Diagnosis. Male only likely to confuse with S. cryptella, but separated by smaller size and darker head of thymi. Females could in addition be confused with the slightly larger S. subnitidella, but have less antennal segments, in doubt the genitalia should be examined. Occasionally the presence of two spots makes thymi resembling a small S. eurema. Other species of same size and colour-pattern belong to Stigmella and are recognized by typical Stigmella collar and absence of generic characters of Trifurcula. Male genitalia differ from those of S. headleyella by different shape of vinculum, gnathos, valvae and cornutus. In southern central and south Europe there are several, partly undescribed species with only slightly different genitalia.
Male. Wingspan: 4.0-5.0 mm. Head: frontal tuft and collar yellowish brown to fuscous; scape white, with some brown scales, small; antenna with 28-32 segments. Forewing greyish brown to fuscous, sometimes with a more or less distinct tornal spot; in addition some white scales on costa might occasionally resemble a second spot; cilia-line distinct, terminal cilia silvery white. Hindwing: grey. Abdomen with grey anal tufts.
Female. Wingspan: 4.0-4.5 mm. Antenna with 23-26 segments. Male genitalia. Vinculum only slightly narrowed anteriorly, with obscure emargination. Uncus pointed. Gnathos with broadly triangular central element. Valva triangular, with very narrow and straight distal process at tip. Aedeagus with one very long curved, pointed cornutus, more than half aedeagus length, and small lateral spines, near phallotreme.
Female genitalia. T VIII with a few setae; anal papillae with 13-14 setae. Bursa narrow, with indistinct narrow signa. Ductus spermathecae with long straight basal part and coiled terminal part with 7-7½ convolutions.
Described in detail by Borkowski (1970a). Host plants: Thymus pulegioides, T. pannonicus, and T. glabrescens. Egg: usually on under-surface of leaf. Larva: not described. Mine : a narrow long gallery with broken linear frass, usually occupying two leaves and the petioles or part of the stem between them. Cocoon: brownish.
Not in Scandinavia or Finland. - Very local in central Europe, confined to extremely sunny and hot, usually calcareous, habitats. Known with certainty from Halle in East Germany, southern Poland, surroundings of Budapest and eastern part of Austria (mines, Van Nieukerken, unpublished).
Voltinism: bivoltine, larvae from mid-June to early July and mid-July to early August. Adults reared in May-June and July.
Description based on van Nieukerken and Johansson (1990)