174 \ INTRODUCTION 
‘C, Flies or Two-winged Insects (Diptera). 
In Hermann Miiller’s opinion (‘ Fertilisation,’ p. 36) most species of flies probably 
visit flowers, but this only applies to the first main division of the order, the Diptera 
brachycera, which are of very great importance in relation to pollination, while 
the species of the second great division, the Dzptera nematocera, are almost useless 
in this respect, only some of the Tipulidae being common visitors of flowers, while 
a few tiny moth-flies (Psychodidae) pollinate Aristolochia, Arum, Adoxa, and 
Chrysosplenium. It will therefore suffice to describe the organs employed in pro- 
curing nourishment from flowers, and the way they are used in those flies which 
are of special importance for the transference of pollen. I think that this cannot 
be done better than by following the admirable account given by Hermann 
Miiller (‘ Fertilisation,’ pp. 36 et seq.). 
2 
; yy iin ty, 
Fic. 72. Mouth-parts of Eristalis (x 7) (after Hermann Miller). (1) Head of E. arbustorum, with 
retracted proboscis; from the side. (2) Ditto, from below. (3) Ditto, with extended proboscis ; from 
above. (4) Extended proboscis of E. tenax; from the side. . (5) Ditto, from below. (6) Ditto, from above. 
a, eye; 50, antennae; cc, end-flaps of proboscis; c’c’, their inferior segments; d, groove on the upper 
side of the tip of the proboscis; contractile middle part of the proboscis; y, contractile base of 
proboscis; 4, upper lip (labrum), grooved below to receive the unpaired piece (2), which probably 
represents the two fused mandibles; 2, maxilla; 7, maxillary palp; 7 2, edges of the cavity on the 
under-side of the head into which the entire proboscis is withdrawn ; 7, occipital foramen. 
The family of Hover-fzes (Syrphidae) includes those well-known insects which 
hover as if fixed to a point in the air, then rapidly dart to one side and act as 
before. They contribute far more to the pollination of our native flowers than 
all the other Diptera put together, for most of their numerous and often very common 
species mainly or exclusively depend on flower-food, and in connection with this 
they exhibit very special adaptations for securing pollen and nectar alternately. 
To illustrate the mouth-parts of flies, and their relation to flowers, H. Miiller 
therefore selected Eristalis and Rhingia, two hover-flies which are highly developed 
in this respect, and at the same time very common. . 
In Eristalis the completely extended proboscis is clearly seen to be composed 
of three successive segments (Fig. 72, 4, 5, 6, and Fig. 73, 1)—(1) The membranous 
basal piece (g), which bears at its front end two unpaired (4, 7) and two paired (, 2) 
