STAGES OF ADAPTATION IN INSECTS 193 
Blumenbesuch v. Insekten a. Freilandpfl. d. bot. Gart. zu Berlin,’ Jahrb. kgl. bot. 
Gart. zu Berlin, iii, 1884; iv, 1886: ‘ Beitrige zur bliitenbiol. Statistik,’ Verh. bot. 
Ver., Berlin, xxxi, 1890: ‘ Bliitenbiol. Floristik,’ pp. 386-8, Stuttgart, 1894). Loew’s 
scheme has also been used by MacLeod and by Heinsius for statistical observations, 
and I myself have employed it in several investigations relating to the statistics of 
flower pollination, during which the conclusion has more and more forced itself 
on me that Loew has adopted the right method, though some change may be 
necessary in certain details. 
Loew’s first group—Allotropous Insects—includes Stage I and part of Stage II 
of Verhoeff’s classification. To the second group—Hemitropous Insects—belong 
Verhoeff’'s Stages IV, III, and VI. The third group—the Eutropous Insects— 
corresponds to Verhoeffs Stage V, with a small part of Stage VI. The last of the 
groups established by Loew—Dystropous Insects—includes species which were not 
taken into consideration by Verhoeff. 
Loew’s stages of adaptation of insects to flower visits are defined as follows :— 
I, Allotropous Insects. Unegually and only slightly adapted flower visitors of 
little value for pollination. ‘They either lack special structural characters adapting 
them to flower visits—except such as may be involved in the features common to the 
group—or if any adaptations of the kind exist they are incipient. In addition to 
flower-food the insects of this group also nourish themselves on a great variety of 
other substances (e.g. the social Vespidae, the blood-sucking Empidae, Tabanidae, 
and others), and some of them are destroyers of flowers (e. g. many beetles, species 
of Cephus, and others). Their movements while visiting flowers are mostly irregular, 
only attaining any constancy in the more highly adapted species. Flower forms 
corresponding to allotropous visitors are very sparingly developed, and by no means 
to be regarded as adaptations to such insects alone, though these display marked 
preference for flowers of the kind. 
To this group belong :— 
1. Hymenoptera. Short-tongued digging-wasps (Fossores) and ruby-wasps 
(Chrysididae), most of the true wasps (Diploptera, e.g. Vespa, Polistes), saw-flies 
(Tenthredinidae), and ichneumons (Ichneumonidae). 
2. Diptera. Muscidae, Empidae, Tabanidae, Therevidae, Leptidae, Stratio- 
myidae, Dolichopidae, Bibionidae, and others. 
3. Coleoptera. Dermestidae, Coccinellidae, Nitidulidae, Lamellicornia (in part), 
Melyridae, Cerambycidae, Lepturinae, Oedemeridae, and others—in fact most 
beetles, excepting such as are particularly hurtful to flowers. 
4. Neuroptera, Orthoptera, and Hemiptera. Such of these as are occasional 
flower visitors. 
II. Hemitropous Insects. Partially adapied flower visitors of moderate value 
Jor pollination. The adaptations to successful flower visits are always distinctly 
recognizable, but are much more feebly developed than in the following group. 
The movements while visiting flowers are skilled, but not so regular and 
constant as in Group III. Flower forms specially adapted to these insects 
are rare. 
DAVIS 0) 
