1933] Migratory Flight of Catopsilia eubule 135 
abundant in tropical America, where it probably originated. 
It may never have become completely adjusted to the sea- 
sons of a temperate zone. If this is true, then the south- 
ward migration can be regarded as an adaptation to avoid 
the necessity for overwintering. Some individuals do sur- 
vive the winter, however, either as pupae or as imagines, 
but the scarcity of the species in early summer makes it 
probable that most of them either migrate or perish. This 
suggestion does not explain the frequent migrations ob- 
served in the tropics. 1 
Whatever the cause or purpose of migrating flights in 
Catopsilia may be, the notes confirm the conclusions of Wil- 
liams (’26), that uni-directional flight is the active expres- 
sion of an internal physiological or psychological state in 
the butterflies. This impels them to undertake self-con- 
tributory flight in a particular direction, which neither ob- 
stacles nor wind can prevent or even modify. This state 
may be produced by external conditions, but the flight it- 
self is the result of volition or effort on the part of the but- 
terflies. In this respect it is essentially similar to the 
migrating flight of birds, differing chiefly in its greater 
irregularity and in not being repeated by the same indi- 
viduals. In its seasonal relations, the flight of C. eubule is 
similar to that of Danais plexippus L. (which likewise is a 
species of tropical origin) , but differs in not being preceded 
by any concentration into large aggregations. 
Anyone who observes this species in migration in the 
future can aid materially in solving its problems by mak- 
ing records of the pertinent facts in as great detail as 
possible. Such observations ought to be made in many 
localities and over a period of years. It is only through the 
co-operation of many observers that the mystery of the 
ir rhe late Walter Heape, in an exhaustive work on animal migra- 
tion and related phenomena (’31), distinguishes between emigration, 
in which no return flight is made, and true migration, in which a re- 
turn flight is always made and which recurs periodically. He would 
therefore class the present flight of eubule as an instance of emigra- 
tion. Heape believes that overcrowding and shortage of food is the 
chief cause of emigration in butterflies, but it is not clear whether he 
refers to shortage of food plants for the larvae or of flowers for the 
adults. He recognizes sexual condition as a contributing factor, 
which weakens somewhat the distinction he makes between the two 
kinds of behavior. 
