2 [June, 



weeks juet at that time, and one larva made such progress that in a 

 fortnight it was full-fed, when it spun a very slight cocoon on the 

 gauze and turned safely to pupa. By this time two more larvae were 

 full-fed and left the food-plant for the gauze, the rest being fully half 

 grown, when a change of weather came, with wind, heavy rain, and a 

 total absence of sunshine. The larvae were, of course, not exposed to 

 the rain, but the effect of the change was that those full-fed made no 

 attempt to spin up, and the .rest ceased to feed, and in a few days 

 they all fell off the gauze or the plants, dead. After a fortnight of 

 wet weather it cleared uj) and the one pupa produced the moth — a 

 male. 



This seems to supply a key to the whole history of the eccentric 

 casual appearances of this and many other inhabitants of warmer 

 climates on our shores. In obedience to some singular instinct that 

 impels insects when becoming too numerous in their natural homes to 

 emigrate to " fresh fields and pastures new," they, contrary to their 

 ordinary habits, cross land or sea, arriving, of course, very often in 

 some inhospitable clime, where — if not at once captured and made 

 native specimens of- — they very likely soon fall victims to some pitiless 

 storm of wind and rain. But supposing both these risks to be 

 avoided, the moth — if an impregnated female — in due course lays its 

 eggs, which most probably hatch, and the young larvae are left — like 

 Mark Twain — "friendless orphans in a foreign land." If the tem- 

 perature happens to be lower or the weather wetter than the natural 

 constitution of the species is able to endure, the difficulty is settled at I 

 once- — the young larvae die without even attempting to feed, but if ■ 

 matters are more favourable, the strongest of them struggle along, and 

 if fairly favoured by the weather a few of them may reach the perfect 

 state ; if quite unusually favoured by the weather a large proportion 

 of them may do so, producing those remarkable instances of the 

 sudden appearance in numbers of a species usually rare. Such good 

 fortune rarely extends to a second season and the species becomes a 

 rarity again or is even probably exterminated here, to be renewed at 

 some future time by the same instinct of migration. In cases such 

 as these it appears to me that sunshine means life, and its absence 

 destruction, to the larvae, and that by this simple and obvious influence 

 the extension of species beyond their assigned limits is practically 

 prohibited. 



It also happens sometimes that the immigrant, following instinc- 

 tively its inherited habit, attempts to produce an additional brood in 

 the year, over what the climate will allow. 



