i6o INTRODUCTION 



concealment of nectar in the flowers that they prefer to visit. Some of the relative 

 facts are as follows : 



Length of Proboscis. 



Species of Prosopis 1-1-25 mm. 



Halictus 15-6 ,, 



Anthrena 2-7 ,, 



Apis mellifica ........ 6 ,, 



Eucera longicornis 10-12 ,, 



Anthophora retusa L 1 5-1 7 



acervorum Z 19-21 



The proboscis of the male humble-bee is 1-2 mm. shorter than that of the 

 worker. The former therefore prefers social flowers to those of any other class, 

 since in these the nectar is easy to get, while the female humble-bees have a 

 preference for flowers belonging to Class H (Loew, ' Blumenbesuch,' I, p. 19). 



The differentiation of humble-bee colonies into three different castes that 

 develop successively ( ? , 5? > $ )> limits each of them to a small set of flowers deter- 

 mined by the times of flowering. The males mostly appear in July, and are 

 therefore debarred from visiting the flowers of spring and part of summer. Plants 

 which bloom very early in the year, e.g. Salix and Pulmonaria, are visited only 

 by queens that have survived the winter, for the first workers do not emerge till 

 a full month after the foundations of the nest have been laid. It is only the 

 queens, therefore, that are to be found on flowers at all times of the year. Even 

 these become increasingly rare as autumn approaches, for the old foundress queens 

 gradually die oflf, and the young queens either do not leave the nest at all or 

 wander about idly on flowers without gathering pollen (Loew, 'Blumenbesuch,' 

 I, p. 21). 



It follows that the activity of the female bees as regards flowers differs in 

 many ways from that of the males. This is most markedly the case in Bombus 

 Gerstackeri Alor. (=B. opulentus Gerst., non Smith). In this species the females, 

 as v. Dalla Torre points out (Kosmos, 1886), only visit Aconitum Lycoctonum, 

 while males and workers visit as exclusively the blue species of Aconitum, especially 

 A. Napellus. 



