DISTRIBUTION OF INJURIOUS INSECTS 13 



Thus again we find great damage done by an introduced insect. 

 In England it never will be a very serious pest, because the 

 chief damage it does is to autumn-sown corn, which in America 

 is sufficiently up to allow of attack by the second brood, but with 

 us a second brood would die off before the seed had germinated. 

 This pest may be disseminated in two ways — one on the straw, 

 as we see happened in regard to America, and another with seed 

 com, for some of the puparia may be found in it, although most 

 come away in the " tailings." 



Here again we find that somehow the natural enemies were 

 not imported with the pest into America. For with us and in 

 Russia the puparia are frequently found parasitised by chalcid 

 flies to such an extent that one breeds out often more parasites 

 than flies in this country ; hence it is kept in check with us. 



The Wheat Midge {Diplosis tritici) of Europe, also found in 

 North America, doubtless owes its origin there to similar factors. 



Of the pests of animals we may still more expect to find a 

 wide distribution due to artificial causes. 



The Sheep Scab Mite {Psoroptes communis, var. ovis, Fusten- 

 berg) has undoubtedly been spread to all sheep-farming countries 

 with the imported stock. Thorough quarantining would have 

 prevented this, such as the regulations which are in force in 

 most countries now concerning the introduction of animals. 

 We can see similar results in regard to the insect enemies of 

 stock. The Sheep Nasal Fly {(Estrus ovis), which lives in its 

 maggot state in the nasal cavities and sinus of the sheep, has 

 been spread with the sheep, and we now find it in America and 

 Austraha. The same has happened with the Ked (Melophagus 

 ovinus). 



Of all human injurious insects the Mosquitoes, or Culicidce, 

 stand foremost in this subject of artificial distribution. This is 

 not so much so on account of the great importance they are to 

 man as annoying agents and as the means of conveying such 

 diseases as yellow fever, malaria, filariasis, and dengue fever, as 

 on account of the wide knowledge we have of their distribution, 

 and the means by which they have been and still are spread over 

 the globe. It is only natural to expect that those with a wide 

 distribution are intimately connected with man ; such is what 

 we find to be the case. It is those which we may class as 

 domesticated mosquitoes which we find have a wide range. The 

 three most noticeable are the Yellow Fever Carrier {Stegomyia 



