8
THE PRESIDENTIAL ADDRESS.
cient explanation of many facts with which we are familiar. In this
country we have been frequently visited by migratory swarms of
various insects, such as the "Painted Lady" and "Clouded Yellow"
butterflies, the larger hawk-moths, dragon-flies, lady-birds, plant-lice,
and even grasshoppers and locusts, but not by these last in the vast
hordes known to other lands. The low specific gravity of the insects
themselves and the occurrence of heavy storms or long-continued
winds may explain this as an involuntary act; but the phenomenon
happening so frequently with certain species only, and the occur-
rence of immense numbers (really flocks) of otherwise solitary
insects, does not permit this explanation to satisfy me. See, for in-
stance, the interesting description of the arrival of a flock of our com-
mon white butterflies (Pieridae) at Brighton reprinted from the
"Entomologist," in Newman's "British Butterflies" (p. 166).
Doubtless many of the insects met with at sea, and as stragglers in
new localities, are involuntary migrants carried by violent winds, or
have otherwise received assisted passages "to fresh woods and pas-
tures new." I cannot believe this of such cosmopolitan species as
the "Painted Lady" (Vanessa cardui) amongst butterflies; the
"Silver-Y" moth (Plusia gamma) that accompanied it across Europe
in such numbers to our shores in 1879 ;15 that weak-flying and rare
British, but very widely distributed moth, Deiopeia pulchella, which
has so many times been caught at sea long distances from any land ;
the "Death's Head" and "Convolvulus Hawk" moths, and the rare
and uncertain Deilephila and Chaerocampae; so with certain beetles,
dragon-flies, and locusts, of which we have occasionally been visited
by two species in Britain. Mr. Distant's paper10 on the geographical
distribution of that arch-wanderer Danais archippus, which has
several times lately been captured in Britain, deserves perusal.
In the Second Report of the United States Entomological Com-
mission upon the Rocky Mountain Locust, issued in 1880, Mr.
Cyrus Thomas has some extended observations on the "causes of
migration" in this remarkable insect (l.c, pp. 103-108). In his
opinion, excessive numbers or want of food are conclusively proved
to be neither remote nor immediate causes, nor has the prevailing
course of the winds anything to do with their migration. In the
Third Report, published in 1883, we again read, "Winds have much
to do with the direction taken by swarms, but not with their
migrating" (l.c, p. 20). Prof. C. V. Riley says in his Report of the
15 Entom., vol. xii., p. 284.
16 Trans. Ent. Soc, Lond., 1877, pp. 93-104.