188 INSECTS AND MAN 



would appear to consist of about one million females and 

 five hundred thousand males." 



British sheep are often attacked by the castor-bean tick, 

 Ixodes ricinus, which is notorious as being the vector of 

 piroplasmosis of cattle and dogs ; whilst in Roumania, 

 Italy, Turkey, and France, the tick, Rhipicephalus bursa, 

 transmits ovine piroplasmosis or carceag; and in South Africa 

 the bont or variegated tick, Amblyomma hebrceum, trans- 

 mits a disease of sheep and goats known as " heartwater." 



SOME ENEMIES OF POULTRY 



The external parasites of domestic fowls belong to many 

 orders and are comprised of various species. In America, 

 for instance, the domestic hen is known to be infested by 

 eighteen species of mites, eight species of bird lice, two 

 species of fleas, and one species each of bugs, ticks, and flies. 

 Add to this, the bird lice found on other domestic fowls, 

 which comprise five species on the goose, four each on the 

 turkey, duck, and pea-fowl, three on the guinea-fowl, and 

 seven on the pigeon, and we get a considerable number. 



One of the most striking facts concerning the bird lice, 

 or Mallophaga, as they are called, is that similar species 

 occur on domestic fowls of the Old and New World. 

 Seeing that these insects cannot fly, and can only pass from 

 one host to another when in actual contact, the fact is the 

 more remarkable. Various explanations have been enunci- 

 ated to account for this phenomenon ; according to one 

 author, "the parasitic species have persisted unchanged 

 from the common ancestor of two or more distinct but 

 closely allied bird species." In other words, these species 

 of Mallophaga existed on the ancestors of their bird hosts, 

 and have persisted ever since, without change, although 

 the hosts may have become modified into different species. 

 If this explanation be the correct one, it is easily accounted 

 for by the fact that although the host may change, the 



