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PYRAMEIS CARDUI AND THE JUNE RAINFALL 



OF 1906. 



By Robert Adkin, F.E.S. 



The influence of meteorological conditions upon our alien 

 lepidopterous fauna is a subject of deep interest, but it is an 

 exceedingly complex question, and so beset with side issues, each 

 capable of exerting an influence for good or evil, that one is in 

 the majority of instances reluctant to assign cause to effect. 

 Occasionally, however, some special circumstance appears to 

 stand out so clearly that whatever possible explanations might 

 be given, we may, with some measure of certainty, fix upon it 

 as a well-defined cause for some particular phenomenon. The 

 failure of the multitudes of Pyrameis cardui that appeared on 

 our southern and south-western coasts in the spring of 1906 to 

 produce a corresponding abundance of the species in autumn 

 appears to be a case in point, the assumption being that the 

 exceptional weather conditions prevailing at a critical time in the 

 species development was the cause of the failure. 



The available records show that the species was met with 

 sparingly in many places near the south-east and south-west 

 coasts during the last week in May. There can be little doubt 

 that the individuals then seen were the early arrivals of a great 

 immigration which struck the Kent and Sussex and the Cornish 

 and Devon coasts on June 2nd and 3rd and spread inland, distri- 

 buting themselves over the southern portions of the country ; those 

 arriving on the Cornish and Devon coasts apparently taking a 

 north-easterly course along the Bristol Channel and were trace- 

 able as far as the southern parts of Monmouthshire, while those 

 which reached the Kentish and Sussex coasts appear to have 

 spread themselves out over the south-eastern counties and the 

 London district. 



The weather over the southern portions of England during 

 the greater part of the month of June was fine and suitable for 

 the butterflies' egg-laying, the range of temperature being fairly 

 high, with a full amount of sunshine and small rainfall. But 

 on the night of the 28th practically the whole of England south 

 of a line drawn from the Bristol Channel to the Wash was 

 visited by a cyclonic rain of exceptional violence, an area of 

 approximately twenty-two thousand square miles receiving from 

 one to two inches of rain, while within this a roughly triangular 

 patch having its apex near Wisbeach (Cambs) and its base ex- 

 tending from near Croydon (Surrey) to Wallingford (Berks), an 

 area of some two thousand six hundred square miles, received 

 from two to upwards of two and a half inches of rain, practically 

 the whole of which fell within twelve consecutive hours. To 

 put the matter briefly : the southern half of England received in 

 the space of a single night an amount of rain equal to that 



