DIPTERA OR TRUE FLIES. 211 



extremity, and are leech-like in movement. They pupate on the 

 leaves where they have been feeding in globular cases or puparia 

 (fig. 103, 3 and 6). The typical genus is Syrpkus, of which 

 there are a great number of species found. Most have broadish 

 flat abdomens, often almost transparent, with yellow and black 

 bands, and extremely delicate pale-coloured legs. S. ribesii and 

 S. balteatus are perhaps two of our commonest -species living on 

 plant-lice. 



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

 Humble-bees (V. bombylans) and Wasps (F. zonaria), when 

 they mimic the colour and appearance of the insects they live 

 amongst. 



Warble-flies, or CEstridae. 



This family of flies is most obnoxious to our domestic and 

 wild mammals, upon which they live as parasites during their 

 larval period. The larvse 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 H. lineata), the Sheep Nasal-fly {CEstrus ovis), and 

 the Horse-bot (Gastrophilus equi), which we will deal with in 

 the above-mentioned order. 



Ox-WAEBLES OE BoT-FLY (HtPODEEMA BOVIB AND H. LINEATa). 



These two species, which are very similar in appearance, at 

 one time caused serious loss amongst 

 beasts : the annual amount, according 

 to Miss Ormerod, was something like 

 £7,000,000. But, thanks to her great 

 energies, this serious malady has been 

 checked, although it stiU does a great Pra. io4.-ox-wakble 



' ° ° {Hypoderma boms). 



amount of harm. The Ox -Warble fly 



(fig. 104) is like a small hairy humble-bee in appearance, gaily 



coloured with thick hair of yellow, black, and red bands on the 



