﻿178 THE ENTOMOLOGIST. 



British Islands from the mainland of Continental Europe cannot 

 he regarded as imposing an obstacle that would not be easily 

 overcome by migratory instincts thus engendered. If, however, 

 the sphere of our observations is confined entirely to these 

 Islands, we have great difficulty in detecting cases of probable 

 immigration; for although the evidence may strongly favour 

 migration, it may not be conclusive, and the circumstance might 

 be referable to other causes. But if we find insects moving in 

 numbers in situations where their presence cannot be accounted 

 for in any other way, we may safely conclude that they are in a 

 state of migration ; the meeting with a swarm of Deiopeia pul- 

 chella in Mid-Atlantic is a case in point, and many other instances 

 are on record referring to other species, and extending even to 

 our shores. The reports furnished, through the courtesy of 

 Mr. Cordeaux, by sundry Lighthouse keepers, although not 

 numerous, are of great importance ; those from Guernsey and 

 Heligoland, referring to vast flights of Plusia gamma, Hybernia 

 defoliaria, &c, passing frequently in a westerly direction, have a 

 distinct bearing upon the subject. 



On the other hand, there is the probability that the sur- 

 roundings of a species during its metamorphoses would affect its 

 subsequent abundance or scarcity : the evidence under this head 

 is at best incomplete, but as far as it goes it is of importance. 

 The females of many species deposit large numbers of ova, yet we 

 do not find a corresponding annual increase in the number of 

 the imagines of such species ; their natural enemies hold them 

 in check. If, therefore, we are to account for the cases of occa- 

 sional abundance by purely local influences, we must find some 

 irregularity in the working of Nature. It has been suggested 

 that the insect may remain in one of its quiescent stages for an 

 unusually prolonged period, — for instance as pupa, numerous 

 cases are on record, but they tend to show that the second year 

 is seldom exceeded, and in such cases as it is the number of 

 emergences are proportionately smaller. But perhaps the most 

 important of local causes is that of meteorological conditions : 

 mild winters and cool summers are said to be detrimental to 

 Lepidoptera, while hard winters and sunny summers are held to 

 be favourable, and so forth ; observation appears to prove that 

 such is the case, but that the effect is upon Lepidoptera generally 

 rather than upon a few species, and it is worthy of remark that 

 some of the cases of greatest abundance of Va?iessa cardui and 

 Plusia gamma have occurred in cold wet seasons, when other 

 species have been actually much less common than usual. 



As examples of species exhibiting tendencies to occasional 

 great abundance the following may be mentioned : — Hybernia 

 defoliaria, H. aurantiaria, Oporabia dilutata, Cheimatobia brumata, 

 C. boreata, and Orgyia antiaua. These all have apterous females, 



