The most characteristic result of the presence of the larve in the mine are its excrements. Only in old, long vacated mines rain may have washed out the frass; the mines then generally look whitish. And of course, there is a rather small number of mines from which the frass is ejected. The colour of the frass is (in fresh mines) very constant, ranging from warm brown to black or greenish. Mostly the frass consists of rather dry grains or pellets. Fly larvae may produce frass that sometimes is semi-liquid (Weiss, 2006a).
Stigmella aurella on Bramble: frass
In this family all transitions may be found, from discrete dry grains to pearl chains and long strings; there are even species where the corridor is filled with deliquescent frass.
Liriomyza congesta on Vetch
Agromyzid larvae lay on their sides in the mine, mowing the leaf tissue with their sickle-shapled jaws in a vertical movenent. Because their is somewhat banana-shaped, both mouth and anus are at either one or the other side of the corridor. They regularly turn over from one side to the other; the result is that the frass is deposited in two parallel rows along the sides of the mine. This double pattern is an excellent tool to recognise in an corridor mine the work of an agromyzid.
Amauromyza lamii on Hemp-nettle: initial corridor
The tempo of turning of the larvae differs from one species to the other. A species like Agromyza alnivora seems to turn over after each bite.
Agromyza alnivora on Alder
Rarely, as in some beetles, the frass consists of thin rods or even long threads. In that case their mines can be confused with those of Eriocrania-species, that produce frass in an almost continuous line.
Eriocrania sangii on Birch: frass
The larvae of some species of Nepticulidae sway their abdomen end rhytmically from side to side; the result is that their frass is deposited in arcs; this frass pattern is called “coiled frass” in English literature.
Stigmella oxyacanthella on Hawthorn
When contrariwise the anus remains positioned in the middle of the corridor the resulting frass line is quite narrow. In many cases the frass pattern (coiled or not, widely dispersed or narrow) is crucial for the identification, although there are also species that show all transitions.
Stigmella trimaculella on Black Italian Poplar
where to put it?
Sometime, like in sawfly (Tenthredinidae) mines, the frass is scattered as loose grains; in old, dried out mines it is amassed as ground black pepper in the lowest part of the mine. In tentiform mines (made by Gracillariidae) the frass mostly is stacked in a neat heap, separated by a silken curtain from the rest of the mine. But much more often the frass is stuck to the epidermis. In many moths with their full depth mines, it is glued to the upper epidermis. It is surprising that this happens both in species where the larvae lie on their back (e.g. Ectoedemia septembrella) but also in the case where the larvae lie belly-up, like Stigmella aurella (top foto). Yet, the frass deposition behaviour i seems to cross taxonomic lines: Ectoedemia albifasciella has its frass in loosely packed grains!
Ectoedemia septembrella on Square-stalked St John’s-wort: ceiling of the blotch, with frass grains stuck to it
Ectoedemia albifasciella on Oak, opened mine
Agromyzidae larvae, that lie on their side and make shallow mines, glue their frass either to to the epidermis or to the floor. Wether there is a taxonomic pattern in this behaviour, and wether the pattern is constant for each species, remains to be studied.
Phytomyza pubicornis op Goutweed, opened mine: frass on the epidermis
Aulagromyza luteoscutellata on Honeysuckle, opened mine: frass on the floor
Rarely the frass is deposited as tarlike drops on the floor of the mine; the ceiling remains absolutely clean. This happens in weevil Isochnus sequensi but just as well in the moth Cameraria ohridella.
Isochnus seqensi on White Willow
A small number of species make holes in their mine to expel most of the frass. Examples are species of the genera Atemelia and Coptotriche. Here the holes are tiny and difficult to loccate. In the sawfly Heterarthrus nemoratus the opening is exactly in the leaf margin.
Heterarthrus nemoratus on birch
The nepticulid Ectoedemia subbimaculella makes a wide tear in its blotch mine.
Ectoedemia subbimaculella on oak
Epermenia chaerophyllella on Cow Parsley