INDIRECT BENEFITS DERIVED FROM INSECTS. 203 



itself of the tube of the leaf instead of a burrow. Yet we know of no such 

 strange deviation from natural instinct, which would be the more remarkable, 

 because the insect was European, while the plant was American, and grow- 

 ing in a hot-house. And, at any rate, it does not seem very likely that 

 the insect would commit her egg to the tube without having previously 

 examined it ; in which case she must have discovered it to be half full of 

 water, and consequently unfit for her purpose. It is not so wonderful that 

 many large flies should, as Professor Barton informs us, drop their eggs 

 into the Ascidia furnished with dead carcasses ; and it seems very probable 

 that Dytisci oviposit in them ; for the Squilla, which Rumphius found 

 there, was probably one of their larvae, this being the old name for them.^ 



However problematical the agency of insects caught by plants as to 

 their nutriment, there can be no doubt that many species perform an 

 important function with regard to their impregnation, which indeed without 

 their aid would in some cases never take place at all. Thus, for the due 

 fertilization of the common Barberry {Berheris vulgaris), it is necessary 

 that the irritable stamens should be brought into contact with the pistil by 

 the application of some stimulus to the base of the filament ; but this 

 would never take place were not insects attracted by the melliferous glands 

 of the flower to insinuate themselves amongst the filaments, and thus, 

 while seeking their own food, unknowingly fulfil the intentions of nature 

 in another department.^ 



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

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

 Kolreuter, 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 emi- 

 nens, 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 (^Saftmaal) or spots of a difl:erent color 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 back- 

 wards ; and in doing this is indeed pressed by the petal to the arcus emi- 

 nens ; but it is only to its lower or negative surface, which cannot influence 

 impregnation. She now takes her way to the second petal, and insinua- 

 ting 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 



' Mouffet, 319. 2 Smith's Tracts, 165. Kolreuter, Ann. of Bot. ii. 9. 



