992 Parasitic Insects. 



Tlie occurrence of the (jestriis fly is limited to certain locali- 

 ties. Usually one finds the i\y from Scandinavia to Southern 

 Europe ; further it is prevalent over Asia, Africa, North Amer- 

 ica and Australia (Strose). But the local prevalence of the 

 oostrus depends essentially on external circumstances. Thus 

 the fly plague is observed exclusively in those regions where 

 the cattle are out day and night from the beginning of spring 

 on, or are driven there in the morning hours. On the contrary 

 those regions remain free from the pest where no pasturing is 

 done or where the grazing only begins in autumn, or where 

 the cattle are driven out to pasture only in the midday hours 

 (as in many localities of South Germany). Factors that are as 

 yet unknown, perhaps climatic or certain conditions of soil 

 also appear to be of influence, for only in this manner can the 

 observation be explained that certain neighborhoods are avoided 

 by the flies, although imported cattle infested with oestrus swell- 

 ings remain on the pastures during the whole summer. On the 

 other hand the spread of the fly by cattle with oestrus swellings 

 cannot be denied in all cases (Strose), 



In Germany warbles are most frequently met with in Eastern Prussia, in some 

 districts of Western Prussia and in the province of Posen, in a considerable part of 

 Pomerania, Brandenburg and Altmark, in the greater part of Sclileswig-Holstein, 

 in the whole of East Friesland, Oldenburg, in several districts of Westphalia and 

 of the Ehine country. 



The oestrus larvae living in the subcutaneous connective 

 tissue produce nodules which may become as large as walnuts 

 and in which they lie imbedded in pus. In the skin the swelling 

 gradually increases in size and shows an opening which is at 

 first about the size of a pin's head, but later on 4 to 7 mm. broad 

 and circular. This is produced l)y the boring movements of the 

 larva, which in its second stage of evolution is already provided 

 with a breathing apparatus, and on that account requires air. 

 From the opening at first a sero-purulent secretion issues, which 

 mats the hair in the neighborhood. At a later stage the black- 

 looking hind end of the larva is close beneath the opening and 

 the larva itself may be expressed from the swelling. It is 22 

 to 27 mm. long, up to 15 mm. thick and according to its develop- 

 ment white, gray, or grayish black. The formation of the swell- 

 ings begins at times as early as in the month of January, gen- 

 erally, however, in the months of February and March, and 

 exceptionally later. They disappear a few weeks after the 

 emerging of" the larvae, from the end of April to the beginning 

 of June, and leave no trace behind them. Isolated warbles can, 

 however, be noticed in the month of July in any neighborhood 

 where pasturing begins late. Now and then the larva? die under 

 the skin, or after tlie exit of the larva the cavity of the nodule 

 is filled with granulation tissue in which lime salts may be de- 

 posited, and in this way hard nodes persist in the skin (Cas- 

 parini). 



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