176 INTRODUCTION 



dimly visible through the skin (Fig. 72, 4 /'). 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 ; / and e are probably equivalent to submentum and 

 mentum. The end-flaps (c c) are probably the modified paraglossae borne by the 

 ligula, though Burmeister holds them to be labial palps. 



Muller (p. 38) explains how these structures are disposed : — i. 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, i, ^) 

 and chitinous mandibular piece (z) to form a tube, which is then bent down so as 



' In Eristalis tenax, which attains a length of 15 mm., the outstretched proboscis is 7 to 8 mm. 

 long. In E. arbnstorum, which is 10 mm. long, the proboscis is 4 to 5 mm. long. 



