240 Mr. J. Blackwall on the Structure , Functions 



light construct nets which do not exhibit the slightest irregularity 

 of plan or defect of structure *. 



Dr. Lister supposed that spiders are able to retract the lines they 

 spin within the abdomen, and whoever minutely observes the Ep'eiroBi 

 when fabricating their snares, will almost be induced to entertain the 

 same opinion. The viscid line produced by these spiders in their 

 transit from one radius to another is sometimes drawn out to a much 

 greater extent than is necessary to connect the two, yet, on ap- 

 proaching the point at which it is to be attached, it appears to re- 

 enter the spinners, till it is reduced to the exact length required. 

 This optical illusion, for such it is, is occasioned by the extreme 

 elasticity of the line, which may be extended greatly by the applica- 

 tion of a slight force, and on its removal will contract proportionally. 

 By this property the viscid spiral line is accommodated to the fre- 

 quent and rapid changes in distance which take place among the 

 radii when agitated by winds or other disturbing forces, and by it 

 insects, which fly against the snare, are more completely entangled 

 than they otherwise could be without doing extensive injury to its 

 frame- work f. 



Complicated as the processes are by which these symmetrical nets 

 are produced, nevertheless, young spiders, acting under the influence 

 of instinctive impulse, display, even in their first attempt to fabricate 

 them, as consummate skill as the most experienced individuals. 



Although spiders are not provided with wings, and, consequently, 

 are incapable of flying, in the strict sense of the word ; yet, by the 

 aid of their silken filaments, numerous species, belonging to various 

 genera, are enabled to accomplish distant journeys through the at- 

 mosphere. These aerial excursions, which appear to result from an 

 instinctive desire to migrate, are undertaken when the weather is 

 bright and serene, particularly in the autumn, both by adult and 

 immature individuals, and are effected in the following manner. 

 After climbing to the summits of different objects, they raise them- 

 selves still higher by straightening the limbs; then elevating the ab- 

 domen, by bringing it from the usual horizontal position into one al- 

 most perpendicular, they emit from the spinners a small quantity of 

 viscid fluid, which is drawn out into fine lines by the ascending cur- 

 rent occasioned by the rarefaction of the air contiguous to the heated 

 ground. Against these lines the current of rarefied air impinges, till 

 the animals, feeling themselves acted upon with sufficient force, quit 

 their hold of the objects on which they stand and mount aloft. 



Spiders do not always ascend into the atmosphere by a vertical 

 movement, but are observed to sail through it in various directions ; 

 and the fact admits of an easy explanation when the disturbing 

 causes by which that subtile medium is liable to be affected are taken 

 into consideration. A direction parallel to the horizon will be given 

 by a cuvjeat of air moving in that plane ; a perpendicular one, by 



* Zoological Journal, vr'l. v. p. 181-188. Transactions of the Linnsean Society, 

 vol. xvi. p. 477-479. Researches in Zoology, p. 253-270. 

 f Researches in Zoology, pp. 267, 268. 



