MIGRATION AND DISPERSAL OF INSECTS! LEPIDOPTERA. 127 
Migration and Dispersal of Insects: Lepidoptera. 
Bye LU HE. Sen 
The palm as a migrant among butterflies must, however, in 
the Palearctic region, be awarded to the cosmopolitan Pyrameis 
cardut, 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 Palearctic 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 larvee 
by the early frosts, and thus bring about, as does Colias edusa, 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 yamma. 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, we may note that P. gamma, 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 larvee 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, Pyramets cardut, 
&e. 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. yamma rising from the flowers 
when disturbed ‘“‘in swarms.’’ Slater records that on August 18th 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.”” McRae states 
that at Bournemouth P. cardui was in August swarming in thousands, 
and P. yamma 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. yamma 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 
