MOTIONS OF INSECTS. 463 
and Reduvius have also these vesicles — which are armed with two claws 
—on all their feet." 
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 (Araneide), 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; 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 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 fails against them as they creep along,”® 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.* 
Tt 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 (Phalangium). 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 
antennz 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 flying. T 
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. TI shall therefore divide flying 
Insects into those that fly without wings, and those that fly with them. 
1 Do Geer, 96, t. v. f. 18, 14, 17. 19. t. vi. f. 2. 5. 
; nee caterpillars of many Lepidopterous insects possess the same power. 
5. 
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.) 
