THE MILKWEED BUTTERFLY 
37 
This record brings the number of examples seen in Britain 
up to August ist, 1934, to 64. 
Respecting the appearance of D. plexippus in this country, 
I published the following note in The Entomologist , Vol. LIV, 
P- x 45 > J un e, 1921 : “ Hitherto it has always been considered 
that the occurrence in the British Isles of Danaus plexippus 
has, so far as known, been due to voluntary immigration. 
But as I previously stated {Field, April 17th, 1915), it is 
probable that the specimens which have been seen and 
captured in this country have received assisted passages 
across the Atlantic on board ships. But at that time no proof 
existed, as no observations of the kind had been recorded. 
It is, therefore, with considerable interest that I am now able 
to place on record the following facts regarding the appearance 
of this American species in England under certain conditions, 
but whether such apply to all the thirty specimens observed 
and captured in the British Isles, and others elsewhere in 
Europe, cannot be ascertained. For the following very 
interesting facts I am indebted to Mr. Geo. P. Pearson, who 
writes to me as follows : * While crossing on the way to 
Jamaica I learned that the purser of the boat (Mr. F. W. 
Buxtin) was a collector. I soon made friends with him and 
asked him how plexippus gets over to England. He at 
once said : “I will tell you. When we leave Norfolk, Va., 
in the autumn, there are always a lot of them flying about 
the potato locker, which is on deck. There are so many of 
them that they get shut in and live on such nourishment as 
they can extract from the potatoes ; when they arrive in 
dock in England they escape. Last summer we had two all 
the way over that never left the ship until we arrived at 
Tilbury, and I saw them fly away there." 1 " Possibly this 
may to some extent account for the apparent extreme scarcity 
of this butterfly in Ireland, where only two examples have 
been observed, one in October, 1916, and one in September, 
I 933- Otherwise it might be supposed that the western coast 
of Ireland would be the most accessible landing-place for these 
insects after their long flight across the Atlantic, had they 
migrated without assistance. In any case, the above throws 
a new light on the subject. Yet there is not the slightest 
doubt that a big voluntary migratory flight of these butterflies 
