufficiently adhesive to retain any unwary fly Avhich 

 comes in contact with it. A net of average dimensions is 

 estimated by Mr. Blackwali, to contain 87,360 of these glo- 

 bules, and a large net of fom-teeu or sixteen inches in diameter, 

 120,000 ; and yet such a net will be completed by one species 

 {Epeira apoclisd) m about forty minutes, on an average, 

 if no interruption occur.* In ordinary circumstances, the 

 threads lose their viscidity by exposure to the air, and require 

 to have it renewed every twenty-four hours. Any observer, 

 by scattering a little fine dust over the web, may satisfy 

 himself that it is retained only on the circles where the minute 

 globules are placed, and not upon the radii.f If the globules 

 are removed, both lines are unadhcsive ; but in other respects 

 they are essentially different, the circular lines being trans- 

 parent and highly clastic, while the radial lines are opaque, 

 and possess only a moderate degree of elasticity. The astro- 

 nomer finds the opaque silk of the radial lines and of the 

 egg-bag a convenient substitute for platina wires in the tele- 

 scopes attached to his instruments ; but the silk of the cuxulai- 

 lines being transparent, is, from that circumstance, unsuitable 

 for his purpose-! The nets of some Spiders are constructed 

 under water — the secretion being insoluble — and are spread 

 out for the capture of aquatic insects. 



A great deal of false commiseration has been bestowed 

 npon the flies which fall victims to the voracity of the Spider, 

 Avho has accordingly been regarded as " Cunning and fierce, 

 mixture abhorred." But considered ai'ight, there is no cruelty 

 in any animal exercising, for its support, those powers with 

 which it has been endowed by its Creator. It does not kill 



• Trans. Linnican Society, vol. xvi. page 478. 



t Kirby and Spence, vol. i. page 419. 



X Tliis fact has been very kindly communicated to us by the Rev. 

 Dr. Robinson, Armagh Observatory. The silk there employed is procured 

 from the egg-bags of the common Garden Spider (^Epeira Diadema). 



