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X1V 
not checked, may threaten the welfare or perhaps the existence 
of the food-plant over a wide area. 
The cause here spoken of may operate and the instinct to 
emigrate may be liberated every year or only in exceptional 
years. Thus Libythea labdaca Westw., appears to migrate 
so regularly in 8. Nigeria that Mr. C. O. Farquharson wrote 
to me (May 3, 1917): ‘*In some places the natives take the 
appearance of the migrants as a sign that the planting season 
for such crops as maize and other annuals has begun, which is 
equivalent to saying that the rains have definitely set in.” 
On the other hand, the Rev. K. St. Aubyn Rogers records that, 
near Mombasa, Libythea laius Trim., and other butterflies 
(Catopsilia florella F., is, however, an annual migrant) appear 
only in occasional years (1899 and again in 1911) after a 
period of prolonged and severe drought. Mr. Rogers con- 
cluded ‘* that butterflies which are usually non migrants may 
be stimulated by abnormal conditions to become migrants ”’ 
(Proc. Ent. Soc. Lond., 1912, pp. xevii—xcix). 
The view here adopted—that the exhaustion of the larval 
food-plant is the essential cause of migration—is, I believe, 
the opinion usually held. It is certainly the one suggested 
by Col. N. Manders in Trans. Ent. Soc., 1904, pp. 705, 706, 
by Boisduval as quoted by Herbert Druce in * Biologia Centrali- 
Americana,” Heterocera, ii, p. 3, by Spruce in Journ. Linn. 
Soc., Zool., ix, pp. 3855-357, and by Roland Trimen in * South 
African Butterflies,’ vol. i, 1887, p. 32, n. 1. 
Evidence that the food-plant is actually exhausted at the 
time of migration and over part of the area traversed is brought 
forward by Spruce (l.c.), Manders (vbid., p. 704), and Mrs. 
Barber as quoted by Trimen (I. c.). This latter evidence, 
obtained in Griqualand West in 1881, is especially valuable, 
as it is apparently an account of the condition. which precedes 
and leads to the emigration of Catopsilia florella. The cater- 
pillars had stripped the leaves from their food-plant Cassia 
arachoides “and then devoured the young shoots, and even 
the bark of the stems.” Dr. G. A. K. Marshall, D.Sc., tells me 
that he has observed the same conditions in a Rhodesian valley, 
he believes on the road from Salisbury to Hartley, and here 
the emigration of C. jlorella had begun. 
