Wing-reduced Diptera are often characterized by having other body extremities shortened as well, namely the antennae and legs. The antennae in the male of R. (R.) incapax, however, are very long, and the legs are considerably slender (Figs 4–5), corresponding to those of some fully-winged species of this genus. It is worthy of mention that, in R. (R.) incapax, reduction of the wings has not yet achieved a strongly brachypterous appearance, but rather seems to have advanced towards what could be termed a stenopterous condition. As to the wing length, the species in question shows a certain resemblance to some unrelated species of Limoniidae with shortened wings, probably flightless, such as females of Phylidorea (Phylidorea) heterogyna (Bergroth, 1913), or both sexes in Molophilus (Molophilus) ater (Meigen, 1804). In the latter two species, in contrast to R. (R.) incapax, the legs are correspondingly shorter and stouter. On the other hand, in Dicranomyia (Dicranomyia) reductissima (Alexander, 1952) [= lindrothi (Tjeder, 1963)] (Limoniidae), described from Tibet (as lindrothi from Alaska), a species micropterous in the male (female unknown), the legs are very slender and the antennae normally developed (Tjeder 1963). [Note: A form characterized as stenopterous in both sexes, interpreted as belonging to this species, was recorded from Mongolia and Tuva Region in Russia (Savchenko 1972).] Another example of long-legged brachypterous species may be Symplecta (Symplecta) holdgatei (Freeman, 1962) from the Gough Island in the Atlantic Ocean (Jones et al. 2003). This indicates that reduction of the wings and shortening of the antennae and legs may not necessarily be correlated and may be subject to different selective pressures.

Hackman (1964) proposed several categories of wing-reduced Diptera according to the climatic conditions of their habitats and modes of their lives. In addition to a few comparatively curious cases related to parasitism, unknown in tipulomorphans, wing reduction is mostly explained as adaptation to severe environmental conditions. These include, above all, low temperature inhibiting flying activity (see also Byers 1961, Mani 1962, Martinovský and Starý 1969) under arctic, alpine or nival conditions. For some insular species or those living on the seashore, strong winds may favour selection towards wing reduction as a factor generally hindering the flight, not explicitly preventing the insects from being blown out to sea, as sometimes suggested (cf. Huxley 1945 ex Hackman 1964). In Tipulidae, brachyptery of some females (Tipula, mostly subgenus Vestiplex) may be correlated with their deep-boring mode of oviposition, in relation to low temperature, or wind exposure, or both (Hemmingsen 1956).

Rhabdomastix (R.) incapax, however, occurs in the warm Mediterranean subregion, at moderate altitudes (465–508 m), and on inland sites sheltered from wind. Hence, wing reduction resulting from adaptation to the life in terricolous habitats, particularly among dense low vegetation, may be the case for this species (cf. Hackman 1964; as known in Molophilus ater). This could indicate that although the size of wings has limited dispersal ability, but the habitat closer to water might have helped the species to disperse by water carrying. Wing reduction in R. (R.) incapax may have been also supported by combination of high humidity and secretive life on sandy to muddy ground at bases of low graminoid plants, similar to what is observed for Crumomyia pedestris (Meigen, 1830) (Sphaeroceridae), inhabiting marshland habitats (Roháček 1975, 2012).

All European Rhabdomastix species, in fact their larvae, require flowing water, and the adults live under riparian conditions, being closely associated with sandy or gravely banks of streams and larger rivers, overgrown with scattered, low vegetation. Even some fully-winged species may be observed sitting or crawling about on plants or on the ground, but are rarely seen flying (Starý 2003). A few specimens of R. (R.) incapax were at first discovered by us by sweeping a low riverbank vegetation, but the best technique to collect this species is by crawling on all fours, pulling apart tufts of plants and then aspirating specimens directly into the pooter (Fig. 7). The adults of R. (R.) incapax were found in the growths of the spikerush (Eleocharis palustris) and a small rush (Juncus sp.). Patches of these plant species occurred on sandy to muddy banks of the Éleme river (Figs 6, 10). Among this low vegetation, the adults, mostly males, significantly outnumbering females, were moving rather quickly, and when stimulated by our disturbance, they climbed up onto plant stems, making sometimes short jumps from these down on the wet ground.

Rhabdomastix (R.) incapax was collected not only at the Éleme river, but one specimen (see Material examined) was also swept from the vegetation on another site, approximately 6.8 km south-east of the type locality, with a somewhat different habitat. This included a slowly flowing brook (Fig. 11) with marshy littoral vegetation, containing, among other plants, low growths of a small Juncus sp. This indicates that the species may be more widely distributed in Sardinia, and not restricted only to banks of larger rivers.