Watt. — Leaf-mining Insects of New Zealand. 203 



deposited against, and remains attached to, the roof of the mine. Indi- 

 vidual leaves of 0. nitida may contain many mines, but these never inter- 

 fere with one another, and, however cramped the room at disposal, a 

 gallery will seldom ever cross itself, tending rather to become closely 

 vermiform, with its loops applied to, but not encroaching upon, each other. 

 I have rarely found more than three or four mines in any one leaf of 

 0. macrodonta ; and frequently the mines on this plant cause the leaf-tissue 

 in their immediate vicinity to become discoloured with a reddish tinge. 

 The length of time occupied in constructing the mine is short — two or three 

 weeks in all. T have several times noted ova, and on returning some three 

 weeks later have found nothing but empty mines. The colour of the mine 

 is light brown, and the black frass under the thin transparent cuticle causes 

 it to become very conspicuous. On holding the leaf up to the light the 

 mine is a pretty and striking object. When full-fed the larva emerges 

 through a semicircular cut in the roof of the gallery at its terminal part, 

 and makes its way to the ground. There remains a curious fact to be noted : 

 In the case where the ova are laid on the leaf-buds of 0. nitida it is not the 

 upper surface of the leaf to which they are attached, this surface being 

 snugly tucked away in the interior of the bud ; it is to what will become 

 the under-surface of the leaf, and amongst the thick covering of hairs that 

 protect it at this time, that the ova are firmly cemented. No matter 

 whether it be the upper or lower surface to which the egg is attached, the 

 mine is always constructed close against the actual upper surface. The 

 midrib here, even in these small succulent leaves, prevents, in all but a few 

 cases, the larva from mining from one half of the leaf to the other ; the 

 consequence is the mine becomes closely looped backwards and forwards 

 in the direction of the long axis of the growing leaf, so as to fill almost 

 completely one half of the leaf. In this looped vermiform fashion the 

 mine has progressed from the margin of the leaf towards the centre, as 

 though the larva were aware that any other mode of tunnelling would 

 cut off the sap-supply and leave it short of food. The mine usually 

 progresses in a looped, vermiform fashion, from the margin of the leaf to 

 the centre : but in very young leaves it often happens that when the thin 

 half has been destroyed the tunnel actually enters the midrib. The course 

 then taken is always downwards, the point of emergence lying 1 in. or less 

 below the base of the leaf. 



It is rare to find more than one larva mining in any one of these young 

 leaves. These mines have a curious effect upon the subsequent growth 

 and shape of the leaves. Practically one half of the leaf has been destroyed, 

 but the other half has all the time had its sap-supply (even when the larva 

 was in the stem, for the mine is very small, probably one-third of the 

 transverse diameter of the stem), and has grown accordingly ; the result 

 of one half of the leaf growing and the other not has been to cause it 

 to become curled around the axis. During November a large number of 

 these curled leaves are to be found on the food-plant. Except in the very 

 young leaves there is little of the mine to be seen on the under-surfaces ; 

 in the older leaves there may be a slight darkening of the colour along the 

 track of the mine, and also the surface may be slightly elevated. The 

 entire mine is in the spongy parenchyma of the leaf and leaf-stem. 



The Larva. 

 Length when full-grown, about 4 mm. The ground-colour is very pale 

 green, with a comparatively broad dark-green central line (intestinal canal 



