IS87.] ]13 



hawkbit might be said to have been alive with them, for their pugnacious instiucta 

 kept them in a constant turmoil : the flowers appeared to be a great attraction, for 

 they were continually settling upon them. In contrast to the abundance of P. 

 brassiccB and rupee, napi was almost a rarity. I may mention that last year P. rapes 

 appeared also in pretty strong force, although not to compare in numbers to this. 

 Last autumn I collected fx'om plants of Brassica in the garden about fifty full-grown 

 larvae of P. rapce ; about one-third of these pupated all right, but the others 

 produced hundreds of the larvte of the usual parasites, from which this summer they 

 emerged in legion, and this appears to be One effectual means of keeping P. rapcB in 

 check.— A. Elliot, Caverton, Roxburgh, N. B. : September 14th, 1S87. 



3Iigration of insects. — I am rather surprised to see in the Ent Mo. Mag. for 

 September that the immigration of White Butterflies is strongly supported by 

 several Entomologists. It seems to me that this idea, though, perhaps, occasionally 

 correct, is somewhat overdone. If your readers had walked with me, say between 

 Lewes and Eastbourne, on a day in September, 1886, I think they would not for a 

 moment have doubted the possibility or pix)bability of one raising a swarm of both 

 Pieris brassiccB and P. rapce for ourselves. Walls, &c., were covered with the larvee, 

 and the cabbages in the gardens were reduced to simple ribs. Considering the gi'eat 

 fecundity of insects, it is, perhaps, surprising to us that so few reach the imago state 

 until we begin to calculate the number of coincident circumstances necessary for that 

 end and the great odds against it. Suppose, for instance, ten coincident favourable 

 circumstances necessary for the perfection of every specimen of a brood — a very 

 moderate calculation — it is probable that ordinarily only, perhaps, two or three of 

 these favourable circumstances actually occur. In that case we see the insect hi its 

 usual or average state of abundance. Should only one circumstance be favourable — 

 the others unfavourable or even neutral — the insect is rare. On the contrary, should 

 eight, nine, or ten circumstances be all favourable, the insect far exceeds its usual 

 numbers and swarms. What the particular circumstances are that influence tlie 

 abundance or paucity of various species we but imperfectly know, but an)ong them 

 may be instanced absence or abundance of parasites, absence or presence in varying 

 degrees of moisture, dryness, heat, cold, or storms at various critical periods, the 

 direct or indirect action of man, &c., &c. 



That partial migration caused by over-crowding and shortness of food takes 

 place no one will deny, and this explains the occasional presence of great numbers 

 on the sea coast, as every movement in that direction is stopped and the species 

 becomes as it were heaped there. If there is an extra abundance of Aphides, or 

 glow-worms, or the more sluggish Coleopteva which frequently happens, or even 

 of the local species of Lepidoptera, no one thinks of flying to immigration for a 

 cause, but considers at once that local conditions have produced it. Then why 

 reason differently with other species ? 



Some years ago I saw Colias Eduna by dozens drying their wings in clover fields 

 — evidently true British bred and born — yet all the while there was the usual talk 

 of " immigration." If imniigi-ation is so potent a factor in producing our insect 

 fauna, why do we not more often see the various strong flying butterflies and moths 

 of neighbouring Europe which species may be numbered by scores? — J. H. A. Jennee, 

 4, East Street, Lewes : September 5th, 1887. 



