i62 INTRODUCTION 



already shown, their fitness to suck nectar which is more or less deeply seated. 

 The statistical investigations made by Hermann Miiller, Loew, MacLeod, and myself 

 have shown that short-iongued bees prefer flowers with nectar that is exposed, or not 

 very deeply concealed, to all other kinds. They markedly affect (as do long-tongued 

 wasps) yellow and white hues, especially social flowers and flowers with half- 

 concealed nectar which are so coloured. To a lesser extent they seek out con- 

 spicuous social flowers, and are still less partial to flowers with completely concealed 

 nectar, those with exposed nectar, and pollen flowers. The deeply seated nectar 

 of bee flowers, humble-bee flowers, and lepidopterid flowers is inaccessible to 

 them, and they therefore only occasionally appear on these as thieves (Knuth, 

 ' Blutenbesucher,' II, p. ii). As Loew (' Einftihrung,' p. 378) points out, the 

 difference in time of appearance between the two sexes has an influence on pollina- 

 tion in the case of the two short-tongued genera (Andrena and Halictus), which 

 are regarded as the most important flower visitors of their kind. In Andrena 

 both sexes appear at approximately the same time, while in Halictus only females 

 are to be seen flying about in spring, and these are followed by parthenogenetic 

 summer and autumn generations, the males of which are on the wing till autumn, 

 when they perish, while the fertilized females survive the winter. The Andrenae 

 seen in spring make up the chief contingent of the visitors to willow catkins, while 

 the males of Halictus, which appear late in the year, visit late-flowering Composites 

 by preference. The times of appearance of the insects and the flowering periods 

 of the flowers, therefore, exhibit distinct mutual relations. 



It has already been pointed out (p. 95) that certain species of Andrena visit 

 almost or quite exclusively certain species of flowers, and that the species of Prosopis 

 seek out by preference flowers with a strong odour. 



The short-tongued bees of the Alps (H. Miiller, 'Alpenblumen,' p. 520) are 

 very distinctly superior to wasps as regards adaptation to flowers. In species where 

 the nectar is externally visible (E and EC), wasps make up more than half of the 

 visitors, not a quarter of them being bees. The latter most decidedly prefer flowers 

 with completely concealed nectar, although their visits are divided among flowers 

 in all stages of specialization. 



The visits of long-tongued bees, in accordance with their structure, are chiefly 

 paid to bee and humble-bee flowers, and also to brightly coloured Composites; in 

 less degree to lepidopterid flowers, and to white or yellow social flowers with con- 

 cealed nectar; and still less to flowers with half-concealed nectar and to pollen- 

 flowers. Flowers with exposed nectar are visited by them very rarely indeed 

 (Knuth, ' Bliitenbesucher,' II, p. 11). They show a strongly marked preference for 

 red, blue, and violet colours. 



Solitary long-tongued bees were never met with in the Alps (H. Miiller, 'Alpen- 

 blumen,' p. 521) on flowers with freely exposed nectar, and only one-tenth of their 

 visits were paid to flowers with nectar that was clearly visible though not quite fully 

 exposed. On the other hand, 85 % of their visits were to flowers with completely 

 concealed nectar, and almost half of these were typical bee flowers. 



Social long-tongued bees are influenced ('Alpenblumen ') by the necessity for rifling 

 as completely as possible the maximum number of flowers, for with increase in 

 the number of individuals the demand for food increases. Even flowers with 



