DIPTERA OE TRUE FLIES. 



233 



with a narrow anterior extremity, and are leech-like in move- 

 ment. They pupate on the leaves where they have been feeding 

 in sac-shaped cases or puparia (fig. 128, 3 and 6). The typical 

 genus is Syrplucs, of which there are a great number of species. 

 Most have broadish flat abdomens, often almost transparent, 

 with yellow and black bands, and extremely delicate pale- 

 coloured legs. S. rihesii an<l S. baltcatus, and the black and 



Fig. 128. —Hover-flies {syrphidw). 



1, Syrphus baltficU-us ; 2, larva devouring aphis; 3, pupa; 4, Catahomha jnjra^tri; 5, 

 larva ; (j, jiupa ; 7, Syrplius ribesH, 



white Catahomha pyrastri, are perhaps our commonest species 

 living on plant-lice. Some also feed on Scale insects [Coccidai). 

 Other genera are parasitic (Volucella), living in the nests of 

 Humble-bees (F. hombylans) and Wasps {V. zonaria), and 

 mimic the colour and appearance of the insects they live 

 amongst. 



Warble-flies, or (Estridae. 



This family of flies is most obnoxious to our domestic and 

 wild mammals, upon which they live as parasites during their 

 larval period. Some also live upon man. The larvas are called 

 " bots," and live under the skin, in the head, and in the 

 stomach and intestines of animals. The most injurious species 

 are the Ox-warbles {Hypoderma bovis and II. lineata), the 



