296 INDIRECT BENEFITS DERIVED FROM INSECTS. 



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 

 ofl" 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 

 cmincns; 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 iu tins man- 

 ner 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. — Mr. 

 Sprengel found, that not only are insects indispensable 

 in fructifying the different species of Iris, but that some 

 of them, as /. 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 ^. 



Aristolochia Clematitis, according to Professor Will- 

 denow, is so formed, that the anthers of themselves 

 cannot impregnate the stigma ; but this important affair 

 is devolved upon a particular species of Tipula ( T.pen- 

 nicornis). The throat of the flower is lined with dense 



* Chr. Conr. Sprengel Entdecktes Geheimniss, &fc. Berlin IT93, 4to, 

 quoted in ^nn. of Bol. i. 414. 



