188 MR. T. H. CORRY ON ASCLEPIAS CORNUTI. 
being accomplished by the foot remaining included between the ale while a simple, 
quiet dragging motion of the leg upwards in the alar chamber takes place in a perfectly 
vertical direction. As the apex of each of the ale terminates immediately above the 
lower border of the widely expanded base of the black corpusculum, it follows of necessity 
that when the foot reaches the superior end of the alar chamber in which it has been 
guided, one at least of the two hooked claws upon it, or some part of the foot in the case 
of Diptera, must easily enter the hollow cavity of the corpusculum, which lies in such a 
position that this result is inevitable*. If the leg is then drawn further upwards, part 
of it travels along the fissure on the anterior side of the corpusculum, the hooked claw 
of the tarsus being firmly caught and held by the projecting margins which bound the 
chink, and which gradually converge superiorly, so that a wedge-shaped fissure is the 
result. Its foot and leg in this way become so firmly fastened that the insect, finding 
itself fairly and completely caught, begins to make efforts to free itself, and in its further 
movement draws its foot forcibly outwards and slightly upwards. Ву so doing the insect 
detaches the whole corpusculum from its excreting furrow, to the bottom of which this 
body is not at this stage in any way adherent, and then draws it outwards likewise, 
together with the pair of pollinia which are attached to it by means of its “ appendages,” 
and which are pulled out of their open pouches and carried away (fig. 19). If, however, the 
leg is, on the contrary, pushed downwards, the insect is unable to bring away the pollinia ; 
but if it is strong, it tears the corpusculum away from its “appendages” and escapes 
with it; while if it should happen to be small and weak, as e.g. a small Fly or an Ant, 
it is unable to do this, and remains hanging to the corpusculum, being in this way kept 
a prisoner, and must either release itself by detaching a leg, which may be then found 
attached to the corpusculum, or, being unable to do so, e.g. in the case of the Ant, it 
perishes by starvation t. The corpusculum is not, then, as it is commonly said to be, 
cemented or adherent to the insect's leg by any viscous matter. On the other hand, the 
connexion of the two is owing simply to the fact that the limb of the insect, entering the 
hollow cavity of the corpusculum in a position determined by the surrounding parts, is 
firmly caught fast in its fissure, owing to the peculiar configuration of the latter, which, 
as has been shown, is wide at first inferiorly, and rapidly narrows upwards to a point. 
Further, in the expanded flower the corpusculum is not at all adhesive or viscid ў, but 
is hard and possesses a perfectly firm, smooth surface. Its shining appearance has per- 
haps led those ignorant of its real mode of formation and the subsequent changes whieh 
it undergoes to conjecture that it was sticky. "The adhesion, then, cannot be due to any 
cementing liquid, neither is the vertical fissure on the anterior surface of the corpusculum 
* [The hooks of the tarsi have very little to do with the retention of the leg or withdrawal of the pollinia, as it 
often happens that the proboscis, or even single stout hairs on the legs of an insect, enter the alar chamber, and are 
caught by its elastic sides, and so withdraw the pollinia.—N. E. Br.] 
t The operation of extraction by insects may be easily observed by the simple expedient of placing the hand over 
the blossoms at some distance above them, when, the insects which have settled there being startled by the shadow 
and attempting to fly upwards, their feet become caught within the ale on which they are resting for support, and 
thus the pollinia are extracted. 
+ As stated most recently by C. Darwin, vide ‘The Effects of Cross and Self-fertilization in the Vegetable 
Kingdom,’ p. 375. : 
