456 MOTIONS OF INSECTS. 
(Dytisci), as given for sexual purposes.! The three first joints of the anterior 
tarsi of many of the larger rove-beetles (Staphylinus L.) are dilated so as 
to form, as in the last-mentioned insects, an orbicular patella, but covered by 
cushions. Since in them this is not peculiar to the males, it is probably 
given that they may be able to support their long bodies when climbing, 
But the most remarkable class of climbers consists of those that are 
furnished with an apparatus by which they can form a vacuum, so as to 
adhere to the plane on which they are moving by atmospheric pressure, 
That flies can walk upon glass placed vertically, and in general against 
gravity, has long been a source of wonder and inquiry; and various have 
been the opinions of scientific men upon the subject. Some imagined that 
the suckers on the feet of these animals were sponges filled with a kind of 
gluten, by which they were enabled to adhere to such surfaces. This idea, 
though incorrect, was not so absurd as at first it may seem ; since we 
have seen above in many instances, and very lately in that of the Sinin- 
thurus fuscus, that insects are often aided in their motions by a secretion of 
this kind. Hooke appears to have been one of the first who remarked that 
the suspension of these animals was produced by some mechanical con- 
trivance in their feet. Observing that the claws alone could not effect 
this purpose, he justly concluded that it must be principally owing to the 
mechanism of the two palms, pattens, or soles, as he calls the suckers ; these 
he describes as beset underneath with small bristles or tenters, like the wire 
teeth of a card for working wool, which having a contrary direction to the 
claws, and both pulling different ways, if there be any irregularity or yielding 
in the surface of a body, enable the fly to suspend itself very firmly. That 
they walk upon glass he ascribes to some ruggedness in the surface ; and 
principally to a smoky tarnish which adheres to it, by means of which 
the fly gets footing upon it.? But these tenter-hooks in the suckers of 
flies, and this smoky tarnish upon glass, are mere fancies, since they can walk 
as well upon the cleanest glass as upon the most tarnished. Reaumur also 
attributes this faculty of these animals to the hairs upon their suckers.’ 
That learned and pious naturalist, Dr. Derham, seems to have been one of 
the first who gave the true solution of this enigma. “ Flies,” says he, 
“besides their sharp hooked nails, have also skinny palms to their feet, 
to enable them to stick on glass and other smooth bodies by the pressure of 
the atmosphere.” * He compares these palms to the curious suckers of male 
Dyftisci, before alluded to, and illustrates their action by a common practice 
of boys, who carry stones by a wet piece of leather applied to their top. 
Another eminent and excellent naturalist, the late Mr. White, adopted 
this solution, He observes that in the decline of the year, when the 
mornings and evenings become chilly, many species of flies retire into 
houses and swarm in the windows; that at first they are very brisk and 
alert ; but, as they grow more torpid, that they move with difficulty, and 
are scarcely able to lift their legs, which seem as if glued to the glass ; and 
that by degrees many do actually stick till they die in the place. Then, 
noticing Dr. Derham’s opinion as just stated, he further remarks that they 
easily overcome the atmospheric pressure when they are brisk and alert. 
1 See Kirby, in Fauna Boreali- Americana, on various modifications of these foat- 
cushions amongst some tribes of beetles, 
2 Microgr. 170. 5 iv. 269, 
4 Physico-Theol, ed. 18. 868. note 
