216 EVENINGS AT THE MICROSCOPE. 



Now the microscope, or a powerful lens, will reveal the 

 cause of this difference ; the threads of the cables and 

 radii are perfectly simple, while the spiral threads are 

 closely studded with minute globules of fluid, like drops 

 of dew, which, from the elasticity of the thread, are 

 easily separated from each other.* These are globules of 

 viscid gum, as is easily proved by touching one or two 

 with the finger, to which they will instantly adhere ; or by 

 throwing a little fine dust over the nets, when the spirals 

 will be found clogged with dirt, while the radii and cables 

 remain unsoiled. It is these viscid threads alone that 

 have the power of detaining the vagrant flies which acci- 

 dentally touch the net. 



The diversity in the secreting organs already alluded to, 

 as well as in the spinnerets, is no doubt connected with 

 this difference in the character of the silk ; and it is 

 worthy of remark, that this diversity is greatest in such 

 Spiders as the Upeirce, which spin geometric nets. 



Immense is the number of globules of viscidity that 

 stud the spiral circles of one of these nets. Mr. Blackwall, 

 the able and learned historian of the tribe, has estimated 

 that as many as 87,360 such pearly drops occurred in 

 a net of average dimensions, and 120,000 in a large net 

 of fourteen or sixteen inches diameter ; and yet a Spider 

 will construct such a net, if uninterrupted, in less than 

 three-quarters of an hour. 



Scarcely less admirable are the ease and precision with 

 which the little architect traverses her perpendicular or 

 diagonal web of rope ; a skill which leaves that of the 

 mariner who leaps from shroud to backstay in a ship's 

 rigging immeasurably behind. To understand it, however, 

 in some measure, look at this last joint of one of the feet 



* Mr. Bichard Beck (" Trans. Micr. Soc.," ix. 17) has ascertained that 

 the spiral thread is, when first spun, of uniform thickness ; but, after 

 some exposure, the viscid matter spontaneously accumulates at most 

 regular distances, and forms itself into globules, which are larger and 

 smaller alternately. It is a beautiful example of molecular attraction. 



