268 



MOTIONS OF insects. 



insects ascend and descend by means of a thread that issues 

 from them 1 ; but perhaps every one has not remarked — when 

 they wish to avoid a hand held out to catch them, or any 

 other obstacle — that they can sway this thread from the per- 

 pendicular. When they move up or down, their legs are 

 extended, sometimes gathering in and sometimes guiding their 

 thread; but when their motion is suspended, they are bent 

 inwards. These animals, although they have no suckers or 

 other apparatus — except the hairs of their legs and the three 

 claws of their biarticulate tarsi, to enable them to do it — 

 can also walk against gravity, both in a perpendicular and a 

 prone position. Dr. Hulse, in Ray's Letters, seems to have 

 furnished a clue that will very well explain this. I give it 

 you in his own homely phrase. u They " (spiders) " will often 

 fasten their threads in several places to the things they creep 

 up ; the manner is by beating their bums or tails against 

 them as they creep along." 2 Fixing their anus by means of 

 a web, the anterior part of their body, when they are resting, 

 Ave can readily conceive, would be supported by the claws 

 and hairs of their legs ; and their motion may be accomplished 

 by alternately fixing one and then the other. But you will 

 remember I give you this merely as conjecture, having never 

 verified it by observation. 3 



It may not be amiss to mention here another apterous in- 

 sect that reposes on perpendicular or prone surfaces, without 

 either suckers or any viscous secretion by which it can ad- 

 here to them. I mean the long-legged or shepherd spiders 

 {Phalangiwnri). The tarsi of these insects are setaceous, and 

 nearly as fine as a hair, consisting sometimes of more than 

 forty joints, those toward the extremity being very minute, 

 and scarcely discernible, and terminating in a single claw. 

 These tarsi, which resemble antennas rather than feet, are 

 capable of every kind of inflexion, sometimes even of a spiral 

 one. These circumstances enable them to apply their feet to 



1 The caterpillars of many Lepidopterous insects possess the same power. 



2 65. 



3 Mr. Blackwall, as before stated, conceives that the power possessed by 

 spiders which use no threads, such as Drassus vielanogaster, Salticus scenicus, &c, 

 of walking up polished surfaces, is derived from an adhesive fluid emitted from 

 the tubular hair-like appendages of their tarsi. (Linn. Trans, xvi. 480. 769.) 



