MIGRATION AND DISPERSAL OP INSECTS '. LEPIDOPTERA. 12? 



Migration and Dispersal of Insects: Lepidoptera. 



By J. W. TUTT, F.E.S. 



The palm as a migrant among butterflies must, however, in 

 the Palaearctic region, be aAvarded to the cosmopoHtan Pyrameis 

 cardui, which is distributed throughout almost the whole of the eastern 

 and western hemispheres. It abounds almost every year in the sub- 

 tropical countries of the Old World, and hence vast flocks appear to 

 disperse themselves into the Palaearctic regions, as well as to the south. 

 In Britain it sometimes appears in successive years, and is rarely 

 absent for more than three or four years in succession. Yet, with the 

 exception of an occasional individual, the autumnal progeny that 

 results from the spring immigrants fails to hybernate, and the species 

 cannot establish itself permanently in this country. The autumnal- 

 bred specimens, undoubtedly following the habit engendered in the 

 subtropical home of their parents, lay their eggs (which quickly hatch) 

 and attempt to produce another brood, which is killed off as larvae 

 by the early frosts, and thus bring about, as does Colias edma, their 

 own extermination. 



Before discussing the migration of P. cardui it may be well to call 

 attention to the apparent similarity of the conditions that cause the 

 migration of this species and Plusia (jamma. Not that migrations of 

 these species do not take place independently, but their repeated 

 simultaneous occurrence is worthy of remark. To run through this 

 connection historically would be largely a waste of time, and the 

 following records must be taken as examples of many others. Before, 

 however, giving any records, wo may note that P. (jamma, with us, 

 has no regular season ; it has been seen from January to December in 

 the imago state, and in its abundant years when a late brood of 

 imagines comes out well into October, the larvfe from these feed up all the 

 while food is obtainable, and die off or pupate (according to the severity 

 of the winter) in late November and December. Under any conditions 

 their continuous-broodedness results in their repeated destruction, and 

 here it falls in the same category as our two Coliads, Pyrameis cardui, 

 &c. In 1879, one of the wettest and coldest summers of the century, 

 there was a marvellous incursion of both species in this country in 

 May. Thus Cambridge records that in the Bloxworth district both 

 species were unusually abundant in May and June, and that, in August, 

 they were in the greatest profusion, P. (jamiua rising from the flowers 

 when disturbed " in swarms." Slater records that on August 13th the 

 sea at St. Leonards was scattered over with the moths that were being 

 washed up in lines on the shore, and states that no one seems to have 

 observed whether the moths had come from France, or had been 

 drowned in attempting to leave England. Carrington observed that 

 the sandhills on the Essex coast were infested with the species, the 

 numbers being so great as "almost to pass description." McEae states 

 that at Bournemouth P. cardui vf&s in August swarming in thousands, 

 and P. f/amma in tens of thousands, whilst it was quite evident that 

 the autumnal abundance (arising from the spring immigration) was 

 not confined to England, for Cox records that near the Kursaal, at 

 Ostend, P. (jamma was in shoals, whilst P. cardui was flying by 

 hundreds up and down the streets and on the barren sandhills, whilst 

 Thwaites notes that in Saxon Switzerland the two species were as 



