78 REPORT — 1844. 



placed in situations absolute!}' impen'ious to 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 Epeircs, when fabri- 

 cating 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 approaching the point at which it is to be attached, 

 it appears to re-enter the spinners, till it is reduced to the exact length re- 

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

 elasticity of the line, which may be extended greatly by the application of a 

 slight force, and on its removal will contract proportionally. By this pro- 

 perty the viscid spiral line is accommodated to the frequent and rapid changes 

 in distance which take place among the radii M'hen agitated by Windsor 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 pro- 

 duced, nevertheless, young spiders, acting under the influence of instinctive 

 impulse, display, even in their first attempt to fabricate them, as consum- 

 mate skill as the most experienced individuals. 



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

 capable 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 atmosphere. These aerial excur- 

 sions, which appear to result from an instinctive desire to migrate, are under- 

 taken 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 

 themselves still higher by straightening the limbs ; then elevating the abdo- 

 men, by bringing it from the usual horizontal position into one almost perpen- 

 dicular, they emit from the spinners a small quantity of viscid fluid, which is 

 drawn out into fine lines by the ascending current occasioned by the rare- 

 faction 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 par- 

 allel to the horizon will be given by a current of air moving in that plane ; 

 a perpendicular one, by the ascent of air highly rarefied ; and directions in- 

 termediate between these two will, in general, depend upon the composition 

 of forces. When the horizontal and vertical currents are equal in force, the 

 line of direction will describe an angle of 45° nearly with the plane of tiie 

 horizon ; but when their forces are unequal, the angle formed with that plane 

 will be greater or less as one current or the other predominates. 



The manner in which the lines of spiders are carried out from the spinners 

 by a current of air appears to be this. As a preparatory measure, the spin- 

 ning mammulae are brought into close contact, and viscid matter is emitted 



* Zoological Journal, vol. 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. 



