1894.] 151 



of which is to bring about an increased flow of sap to the part. That 

 a free and over-abundant supply of sap is conducive to the longevity 

 of leaves is shown by the length of time that the branches of recent 

 pollards and still more the shoots thrown up from the stools of felled 

 trees retain their greenness. Hence it might with good reason be 

 argued that the afflux of sap brought about by the irritation of the 

 larva is the sum and substance of the whole business. The explanation 

 certainly is simple enough ; but simple explanations are not necessarily 

 right, and in this case, when all attendant circumstances are considered, 

 and especially the length of time that vitality is retained after the 

 death of the leaf as a whole, I think it will be admitted that something 

 more than mere mechanical irritation must be at work. Looking at 

 one of these green patches, with its margins fading gradually into the 

 surrounding brown area, it is almost impossible to escape the conviction 

 that it is produced by some substance that we may call a poison, or 

 better still, looking at its effects, a preservative, which, taken up by 

 the sap, is carried to the cells, and being appropriated in its progress 

 gets more diluted and attenuated the further it travels. What this 

 substance may be, whether a secretion specially provided for the purpose 

 and poured out from the mouth of the larva, or possibly some excretory 

 substance present in the frass, I am quite unable to say. At any rate, 

 the whole virtue of the operation seems to be exercised whilst the 

 larva is still young, and once accomplished the life or death of the 

 creature is of little or no consequence. 



Exhibited by very many species, in none does it offer a better 

 opportunity for study than in subbimaculella, one of the commonest 

 and best distributed of our species. Passing under the oak trees any 

 time in the autumn, from September to November, it will hardly be 

 possible to overlook its little green patches in the brown leaves lying 

 on the ground. Usually the patches are roughly triangular, and extend 

 from an angle of the midrib for some little way into the corresponding 

 interspace, whilst at or near the apex is the larval blotch. If we look 

 closely, the empty egg-shell will be found on the upper surface of the 

 midrib, or occasionally of a side rib, and proceeding from it a fine 

 gallery that keeps accurately to the side of the rib, and catches and 

 impregnates, we must suppose, the sap in its passage out of the vascular 

 bundles. At any rate, it is whilst the larva is making this preliminary 

 gallery that the whole of its singular influence over the leaf is exer- 

 cised, for not unfrequently a patch is found as large and as green as 

 usual, and yet nothing is to be seen but this preliminary gallery 

 and the dead larva at the end of it. Even more strikine: as a 



