INDIRECT BENEFITS DERIVED FROM INSECTS. 169 
insinuate themselves amongst the filaments, and thus, while seeking their 
own food, unknowingly fulfil the intentions of nature in another de- 
partment. * , ’ 
The agency of these little operators is not less indispensable in the 
beautiful tribe of Jris. In these, as appears from the observations of 
.Kélreuter, the true stigma is situated on the upper side of a transverse 
membrane (arcus eminens of Haller), which is stretched across the middle 
of the under surface of the petal-like expansion or style-flag, the whole of 
which has been often improperly regarded as fulfilling the office of a stigma. 
Now, as the anther is situated at the base of the style-flag which covers 
it, at a considerable distance from the stigma, and at the same time cut off 
from all access to it by the intervening barrier formed by the arcus eminens, 
it is clear that but for some extraneous agency the pollen could never 
ossibly arrive at the place of its destination. In this case the humble-bee 
is the operator. Led by instinct, or, as the ingenious Sprengel supposes, 
by one of those honey marks (Saflmaal) ov spots of a different colour from 
the rest of the corolla, which, according to him, are placed in many flowers 
expressly to guide insects to the nectaries, she pushes herself between the 
stiff style-flag and elastic petal, which last, while she is in the interior, 
presses her close to the anther, and thus causes her to brush off the pollen 
with her hairy back, which ultimately, though not at once, conveys it to 
the stigma. Having exhausted the nectar, she retreats backwards: and 
in doing this is indeed pressed by the petal to the arcus eminens ; but it is 
only to its lower or negative surface, which cannot influence impregnation. 
She now takes her way to the second petal, and insinuating herself under 
its style-flag, her back comes into close contact with the true stigma, which 
is thus impregnated with the pollen of the first visited anther; and in 
this manner migrating from one part of the corolla to another, and from 
flower to flower, she fructifies one with pollen gathered in her search after 
honey in another. Sprengel found that not only are insects indispensable 
in fructifying the different species of Jris, but that some of them, as Z. 
wviphium, require the agency of the larger humble-bees, which alone are 
strong enough to force their way beneath the style-flag; and hence, as 
these insects are not so common as many others, this Iris is often barren, 
or bears imperfect seeds.? Sprengel also contends, that insects are essen- 
tially necessary in the impregnation of Asclepiadee ; in which opinion he 
is confirmed by the conclusive testimony of the celebrated botanist Robert 
Brown, Esq., who states® that there can be no doubt that the agency of | 
insects is very frequently, though not always, employed in the fecundation 
of Orchidee, “but that in those Asclepiadee that have been fully examined, 
the absolute necessity for their assistance is manifest.” 
Aristolochia clematitis, according to Professor Willdenow, is so formed, 
that the anthers of themselves cannot impregnate the stigma; but this 
important affair is devolved upon a particular species of gnat (Cecidomyia 
pennicornis). The throat of the flower is lined with dense hair, pointing 
downward, so as to form a kind of funnel or entrance like that of some 
kinds of mouse-traps, through which the insects may easily enter, but not 
1 Smith’s Tracts, 165. Kélreuter, Ann. of Bot. ii. 9. 
? Chr. Conr. Sprengel, Lntdecktes Geheimniss, &c. Berlin, 1793, 4to.; quoted in 
Ann. of Bot. i. 414, 
a On the Organs and Mde of Fecundation in Orchidee and Asclepiadee. Linn. 
Trans, xvi. 781, 
