VARIOUS DIPTERA 183 



nectar, the species of Bombyllus, Systoechus, and Dischistus are provided with a long 

 proboscis, with which they suck nectar as they hover. With regard to the rapid 

 movement of their wings, and their way of visiting flowers, the Bombyliidae may be 

 compared with the Sphingidae, as well as with the emerald-green and azure-blue 

 Brazilian bees of the genus Euglossa, and with humming-birds. The movements of 

 their wings are so rapid that to our eyes they appear motionless. Although these 

 insects effect pollination much like hawk-moths, and readily visit lepidopterid flowers, 

 we have no special ' bee-fly flowers.' On the other hand, some of the lower flies 

 (Muscidae, gnats), the stupidity and inconstancy of which in visiting flowers was 

 long ago repeatedly commented upon by Sprengel ('Entd. Geh.'), and which do not 

 possess the smallest degree of adaptation to flower-food, serve as the exclusive or 

 almost exclusive agents of pollination for flowers specially adapted to them, such as 

 ' nauseous flowers,' ' pitfall flowers,' ' deceptive flowers,' and so forth. (According to 

 Loew, * Blumenbesuch,' II, pp. 1 14-15.) 



This fact, says Herm. Mtiller (' Wechselbeziehungen,' p. 19), which at first appears 

 very strange, is susceptible, on closer examination, of a simple explanation. The 

 adaptations of insects to procuring flower-food are obviously conditioned by the 

 degree of their dependence upon it, and by the keenness of competition for 

 the spoil, so that the most constant and zealous flower visitors must naturally be 

 the most likely to participate in the results of natural selection. On the other hand, 

 exclusive adaptation of flowers to a narrow circle of visitors can only occur when (and 

 it can occur the more easily the more) such visitors possess certain peculiarities differ- 

 entiating them from all others and rendering flowers otherwise useless and inacces- 

 sible available to them only. Now carrion-, flesh-, and dung-flies, and other Diptera 

 fond of waste products, have tastes unlike those of all other flower visitors, and in 

 accordance with this flowers might readily be evolved, and as a matter of fact have 

 been evolved, which exclusively or chiefly attract Diptera of this kind, while at the 

 same time they repel other visitors, or at least most of them, by inducing disgust. 

 The more highly specialized flies (Bombyliidae, Empidae, Conopidae, Syrphidae), on 

 the other hand, confine themselves exclusively to flower-food, and are (in part) most 

 zealous and intelligent flower visitors, admirably adapted by the possession of a long 

 proboscis to get at even deeply concealed nectar. But in spite of this they do not 

 possess a single peculiarity fitting them to plunder flowers, in which they are not 

 surpassed by bees and Lepidoptera. 



It is easy for the Bombyliidae to plunder flowers with nectar completely concealed 

 at a moderate depth, and even from lepidopterid flowers with a tolerably long 

 corolla-tube they can extract nectar as easily as the Lepidoptera themselves. Bee 

 flowers are also much more readily accessible to them than to the other anthophilous 

 Diptera, and are more frequently plundered by them. On the other hand, social 

 flowers are much less convenient for sucking while hovering, and are consequently 

 only very rarely visited by these insects. In the Alps they were never met with on 

 anemophilous flowers, pollen flowers, and flowers with fully exposed nectar, and but 

 rarely on flowers with nectar partially concealed. Their preference for red, violet, 

 and blue, is so remarkable that they were observed on three times as many flowers 

 of these colours as on white or yellow ones (Miiller, ' Alpenblumen,' pp. 515-17). 



While, therefore, certain Diptera, especially Syrphidae and Bombyliidae, are found 



