140 BLOOD-SUCKING INSECTS 



I was profoundly ignorant, so when I was back in my 

 own library I turned up the late Professor Miall's 

 Natural History of Aquatic Insects 1 and Lieut.- 

 Colonel Andrew Balfour's War against Tropical Dis- 

 ease? and found all that is known about the genus 

 Simulium. Unlike midges, whereof the different species 

 are bred in decayed wood, cattle dung, and other sub- 

 stances, the larva of sand-flies is aquatic ; its legs are 

 so modified that each pair forms a sucker, and it 

 attaches itself by the hind-pair to a leaf or stone under 

 water. It cannot, therefore, move at the surface and 

 breathe atmospheric air like the larva of the gnat, but 

 must rely on a supply of free oxygen in the water. 

 For this reason, Simulium can be bred only in swift 

 or rough streams, whereof the water is thoroughly and 

 constantly aerated. For a description of the strange 

 vicissitudes attending the metamorphosis of this tiny 

 insect, and especially the peculiar mechanism that 

 enables a minute, fragile creature with gauzy wings to 

 emerge from the pupa in the depth of a torrent and 

 escape to the upper air without injury, I must refer my 

 reader to Professor Miall's admirable treatise above 

 mentioned. But there is one statement by that high 

 authority which I cannot reconcile with my experience 

 of British sand-flies. After mentioning that various 

 species of Simulium have proved a serious plague both 

 to human beings and cattle in Hungary and on the 

 Lower Danube, and also to explorers in Australia, the 

 learned professor remarks that ' in Britain the flies of 

 Simulium are perfectly harmless.' Now, assuming 



1 Macmillan and Co., 1895. 



1 Wellcome Bureau of Scientific Research, 1920. 



