178 



INTRODUCTION 



that it does not discover and utilize. Even the deeply hidden nectar-receptacles of 

 Iris are found and sucked by Rhingia. In anthophilous insects, the power to detect 

 hidden nectar increases pari passu with the structural adaptations for securing it. 

 When Sprengel described flies as stupid insects, incapable of finding nectar which lay 

 concealed, that statement applied to the great majority of short-tongued forms, but 

 not at all to forms vjdth long proboscides, such as Syrphidae, Bombyliidae, Conopidae, 

 and Empidae. 



Even in the Syrphidae, however, only a few species have acquired so highly 



specialized a proboscis as Eristalis : the great majority are in a much lower stage 



of adaptation, as shown in Fig. 75. The labium is much shorter, its extensible 



middle joint is wanting, the terminal flaps are swollen and cushion-shaped, and 



there is a corresponding diminution in intellectual power 



in regard to the spoliation of flowers. This is indicated 



by the large amount of variation in the length of the 



proboscis, as follows : — 



mm. 



Syrphus balteatus 2 



„ ribesii 3-4 



Eristalis arbustorum 4-5 



Helophilus trivittatus 6-7 



Eristalis tenax 7-8 



Volucella bombylans 8 ' 



Rhingia rostrata 11-12 



With reference to this dissimilarity in the length 

 of the proboscis of species belonging to the same family, 

 Loew very rightly remarks (' Blumenbesuch,' II, p. 117) 

 that it is necessary to test each Syrphid as regards its 

 stage of adaptation. For this, however, the observations 

 so far recorded are insufiicient. Loew thinks that there 

 may possibly be a continuous series of transitional 

 stages between purely allotropous forms and well-marked hemitropous ones. 



Loew ('Blumenbesuch,' II, p. 116) calls attention to the fact that the more 

 or less well-developed feathering of the antennae of some species is of importance 

 for the transfer of pollen from flower to flower. It is especially well marked in the 

 genus Volucella. The curved pendent dorsal brush of the antennae is more than 

 2 mm. long in V. bombylans, while the lateral branches are about i mm., and 

 numerous pollen-grains are not infrequently found adhering to the dense feathery 

 hairs. As Loew further remarks, the dorsal brush forms the most prominent part 

 of the head, so that the fly in alighting on flowers will — so far as the structure 

 of these permits — in many cases deposit pollen-grains on the stigma. An equally 

 well-developed feathering of the dorsal brush also occurs in Sericomyia and Arcto- 

 phila, while in Cheilosia and Eristalis there are species with naked dorsal brush 

 as well as others where it is feathery. 



Loew further regards the hairs below the eyes of many Syrphidae (Leucozona, 

 Volucella, Sericomyia, Arctophila, Eristalis, and species of Cheilosia) as an adaptation 



Fig. 75. Proboscis of Syrphus 

 halteatzts^ Deg. ; seen from below 

 (after Henr. Miiller). References 

 as in Fig. 73. 



