INTRODUCTION. 
11 
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 application of a slight force, and on its removal will contract propor¬ 
tionally. By this property the viscid, spiral line is accommodated to the frequent 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 framework. 
Complicated as the processes are by which these symmetrical nets are produced, 1 never¬ 
theless 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. 
By contributing to check the too rapid multiplication of insects, from which they chiefly 
derive their sustenance, spiders perform an important part in the economy of nature. They 
devour one another also, the weaker falling victims to the more powerful; and as female 
spiders, with few exceptions, are larger and more vigorous than males, they frequently prey 
upon the latter, sometimes, indeed, immediately after they have received their embraces. 
Their enemies, however, are not limited to those of their own kind; quadrupeds, birds, fishes, 
reptiles, and even insects, destroy them in large numbers. 
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 
atmosphere. These aerial excursions, which appear to result from an instinctive desire to 
migrate, are undertaken when the weather is bright and serene, particularly in 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 abdomen, by bringing it from the usual horizontal position into one almost 
perpendicular, 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 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 current of air moving 
in that plane ,• a perpendicular one, by the ascent of air highly rarefied ; and directions inter¬ 
mediate 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 the 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 are carried out from the spinners by a current of air 
appears to be this. As a preparatory measure, the spinning mammulse are brought into close 
1 ‘Zoological Journal/ vol. v, pp. 181-188. ‘ Researches in Zoology/ pp. 253-270. 
