FLIES 



251 



which it is most destructive. In colour it is green, with whitish lines on the hack 

 and sides and the breathing-pores or spiracles yellowish. In many seasons the moth 

 migrates in great swarms, which in 1879 swept across Europe from North Africa 

 in millions, the invasion extending into England, where it was fortunately checked 

 by heavy rain. 



Flies. 



Flies, of course, are well represented among the field - life of 

 Europe, one of the most common and most mischievous being the great 

 gadfly (Tabanus bovinus), readily distinguishable by its blackish brown colour, the 





■ 



^ Ml :> 





1 



A PAIR OF GREAT GADFLIES. 



black stripes on the breast, the white triangular spots and yellow edges of the 

 segments of the abdomen, and the yellow legs. The grubs are marked with 

 blackish cross-lines, and live underground. The grubs of another group of flies 

 likewise change into pupa? underground, where they are bold and industrious insect- 

 feeders, one of the most important being the hornet robber-fly {Asilus crabroni- 

 formis), which is reddish yellow in colour, with the three first segments of the 

 abdomen deep black. 



The gadflies and bot-flies of the family (Estridce are remarkable on account of 

 their habits ; their larva?, which are covered with a rough shiny skin, living beneath 

 the skin of grass-eating animals, where they cause large abscesses. Among them 

 may be mentioned (Estrus ovis, the sheep bot-fly, the thorax of which is covered 



