ON_ AERONAUTIC SPIDERS. 261 
being drawn out by the ascending current of rarefied 
air into fine lines several feet in length, was carried 
upwards, until the spiders, feeling themselves acted 
upon with sufficient force in that direction, quitted 
their hold of the objects on which they stood, and 
commenced their journey by mounting aloft. 
Whenever the lines became inadequate to the pur- 
pose for which they were intended, by adhering to 
any fixed body, they were immediately detached from 
the spinners, and so converted into terrestrial gossa- 
mer, by means of the posterior pair of legs, and the 
proceedings just described were repeated, which 
plainly proves that these operations result from a 
strong desire felt by the spiders to effect an ascent. 
But what, it may be asked, is the exciting cause of 
this singular propensity? It has been suggested that 
hunger, or an inclination to procure some favourite 
kind of food, may supply the requisite stimulus. 
These suppositions, however, are discountenanced by 
the plump appearance which the animals exhibit, by 
their total disregard of such winged insects as happen 
to be placed within their power, by their utter 
inability to regulate their motions, while afloat, in 
any other manner than by letting out or drawing in 
the lines by which they are conveyed through the air, 
and thus promoting their ascent or descent, by the 
unsuitableness of the lines for securing their prey, 
and, lastly, by the uncertainty when a favourable day 
for their purpose may occur, or even that one may 
