MOTIONS OF INSECTS. 497 



him suspect that it acted like a cupping-glass, and so produced the adhe-' 

 sion.^ This circumstance affords another proof that the foot-cushions in 

 the Orthopiera may act the same part; they appear to be vesicular; and 

 in numbers of specimens, after death, I have observed that they become 

 concave, particularly in Acrida viridissima. 



In Cimbex, and others amongst the saw-fly tribes, the claw-sucker is 

 distinguished by this remarkable peculiarity, that its upper surface is con- 

 cave^, so that before it is used it must be bent inwards. Besides these, at 

 the extremity of each tarsal joint these animals are furnished with a spoon- 

 shaped sucker, which seems analogous to the cushions in the GrylUna, 

 Locustina, &c. ; and, what is more remarkable, the two spurs (calcaria) 

 at the apex of the shanks have likewise each a minute one.^ Various 

 other insects of this order have the claw-suckers. Amongst others the 

 common wasp (^Vespa vulgaris) is by these enabled to walk up and down 

 our glass windows. 



We learn from De Geer that several mites (to finish with the Apterd) 

 have something of this kind. Among these is the cheese-mite (Acarus 

 siro) ; its four fore feet being terminated by a vesicle with a long neck, 

 to which it can give every kind of inflexion. When it sets its foot down, 

 it enlarges and inflates it; and when it lifts it up, it contracts it so that 

 the vesicle almost entirely disappears. This vesicle is between twoclaws.^ 



The itch Acarus (A. scabiei) is similarly circumstanced. Ixodes Rid- 

 nus 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 

 probably maintain themselves in their station, by the assistance of a secre- 

 tion 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^ ; 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 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. "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."'' 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 



1 "De Geer, iii. 7. « Phihs. Trans. 1816, t. xix. f. 3, 4. 



3 PhUn-i. Traas. 1816, t. xix. f. 1—9. * Da Geer, vii. 91. t. v. f. 6, 7. 



6 Ibid. 96. t. V. f. 13, 14. 17. 19. t. vi. f. 2. 5. 



^ Ttie caterpillars of many Lepidopierous insects possess the same power. ^ 65. 



42* 



