1886.] 129 



The Gold Tail Moth (P. similis) was so plentiful in the neighbourhood of 

 Perry Barr that it covered the hedgerows like " flakes of snow." Since 1879 all 

 these insects hare been much less plentiful. 



With respect to V. cardui and P. gamma, their unusual abundance in 1879 

 seemed to have been general throughout the British Isles, for correspondents in 

 Ireland and the north of Scotland inform me that both species were very abundant 

 there during the same year. 



Now I am inclined to think that a considerable percentage of the unusual and 

 sudden appearances of particular insects are referable to migration. 



Vanessa cardui is an insect which possesses a very wide area of distribution, 

 being found in nearly every country in the globe. It is one of those insects in 

 which a strong migratory instinct appears to be developed. Gathering together in 

 enormous clouds they quit the place of their birth and depart in search of " fresh 

 fields and pastures new." 



The Painted Lady is a butterfly that produces two broods in the year, and 

 hibernates in the perfect state. When one of these large migratory hordes reaches 

 this country it generally appears to be composed of those insects that have hibernated 

 such hordes reach our shores in June or about that time. 



If the swarm is a large one the butterflies spread over the Island, if, on the other 

 hand, the swarm is a small one, the range is more restricted. 



The butterflies then breed and die. The first brood is produced in August when 

 the imagines again in their turn deposit ova on the thistles, and die. In our British 

 climate, thistles and kindred plants die early in the autumn, and before the larviB 

 can feed them on the chilly nights have withered the food plants. Starved with cold 

 and hunger, the larva fall victims to our climate, so that the second brood which 

 ought to live over the winter to continue the race over another year never reaches 

 maturity, and the species disappears. It may happen, however, sometimes that a 

 few of the first brood emerge so late that they hibernate or emerge so early that 

 the progeny passes through the various stages and does the same. Either of these 

 occurrences would carry the race on for another season, but it does not seem possible 

 for this to last more than a year or two. The species is then lost until a fresh 

 migration makes it abundant once more. Vanessa Antiopa is another insect whose 

 phenomenal appearances may in my opinion be referable to the same theory of 

 migration, although the differences in colour between British and Continental 

 specimens require a little investigation. 



There are many other insects which, independent of migration, appear sometimes 

 in immense numbers locally, such as Lohophora viretata, Thecla ruhi, and other 

 local species which are usually the reverse of abundant. Then again, others which 

 are usually tolerably abundant, such as Polyommatus Phlcsas, Euchloe cardamines, 

 and Pieris napi are very scarce in certain seasons. 



The recent progress of science forcibly teaches us that the grandest and safest 

 conclusions are best attainable by means of the most minutely accurate observations 

 persistently conducted according to some well arranged and comprehensive plan. 



lu the cases above cited, numberless experiments are being carried 



