DISTRIBUTION OF INJURIOUS INSECTS 59 



taken place mostly from north and south towards the Equator. 

 We find that many temperate-climate insects will live and 

 flourish in sub-tropical and tropical climates, but the reverse 

 only applies within certain narrow limits according to each 

 species. 



It is extremely unlikely that many tropical pests would 

 live and flourish in the warmer climates of Europe, although 

 we have an instance of such in the Yellow-fever Mosquito 

 (Shgomyia fasciata, Fabricius). On the other hand, sub-tropical 

 species may do so, and even penetrate into still more temperate 

 regions — as, for instance, the San Jose Scale {Aspidiotus perni- 

 ciosus, Comstock), which is found spreading as far north 

 as Canada. The Yellow-fever Mosquito (Stegomyia fasciata, 

 Fabricius), however, does not seem to occur farther north and 

 south of the Equator than 48 . It has evidently spread outwards 

 from the Central American States. We also see that the 

 Brown Spotted-Mosquito {Theobaldinella spathipalpis, Rondani) 

 has spread from Europe to the Sudan, and also far into the 

 Cape. 



Of wider distribution still are some insects which attack 

 stored goods. We find the Corn and Rice Weevils (Calandra 

 granaria and Calandra oryza?, Linnaeus) now in almost all 

 countries from the Equator to Norway and New Zealand, 

 because they are so easily carried in grain. 



The manner in which an introduced insect may behave in a 

 new country cannot be foretold. It may increase very rapidly, 

 such as did in California, the Cottony Cushion Scale (Icerya 

 purchasi, Comstock), which came from Australia. On the other 

 hand, it may die out sooner or later, as happened with the 

 Tasmanian Lady-birds (Lets conformis), which I introduced into 

 this country. The Cushion Scale found a comfortable home, 

 and there being none of its natural enemies to prey upon it, the 

 Scale increased at an enormous rate. The Tasmanian Lady- 

 birds, although they survived two seasons, found the climatic 

 conditions unsuited to them, and consequently died out. 



Many insects from the tropics and sub-tropics may be 

 imported to temperate climates, such as our own, and will 

 flourish under glass, where they find congenial heat and 

 moisture. Scale insects, or Coccida?, are particularly prone to 

 do so. Most of the palm- and hot-house Scale insects we have in 

 Britain are foreign importations. Only recently a Mealy Bug 



