150 [July, 



NOTES ON THE EAELIER STAGES OF THE NEPTICULM, 

 WITH A VIEW TO THEIR BETTER RECOGNITION AT THIS PERIOD 



OF THEIR LIFE. 



BT JOHN H. WOOD, M.B. 



(Concluded from p. 98). 



I conclude with a subject intimately connected with the economy 

 of these insects, though scarcely perhaps relevant to the actual 

 purpose of the paper. 



I allude to the singular power these insects possess of delaying 

 the ripening and death of the part of the leaf they are occupying. 

 Mentioning the subject some years ago to Mr. Stainton, he told me 

 that it had first been noticed by Yon Hey den, but I do not think it 

 has received the attention it merits, and, at any rate, I have come 

 across nothing bearing on it in our own publications. It is a most 

 curious and striking phenomenon. The leaf shall have put on its 

 red or yellow autumnal tint, it shall even have dropped from the tree, 

 have died and turned brown, but the area in which the larva is feeding 

 will remain alive and green, not merely for days but for weeks, pro- 

 vided it be not exposed to excessive dryness. 



Now, it is well known that the fall of the leaf is associated with 

 an acid condition of the sap, and the only explanation I have heard of 

 the phenomenon we are considering is, that the mine, cutting like a 

 trench across the leaf, stops the supply of this acid sap to the part 

 beyond, and so preserves it from the change going on in the rest of 

 the leaf. But I question whether there is not an erroneous notion 

 here, for it is not the acidity of the sap that brings about the changes 

 in the leaf, but the changes in the leaf that bring about the acidity of 

 the sap. Those of us who do much rearing in air-tight vessels must 

 have noticed, as the season drew to a close, that the leaves we placed 

 in our bottles, though perfectly green at the time of picking, would 

 not unfrequently, and in the course of a few days, dye the natural 

 autumnal colour, and apparently more readily (it has seemed to me) 

 than if they had remained on the plant and had continued to be fed 

 by the sap. Here is proof, if proof be needed, that the sap can have 

 little to do with the process, except in the way of retarding it, and 

 that it is a property of the leaf tissue itself. This being so, how then 

 does the larva manage to delay the process and keep that part of the 

 leaf in which it is residing green and living ? 



In the nature of things, the presence of the larva among the 

 living cells must act on them as a stimulant or irritant, the first effect 



