MOTIONS OF INSECTS. 463 



and Reduvius have also these vesicles which are armed with two claws 

 on all their feet. 1 



I am next to consider those climbers that ascend and descend, and pro- 

 bably maintain themselves in their station, by the assistance of a secretion 

 which they have the power of producing. You will immediately perceive 

 that I am speaking of the numerous tribes of spiders (Araneidce), which, 

 most of them, are endowed with this faculty. Every body knows that 

 these insects ascend and descend by means of a thread that issues from 

 them' 2 ; but perhaps everyone 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 perpendicular. 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 ap- 

 paratus except the hairs of their legs and the three claws of their biarti- 

 culate 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. " 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." 3 Fixing 

 their anus by means of a web, the anterior part of their body, when they 

 are resting, we 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. 4 



It may not be amiss to mention here another apterous insect that re- 

 poses on perpendicular or prone surfaces, without either suckers or any 

 viscous secretion by which it can adhere to them. I mean the long-legged 

 or shepherd spiders (Phalangiuni). The tarsi of these insects are seta- 

 ceous, and nearly as fine as a hair, consisting sometimes of more than forty 

 joints, those towards the extremity being very minute, and scarcely dis- 

 cernible, and terminating in a single claw. These tarsi, which resemble 

 antennae 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 the inequalities of the surface on which they repose, so that every joint 

 may in some measure become a point of support. Their eight legs also, 

 which diverge from their body like the spokes from the nave of a wheel, 

 give them equal hold of eight almost equidistant spaces, which, doubtless, 

 is a great stay to them. 



The next species of locomotion .exhibited by perfect insects is./Zying. J 

 am not certain whether under this head I ought to introduce the sailing 

 of spiders in the air ; but as there is no other under which it can be more 

 properly arranged, I shall treat of it here. I shall therefore divide flying 

 insects into those that fly without wings, and those that fly with them. 



1 De Geer, 96. t. v. f. 13, 14. 17. 19. t. vi. f. 2. 5. 



2 The caterpillars of many Lcpidopterous insects possess the same power. 



3 65. 



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

 which use no threads, such as Drassus melanogaster, 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.) 



