176 INTRODUCTION 
dimly visible through the skin (Fig. 72, 4 2’). The contractile piece (/) and the 
piece supported by the chitinous plate (e), together form the free front part of the 
lower lip which is grooved above ; f and e are probably equivalent to submentum and 
mentum. The end-flaps (¢ c) are probably the modified paraglossae borne by the 
ligula, though Burmeister holds them to be labial palps. 
Miiller (p. 38) explains how these structures are disposed :—1. when feeding on 
pollen; 2. when sucking nectar; 3. when at rest. 
1. In order to feed on pollen the fly stretches out the extensible proboscis’, 
moving it, as may be necessary, straight forwards, upwards, or downwards, grasps 
with the two end-flaps, as with two hands tied together at the wrists, a little mass of 
pollen, rubs this down to separate grains by a rapid movement of the flaps, and passes 
the grains back by the same movement into the groove on the labium. In this groove 
lies the labrum, which is grooved below, and encloses the chitinous mandibular piece, 
the two being ready to seize the pollen. As soon as this is ground back by the end- 
flaps, the labrum and mandibular piece separate somewhat by means of their bases 
the parts immediately surrounding the mouth, seize the pollen now lying in the 
groove of the labium and thrust it back into the mouth, apparently by working 
against each other longitudinally. After a few seconds the first portion is swallowed, 
and the same series of actions is repeated. When the pollen-grains are united into 
long strings by elastic threads, as in Oenothera, an action of the fore-legs, alternating 
with the proboscis movements just described, is necessary to free the pollen-grains 
from the threads. After the fly has torn away a little mass of pollen from the anther, 
it brings up the fore-feet to its mouth, standing meanwhile on the mid- and hind-legs: 
then taking the cord of elastic threads between the fore-feet, and quickly rubbing 
them together as if washing its hands, it tears the threads asunder, and clears them 
off the proboscis and legs. Sometimes, to free the end-flaps from adhering pollen, 
it takes the proboscis in its fore-feet and draws them along from back to front. 
A remarkable peculiarity of the flaps, clearly shown in Fig. 73, makes them admirably 
adapted for seizing pollen, grinding it down, and passing it backwards. This consists 
in the fact that their apposed surfaces are closely and evenly beset with parallel 
chitinous ridges, by which the pollen-grains are easily held fast and pushed into the 
lower end of the labial groove. 
The peculiarity in question is undoubtedly an adaptation to feeding on pollen, 
for it obviously facilitates this, and it is well marked in precisely those families of which 
the members visit flowers to obtain both pollen and nectar (Syrphidae, Muscidae, 
Stratiomyidae), while it is absent in flies which feed only on nectar (Bombyliidae, 
Empidae, and Conopidae), as well as in gnats, which also are purely suctorial. The 
gad-flies (Tabanidae) have never been seen eating pollen, although their end-flaps 
possess similar chitinous ridges. Since, however, a few species (e.g. Tabanus micans 
and T. luridus) are often found on flowers, it is not improbable that they too are 
pollen-eaters. 
2. In order to suck nectar a hover-fly apposes the grooved labrum (Fig. 73, 1, 2) 
and chitinous mandibular piece (7) to form a tube, which is then bent down so as 
1 In Eristalis tenax, which attains a length of 15 mm., the outstretched proboscis is 7 to 8 mm. 
long. In E. arbustorum, which is 10 mm. long, the proboscis is 4 to 5 mm. long. 
