Location in the leaf
Obviously, the position of the mine in the leaf blade is primarily determined by the oviposition behaviour of the female insect. The oviposiiton site is quite constant in many cases. Often it is the leaf margin, or the egg is placed immediately next to the midrib. Less often the oviposition site is in a vein axil or just someplace on the blade.
In Nepticulidae it often (but not always!) is a reliable diagnostic character wether the egg is placed on the upper or the lower surface of the leaf.
A number of sawfly species oviposit exactly on the leaf margin. Among them are species that are quite stereotyped. Heterarthrus nemoratus places its egg on the leaf margin, about halfway the leaf. In the species the oldest parts of the mine turn wine red, making it easy to reconstruct the development of the mine..
Heterarthrus nemoratus on Birch
Heterarthrus microcephalus places its eggs exclusively on the extreme tip of the leaf (Altenhofer, 1980b)
Heterarthrus microcephalus on Almond Willow
Trachys minutus is quite catholic in its hostplant selection, but oviposition invariably is on top of something acute: the tip of a leaf, or of a leaf lobe, or of a leaf margin tooth.
Trachys minutus on Elm: oviposition (the black drop) on the tip of a leaf margin tooth
a boring start
The first part of the mine may also be hidden within the petiole or the midrib; the larva in that stage lives a borer, rather than as a miner. At a given moment the larva starts making excursions into the leaf blade. Generally during feeding pauzes of when danger threatens the larva retreats into the safety of the initial corridor.
Ectoedmia hannoverella on Black Italian Poplar
Several Ophiomyia-species lave als larvae in the hollow midrib of Liguleae (‘yellow composites’). From there they make broad, lobe-like corridors into the leaf.
Ophiomyia pulicaria on Dandelion
Also for Liriomyza strigata is the midrib decisive for shape and position of the mine. But in contrast to Ophiomyia this larve lives on top of the midrib, not within it. He is not able to withdraw into the hollow vein. Mainly because of that, the lateral veins of the mine of L. strigata contain a normal amount of frass, while in Ophiomyia almost all frass is deposited in the midrib (in the very base, at the border between leaf and stem.)
Liriomyza strigata on Sow-thistle