INDIRECT BENEFITS DERIVED FROM INSECTS. 297 



case the humhle-bee is the operator. Led by instinct, or, 

 as the ingenious Sprengel supposes, by one of those 

 honey marks (Sqftmaal) 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 ulti- 

 mately, 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 emi7iens ; but it is only to its lower or negative sur- 

 face, which cannot influence impregnation. She now 

 takes her way to the second petal, an4 insinuating her- 

 self 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 mi- 

 grating from one part of the corolla to another, and from 

 flower to flower,aihe fructifies one with pollen gathered 

 in her search after honey in another. — Mr. Sprengel 

 found, that not only are insects indispensable in fructify- 

 ing 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 com- 

 mon as many others, this Iris is often barren, or bears 

 imperfect seeds*. 



Aristolochia Clematitis, according ta Professor Willde- 

 now, is so formed, that the anthers of themselves cannot 



' Chr. Conr. Sprengel Entdecktcs Gelmmniss, i)-c. Berlin 17^3, 

 4to. quoted in Ann. <\f Bui. i. 414. 



