ENEMIES OF APHIDES. 123 



It Qiay be said, then, with some certainty, that the following 

 species prey upon Aphides : — Syrphus luniger^ S. topiarius^ Cata- 

 tomba pyrastri, S. balteatiis, S. ribesii, and in all probability Platy- 

 chirus albimanus. Mr. G. B. Buckton, in his British Aphides, 

 mentions the genera, Sccet^a and Cheilosia^ as containing species 

 which are aphis eaters ; but, although I have frequently taken flies 

 of this genera, I have never found any of my aphis-eating larvae 

 develop them. This may be accounted for by the fact that many 

 species of insects are very local. It should perhaps be added, by 

 way of warning, that notwithstanding the particulars which I have 

 been able to give, the recognition of these larvae requires consider- 

 able experience, and cannot be considered as certain until the 

 appearance of the pupa-case fully confirms the diagnosis of the 

 larva. 



[ now come to a very interesting point in the economy of the 

 Syrphidae. In the course of numerous dissections of the flies 

 which I have made during the past two years, my attention has 

 been frequently arrested by the pollen-grains which are invariably 

 present in all parts of the alimentary system, from the oesophagus 

 to the rectum, and even in the excreta. 



The idea that the mouth-organs of the Diptera are only 

 adapted to the imbibition of fluids or juices must be considerably 

 modified, if it be agreed that pollen forms a considerable, if not 

 the greater, proportion of the food of the Syrphidae in a state of 

 nature. And that such is the case I am quite satisfied. My 

 observations of the stomachs of several species of Syrphus^ Eris- 

 talis, and Rhingia leave no doubt that in all these, pollen consti- 

 tutes the bulk of the food ingested, and it is found in the oesopha- 

 gus, in the sucking stomach, in the digestive stomach, and in the 

 alimentary canal in varying conditions, which indicate plainly the 

 progress of digestion. 



In view of the inapplicability of the suctorial parts of the pro- 

 boscis to the ingestion of a solid-like pollen, this organ becomes a 

 fresh object of interest ; and I have endeavoured, by means of the 

 greatly amphfied figure on PI. VIIL, to set before the reader the 

 details of its structure. It will be seen that the split tubes, or 

 " pseudo-tracheae," take their rise on either side from a curved 

 chitinous channel of similar structure, into which any fluid 



