Leaf miners are specialized insect larvae (juveniles) that live between the top and bottom surfaces of leaves. Generally these small larvae would pass unnoticed if it were not for the fact that, by consuming the photosynthetic material within the leaf, they often leave behind a discolored scar. (Often the cavities are primarily near one leaf surface and are only seen easily from one side. Some damage is more visible if you look through the leaf since the mined regions pass more light.) The areas eaten out come in two forms - narrow tunnels, called linear mines, which wander in various patterns through the leaf and large caverns, called blotches, spots which expand as work progresses. Some species produce only linear mines, some produce only blotches and others make linear mines before switching to blotch mines. Linear mines often get wider as the insect grows. In some cases, insect faecal material, called frass, is distributed in the mine in a distinctive pattern. In order to leave the mine, the insect must make a hole in the surface of the leaf - for insects that make linear mines this occurs where the mine is the widest.
Leaf-mining has evolved as a larval strategy in four insect orders. Most of the leaf miners belong to the Diptera (flies) and the Lepidoptera (moths in this case - some of these mine leaves initially then switch to feeding externally). Some Hymenoptera (sawflies) are leaf miners as are some Coleoptera (beetles). All of these orders have a four stage development - egg, larva, pupa and adult.
The holes I noticed in the willow leaves had neat edges and were elliptical, the longest axis being about 3 mm long. Leading up to each hole was a brownish V-shaped mine that usually had its apex very close to the midvein of the leaf. The mine was on the top surface of the leaf, and had a pattern of concentric arcs. Larvae could be seen working inside mines without holes. In each mine, the frass was packed into the hollowed out space previously excavated. Most of the plant material between the surfaces of the leaf had been eaten by the larger larvae, resulting in pale brown circular areas visible from both sides. The edges of some circles had been thinned further by these larvae. On a later visit, I noticed small seeds floating in the air. In fact many were really hanging, suspended on silken threads. Each seed was actually a little case constructed from the top and bottom surfaces of a leaf and held together by silk. Inside each case was one pupating insect. Where each case had been, there was now an small elliptical hole in a leaf.
leaf mines on willow (actually 2 mines running outward from the central vein in the leaf, terminated by a hole in the leaf)
leaf miner at work (on willow) (taken through a microscope - total length of miner less than 3mm)
Ron Lyons (volunteer 1990-1999)