DIPTERA HOVER-FLIES 



177 



to be enclosed in the labial groove. The insect now uses the end-flaps in one of two 

 ways: it either folds them together (as in Fig. 73, i) while the membranous middle 

 piece (/) of the labium is so far retracted that the suctorial apparatus enclosed in the 

 labial groove protrudes in front of the flaps and dips into the fluid to be sucked ; 

 or else it spreads out and flattens the flaps so that their rough inner surfaces are 

 closely applied to the flower, and the tip of the suctorial apparatus protrudes from 

 the end of the labial groove. Flies with swollen, cushioned-shaped flaps (Syrphus 

 balteatus, Fig. 75) usually behave in the latter way, those with long, narrow flaps 

 (Rhingia, Fig. 74) invariably adopt the former. Both pollen-grains and fluid which 

 have been carried into the tube formed by the chitinous pieces h and t, are aided in 

 their passage to the mouth by dilatation of the sucking stomach. The laciniae and 

 maxillary palps seem to play no part either in sucking or in feeding on pollen, and 

 hence must be looked upon as useless appendages. 



3. In order to bring the proboscis into the sheltered rest position the fly draws 

 the musculo-membranous basal piece {g) backwards and downwards, the labrum, 

 mandibular piece, maxillae, and maxillary palps fold together above, and the very 

 contractile middle piece (/) is com- 

 pletely retracted, being thrown into a 

 few membranous folds at the lowest 

 part of the proboscis. The chitinous 

 plate {e) and the end-flaps {c) simul- 

 taneously fold upwards and forwards, 

 and the complicated proboscis (equally 

 adapted for pollen-eating and nectar- 

 sucking) now lies so deeply hidden 

 in the deep cavity underneath the 

 snout-like prolongation of the head 

 (Fig. 72, 1, 2, 2), that at most the end- 

 flaps protrude a little (Fig. 72, 1). If 

 the head is now examined from below 



(Fig. 72, 2), nothing will be seen in the cavity but the end-flaps {^c c'), and beneath 

 these the upper part of the chitinous plate {e\ of which the lower part lies in the 

 folds of skin belonging to the contractile part of the proboscis. 



An advance on these adaptations, furthering convenience in eating pollen, 

 enabling more deeply seated nectar to be sucked, and rendering complete shelter 

 of the proboscis under the head possible, would result from the greater elongation 

 of this organ, with concomitant development of the snout-like prolongation of the 

 head. Such a state of things finds its fullest expression in Rhingia (Fig. 74), where 

 the proboscis (u-12 mm. long) exceeds the whole body (10 mm.) in length, being 

 unsurpassed in this respect by any indigenous fly '. 



This hover-fly takes a foremost place among our native Diptera, not only in 

 regard to the length of its proboscis, but also with reference to its power of detecting 

 deeply hidden nectar. There is scarcely a single flower with nectar it can reach 



Fig. 74. Proboscis of Rhingia rostrata (after Herm. 

 Muller>. (i) Head with completely retracted proboscis ; seen 

 from the side. (2) The same, at the moment the proboscis 

 begins to unfold. (3) The same, with proboscis completely 

 extended. (4) Head with completely retracted proboscis; 

 seen from below; (enlarged twice as much as (i), (2), and (3).) 

 References as in Fig. 72. 



' Bombylius discolor Mik, alone equals it in length of proboscis : Bombylius major L. 

 approaches it (10 mm.). 



