THE VARIATION OF OPORABIA DILUTATA. 121 



temperature, and natural enemies keep their abundance fairly con- 

 stant. A combination of favourable conditions for the increase of the 

 species, however, occasionally presents itself, and then the locusts are 

 developed in abnormal numbers. The competition between themselves 

 for food leads to their dispersal as larvje, and to their mij^^ration as 

 adults. Such emigrants that pass beyond the normal limits of their 

 geographical area are, however, subjected to entirely new conditions of 

 environment, and especially to those of temperature, and this prevents 

 the permanent extension of their area, for their organisation, fitted as 

 it is to respond to the environment whence they came, may be utterly 

 unfitted for that of the new environment in which they find themselves. 

 A few days of unusual cold, humidity, or drought, may be fatal to eggs, 

 larvfe, or adults, and the natural work of extermination is begun. 

 Hence the influx of immigrants rarely results in the permanent exten- 

 sion of the geographical limits of any species, and many countries that 

 have for thousands of years been visited by migrating swarms of a 

 certain species still refuse that species a place in their permanent fauna. 

 It would appear that the insects love comparatively dry soils, in arid 

 and warm districts, and that, therefore, excessive humidity is more 

 likely to be a factor in preventing their permanent spread than exces- 

 sive drought. They seem also to be sensitive to a low temperature 

 (Riley notes that the locusts, late in autumn, are often killed oft" by a 

 frost or snowstorm), and, therefore, a combination of wet and cold 

 atmospheric conditions is more likely to be fatal than any other. 



If, as we suspect, the migration instinct of locusts be more or less 

 directly connected with the food supply, one may readily find a sugges- 

 tion as to whence the more recently evolved orders — as represented by 

 butterflies, moths, flies, &c. — derived their migrating instinct, for 

 these orders have probably been developed from some ancestral form 

 of insect closely allied to the locust of to-day, with a capacious 

 appetite in all its stages. It is evident that, however intimate may be 

 the connection between the migration of locusts and their food 

 supply, the latter cannot now be the direct cause of the migration of 

 butterflies and moths. We have seen, hoAvever, how the original 

 migrating instinct in these species probably came into being, and how 

 old and deep-seated a habit, if we may so term it, is the tendency to 

 migration. 



The Variation of Oporabia dilutata. 



By LOUIS ]3. I'llOUT, F.E.S. 



In working at the genus Oj/orahia, of late years, I have attempted 

 to talnilate the principal forms of <K <lili(tata, Bkh., and find two of 

 the most striking alierrations have never been named. Lampa's 

 varieties {I\nt. Tidskrift, vi.) are not to be used here, as his ncbidata 

 {(Ulutata) has been ascertained to be the aiUitnniata of Borkhausen 

 {addmdaria, B. White). The following is the best classification at 

 which I have been able to arrive : — 

 I. — Grey forms, more or less typical : — 



a. dilutata, Bkh. — Bunds moderately distinct. 

 h. ab. inscriplata, Don. — Bands intensified. 

 c. ab. iiiqjliiriata, Bkh. — Bands almost obliterated. 



(/. ab. qiiadri/asciata, Bkh., ncc Tr.— Ground slightly paler, wings 

 narrower, bands normal (? a superfluous name). 



