214 THE ENTOMOLOGIST. 



to me to afford no support to Mr, Adkin's theory of occasional 

 migratory swarms of insects probably following the annual 

 migration of birds. Although the great geographical range of 

 this bird was recently referred to by Colonel Legge, its annual 

 migration was known to the late Mr. Gould nearly half a century 

 ago, and a very complete history of the species is given in 

 Sharpe and Dresser's ' Birds of Europe.' Sir Walter Buller, the 

 greatest authority on the birds of New Zealand, remarks, " It 

 occurs occasionally on the New Zealand coast, but apparently 

 only as a straggler, and almost always in winter plumage." If 

 such a theory were tenable, it would be more supported by the 

 annual migration to New Zealand of two species of cuckoos, the 

 one from Australia, the other from Polynesia. And other species 

 occur as stragglers, generally at long intervals, but all are 

 endowed with great powers of flight, and no doubt the migratory 

 impulse in birds has been inherited for innumerable ages. The 

 annual migration of birds to the British Islands, and the 

 occasional migration of swarms (or the greater abundance) of 

 certain species of Lepidoptera from the Continent, seem to me 

 to be due in many cases to totally different causes ; with birds 

 it is an heriditary impulse acting in obedience to the law of 

 reproduction ; with Lepidoptera it may, as Mr. Adkin observes, 

 sometimes affect the status of a species, at least such with wing- 

 less females ; yet in New Zealand I observe that the occasional 

 abundance of Lepidoptera is due to the law of interdependance of 

 plants and insects operating with greater force in some seasons 

 than others. The imagos of some species of Lepidoptera are 

 dependant on the blossoms of certain plants for their support, 

 the failure of which in some districts (or a scarcity in some 

 seasons of the food-plants of the larvae) would instinctively 

 impel them to migrate to other districts where such existed, thus 

 causing their numbers to be concentrated within a limited area. 

 The same cause affects frugivorous birds, and in the agricultural 

 districts we have experienced four great irruptions of the native 

 parrakeets within the last twenty years from this cause alone.* 

 In the forest region of Westland some seasons are noted for 

 producing great swarms of insects. In such seasons the insecti- 

 vorous birds invariably appear in greater numbers in the districts 

 most affected, and roam through the forest in large flights, con- 



* The irruptions of Pallas's sand grouse into the British Islands in 1863, and 

 two years ago, is another case in point. If " the occasional migrations of Lepidop- 

 tera follow the routes taken by the birds in their annual movements," it would be 

 of considerable interest to know what caused the dearth of insect food, in the pre- 

 ceding year, which compelled the birds to migrate westward. There is a note in 

 last year's volume of the ' Entomologist ' (now lent to a friend, and I cannot cite 

 it), calling attention to the occurrence in the British Islands of Deili2yMla galii in 

 unusual numbers in both years in which the birds appeared. Although even in 

 these exceptional cases such occurred, it was in all probability due in both instances 

 to the absence of food in their natural haunts, caused by some seasonal 

 derangement. 



