118 BRITISH APHIDES. 



natural checks upon the increase of the last insect must 

 prove far more efficacious than any that can be devised 

 by the economist. 



Some of the most familiar flies of our summer 

 belong to the family Syrphidse. They possess two 

 wings, and hover above our heads in the sunny spots, 

 between the branches of the trees ; and whilst poising 

 themselves give forth either a continuous hum, or a 

 shriller note, according to the size or the species of 

 the insect. From time to time they dart with won- 

 derful rapidity from their favourite spot, and almost 

 immediately return to it to resume their humming 

 note.* 



This hovering has been noticed by De Geer ; and 

 doubtless the action is purely one of pleasure to the 

 insect. 



Curtis mentions the remarkable circumstance that 

 the males alone practise these gyrations ; at least, he 

 found that out of the numerous specimens he captured 

 whilst so employed, he failed to identify a single 

 female. 



The family Syrphus is comprised in the order 

 Diptera, and is divided into numerous genera, amongst 

 which are Scseva and Cheilosia, which alone contain 

 upwards of fifty species. Their mouth-parts are 

 highly organised, and the neuration of their wings is 

 more elaborate than we find in most Muscidse. 



Of the Aphidivorous forms, Scwva pyrastri, S. 

 ribesii, S. halteata, and Cheilosia scripta may be cited 

 as most common. 



The larvae of the Syrphidse are apodous, blind, and 

 leech-like in form. They move but slowly from place 

 to place by means of the minute booklets with which 

 the posterior rings of their bodies are furnished. 



* The musical hum of these insects has been traced by Pi-of. West- 

 wood to a hairy filamentous organ connected with the thoracic organs 

 of respiration. This apparatus is situated at a little distance from the 

 base of the balancer, but how such an organ is vibrated does not clearly 

 appear. — (Westwood's ' Introd. to Modern Class, of Eutom.,' vol. ii, 

 p. 557). 



