52 REPORT ON INJURIOUS INSECTS FOR 1904. 



Neither the flies or the larvae of the Syrphidne should be des- 

 troyed, as being Aphis feeders they do much towards keeping these 

 harmful pests in check. 



THE SHEEP NASAL FLY. 



Oestrus ovis, Linn. 



In Worcestershire this fly has proved very troublesome, a 

 correspondent at Aston Fields and another at Bromsgrove 

 having suffered severely. 



LIFE-HISTORY. 



The flies make their appearance during June and July and 

 are extremely active during the hot days. On cold damp days 

 they are just as inactive, and may be found on reeds and fences. 

 Certain writers have stated that during the imago stage they take 

 no food. Osborn (i) states " It [the fly] has no mouth and 

 cannot, therefore, take any nourishment," on the other hand, 

 Theobald ( 2 ) has " more than once seen them settled on the 

 flowers of the rush and reed, and on Compositae, evidently ab- 

 sorbing the nectar." 



The eggs are usually deposited two or three days after copu- 

 lation. If a longer period intervenes living larvae are deposited. 

 With reference to the deposition of eggs or larvae very divergent 

 accounts have been given by different writers. Mr. Theobald, 

 to whose account I have already referred, states (p. 71) " I have 

 been fortunate by chance to see two of these flies settle on sheep, 

 and in the most lazy way, without causing any annoyance to the 

 future host of their progeny, deposit as many as fifteen eggs around 

 the sheep's nostrils. One cannot mistake these curved ova, 

 and moreover the eggs were not quite mature. On two occasions 

 I have seen these characteristic ' nits ' on the nostrils of sheep . 

 It is not right to say they always deposit young viviparously. I 

 very much doubt if they do so at all in this country. At least we 

 have no authentic records of such having been done. On the 

 other hand, we have Riley's statement, and this must carry 

 great weight. That they only deposit young is not right, thev 

 may do so on the other hand." My own experience leaves no 

 doubt but that both methods take place, and I venture to offer 

 the following explanation that these two should obtain. 



After copulation, if the weather continues hot and dry 

 one invariably finds in two or three days the 12-18 characteristic 

 curved ova on the sheep's nostrils, but when the female is unable 

 to deposit her eggs, as often happens owing to a sudden change in 

 the weather, development proceeds within the body of the parent, 

 then on the return of sunshine, she deposits 5-7 larvae on the 

 nostrils of the sheep. 



1. Insects Affecting Domestic Animals, 1896, p. 104. 



2. Journ. S. E. Agric. Coll. Wye, 1903, p. 71. 



