Among the insects with wings clad in scales, the butterflies are most known and show the greatest variety of scales. In the Diptera, only some families or particular genera of two large groups are known to bear scales on wings, i.e., mosquitoes (Culicomorpha) and moth flies (Psychodomorpha). From among another large dipteran group, the crane-flies (Tipulomorpha), the scales on wings are present only in one small genus, Maietta Alexander, now endemic to the southwestern coast of South America. Herewith, we describe the Eocene ancestor of Maietta, embedded in Baltic amber, Maietta (Hoffeinsetta ) christelae, n. subgen., n. sp. This species and its recent congeners document the evolution of scale cover from scarce, restricted only to the anterior portion of the wing, to complete and dense. A similar parallel evolutionary route was previously described in the Culicidae. The fossil representative of Maietta provides also a further example of biogeographical relationships of Baltic fauna with recent congeners distributed today far from Europe. The present finding prompts a discussion on a possible protective role of scales against sticky surfaces of resin and the same proposes an explanation to a question: why some insects are astonishingly rare in fossil resins; a problem unsolved since 19th century.

All studied specimens are deposited in public institutions. The study was based on an inclusion in Baltic amber (age: Eocene, Lutetian–Priabonian; Grimaldi and Ross 2017) from the private collection of Christel Hoffeins and Hans Werner Hoffeins. The holotype described herein is deposited in the Senckenberg Deutsches Entomologisches Institut (SDEI), Müncheberg, Germany.

Comparative recent specimens: Maietta trimedia Alexander, 1967, Chile, Aysen, 25 km S of Cochuane, 1-2.02.1990, coll. L. E. Peña (housed in ISEA PAS), and the specimens listed in  Santos et al. (2019).

Methods

The specimens were examined with a Nikon (SMZ25) stereomicroscope equipped with a Nikon digital camera (DS-Ri2) at the ISEA PAS. Photographs and drawings were made using a Nikon SMZ 1500 stereomicroscope equipped with a Nikon DS–Fi1 camera at the University of Rzeszów. Usually, 10-30 stacked photographs were combined using Helicon Focus 5.3x64 (Helicon Soft Ltd (c) 2013). The measurements were taken with NIS–Elements D 3.0 software. Drawings were completed by tracing the photographs. The wing venation nomenclature follows Krzemiński and Krzemińska (2003) and the terminology of male genitalia is according to Alexander and Byers (1981).

Some structures of Maietta (Maietta) trimedia Alexander, and a mosquito, Ochlerotatus sp., were analyzed under the scanning microscope Hitachi SU 8010 SEM at the Podkarpacie Innovative Research Center of the Environment (PIRCE) at the University of Rzeszów. For scanning electron microscope (SEM) observations, specimens were attached to aluminum stubs and sputtered with 20 nm of gold using a Turbo-Pumped Sputter Coater Quorum Q 150OT ES.

The article is descriptive, illustrated with 7 plates of drawings and photos and one table (Table 1). The Table 1 is created based on references, as listed in this table.There are three supplementary files: 1. morphology of the recent and fossil species of Maietta; 2. Phylogeny of the genus Maietta; 3. Climate in Eocene and now in localities of fossil and recent Maietta. No codes and special abbreviations are used.