THE ARMATURE OF ORBWEBS : VISCID SPIRALS. 89 



When drawn out by the foot of the spider into the position of x-ab, 

 Fig. 80, how is this viscid matter affected? In order to test the effect 

 of expansion and sudden contraction upon viscid matter si)read along a 

 Hue, I covered tliin bands of India rubber with mucilage, and then sud- 

 denly stretched them. The result was that invariably globules or beads 

 were formed similar in .shape to those upon the spiral of an orbweb. A 

 twist of the band, by rolling it between the finger and thumb, caused the 

 globules to mass equally around the band, a position which they would 

 doubtless have kept permanently, as do the spider's beads, could the muci- 

 lage have hardened as rapidly as the viscid secretion from the spider does. 

 This tendency would jirobably be greatly increased were the spider when 

 emitting the viscid matter to give the line a sharp twist or even impart to 

 it a vibratory motion. At all events, we know that an elastic line drawn out 

 in the above manner and suddenly released is set into vibration, tlie mechan- 

 ical effect of which would certainly be in the direction here indicated. 



However, it is probable that a natural contraction or crystallization of 

 the viscid matter itself would follow when exposed to the air, and thus 

 produce the beads without any mechanical agitation, which 

 Natural nevertheless would undoubtedly assist the action of aggregation. 

 7. Various experiments showed that an elastic line, or elastic band, 



when covered with mucilage and then stretclied, soon becomes 

 threaded with a series of beads not unlike those on the web of the geo- 

 metric spider. The natural tendency of the material is evidently to gather 

 into these minute globular masses. If we suppose this to represent a gen- 

 eral tendency of viscid liquids, we have at once the reason why spiral 

 lines covered with viscid secretion as they are emitted, should soon present 

 the appearance of strings of beads. 



Incidentally it may be said that the elasticity of the beaded spiral en- 

 ables the spider to so graduate the interradials that they are all taut. 

 The fact that the radii converge upon the centre compels a 

 Equaliz- gj-g^^ difference in length between a string drawn at the outer 

 ^'^d' ^ ^^ margin and one at the interior. Does it not imply a great de- 

 gree of mathematical skill to pay out the requisite amount of 

 line as the distances gradually and continuously diminish ? I suppose that 

 the whole matter is solved chiefly by the elasticity of the spiral thread. 

 If the spider were to draw out an equal length of line in every case it 

 would bridge the wider interspace at the circumference and the narrower 

 one at the centre with eciual facility. 



AVhat is the condition of the spiral thread as it escapes from the spin- 

 nerets? Does it show at once the appearance of viscid beads? 

 How Qj.^ jg j^ smooth upon the surface, and do the nodules gradually 



ea sare^^^,^^ after emission? The difficulty of finding a spider at the 

 exact point of spinning necessary for this observation is in itself 

 considerable; to find a web so located as to allow study under a glass of 



