238 NATURAL HISTORY. [CH. XVI. 



eggs upon the backs of oxen, instil into these ani- 

 mals so much dread, that they may be observed, 

 scampering along, with the tail stretched out at full 

 length, until they reach some neighbouring pond 

 of water ; and it is probably these insects to which 

 the poet alludes, when speaking of a flight of gad- 

 flies, although the (Estrus is more commonly known 

 by the name of the botfly, while the term gadfly 

 seems more generally applied to the various species 

 of the Linnsean genus Tabanus. The latter insects, 

 indeed, from their large size, as well as from the very 

 formidable apparatus of lancets with which the 

 mouth is provided, are well capable of instilhng terror 

 into cattle, although they do not appear to torment 

 oxen and sheep so much as they do horses, which 

 are often driven almost wild with the exceedingly 

 painful wounds made by them. We can well speak 

 from experience, for often in our rambles have we 

 been intolerably teased by some of the species, 

 which have continued to hover over us, until they 

 have found an opportunity of settling upon some 

 part of the exposed hand or face, when they would 

 immediately introduce their lancets with a pain 

 equal to that of the sting of a wasp. 



On comparing the figures here given of the mouths 

 of a tabanus and a gnat with those of the domestic 

 fly in a preceding page, a very great diff"erence will 

 be observed in the structure of the two organs. 

 Here, indeed, we find the various parts of the mouth 

 reaching their fullest degree of development, while 

 other parts, of which not even the rudiments were 

 to be observed in the fly, are here completely or- 

 ganized. In. the bloodthirsty gnat, however, we 

 find the same number of organs as in the Tahani^ 

 and it is very remarkable that no other dipterous 

 insect is furnished with so completely developed a 

 mouth, thus showing the intimate connexion of 

 habits corresponding with identity of structure. 

 According to Leuwenhoeck, the sucker of the gnat 



