STAGES OF ADAPTATION IN INSECTS 191 



Hermann Mtiller (' Alpenblumen,' p. 512) has distinguished eight stages of adap- 

 tation among insects which visit flowers : — 



I. Neuroptera, Orthoptera, Thysanoptera, Hemiptera. 



II. Coleoptera. 



III. Diptera which are little adapted to pollination. All the order except the 



families included in IV. Muscidae, Stratiomyidae, Dolichopidae, and so 

 forth, are therefore placed here. 



IV. Diptera which are more or less well adapted to pollination : Bombyliidae, 



Conopidae, Empidae, Syrphidae. 



V. Wasps (Hymenoptera other than bees). 



VI. Bees with short proboscis (Melitta Kirby). 



VII. Bees with long proboscis (Apis Kirby). 



VIII. Lepidoptera. 



Muller's division of Diptera into two groups (III and IV) is based on the follow- 

 ing considerations ('Alpenblumen,' p. 513). — The families of Bombyliidae, Conopidae, 

 Syrphidae, and Empidae are highly specialized in relation to flower-food, for well- 

 marked intelligence and the possession of a long proboscis enable the species they 

 include to secure nectar even when deeply hidden. All the species of the three first 

 families are purely anthophilous when adult, while among Empidae the same thing is 

 at least true for the genera Empis and Rhamphomyia. Some of the other families of 

 Diptera — such as the Dolichopidae, the Stratiomyidae, and particularly the Muscidae 

 — are also constant flower visitors as regards a larger or smaller proportion of their 

 species, genera, and even sub-families, while among the Muscidae there are several 

 forms of considerable intelligence and with a tolerably long proboscis, e.g. Gonia, 

 Ocyptera, Prosena. But as these forms are very limited in number and importance 

 compared with the hundreds of their stupid short-tongued congeners, we are justified 

 — when taking a broad survey — in grouping together the Bombyliidae, Conopidae, 

 Empis, Rhamphomyia, and Syrphidae as Diptera more or less well adapted to 

 pollination, and all the others as Diptera less adapted to it. 



The division of Hymenoptera into wasps, short-tongued bees, and long-tongued 

 bees (IV, V, VI), naturally follows from Muller's account of the structure and habits 

 of these insects, as already given. He ('Alpenblumen,' p. 518) calls attention to the 

 fact that wasps chiefly compete with beetles and short-tongued flies (p. 513), which he 

 describes as being at the lowest stage of adaptation. In the same way he compares 

 — as regards flower visits — the flies which are more specialized as pollinators with the 

 unspecialized bees, for these display marked preference for flowers with partly or 

 entirely concealed nectar. We find therefore that Hermann Mijller partly antici- 

 pates the oecological classification that E. Loew has worked out in such an acute 

 and admirable manner (see pp. 192-5). 



C. Verhoeff" (' Blumen und Insekten auf Norderney,' pp. 176-8) has reduced 

 the stages of adaptation in insects to flower visits from eight to six. His groups 

 IV, V, and VI entirely agree with the corresponding ones of Hermann Miiller. In 

 defining these stages of adaptation he pays special attention to the structure of the 

 mouth-parts, the presence and character of a hairy covering, the size of the body, 

 the activity during visits, and the frequency of these. Verhoefi" distinguishes — 



