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. l 



The agency of these little operators is not less indispensable in the 

 beautiful tribe of Iris. In these, as appears from the observations of 

 Kb'lreuter, the true stigma is situated on the upper side of a transverse 

 membrane (arcus emincns 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 eminent, 

 it is clear that but for some extraneous agency the pollen could never 

 possibly 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) or 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 Iris, but that some of them, as 7. 

 xiphium, 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. 2 Sprengel also contends, that insects are essen- 

 tially necessary in the impregnation of Asclepmdeas ; in which opinion he 

 is confirmed by the conclusive testimony of the celebrated botanist Robert 

 Brown, Esq., who states 3 that there can be no doubt that the agency of 

 insects is very frequently, though not always, employed in the fecundation 

 of QrcMdea, "but that in those Asclepiadecs 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. Kolreuter, Ann. ofBot. ii. 9. 



2 Chr. Conr. Sprengel, Entdecktes Geheimniss, &c. Berlin, 1793, 4to. ; quoted in 

 Ann. of Sot. i. 414. 



5 On the Organs and Mide of Fecundation in Orchidece and Asclepiadece. Linn. 

 Trans, xvi. 731. 



