Halictus simplex Bluthgen 1923

Halictus simplex Blüthgen, 1923 Conservation status EN Distribution This species has a distribution range extending from Spain to Kazakhstan, with occurrences decreasing in northern Europe (Pauly et al. 2016). In Belgium, H. simplex was recorded across most provinces of Wallonia during the last century. Recently, however, observations of this species have been largely restricted to the Ardennes (Pauly 2017). Comparative studies estimate a substantial decline in its area of occupancy, ranging from 50 % to 80 %, between the periods 1900–1969 and 1970–2017 (Drossart 2019). This decline is likely due to habitat destruction, highlighting the need for restoration efforts. To support H. simplex, restoration of thermophilic, nutrient-poor grasslands should be prioritised, with measures promoting extensive land-management practices. Notes This species inhabits calcareous grasslands and a broader range of thermophilic environments with sparse vegetation, including sand and gravel pits (Westrich 2018). Nests are constructed on horizontal surfaces and feature a short, above-ground tube structure (Pesenko et al. 2000). While three specimens were collected in quarries, the other two were found in wet grasslands crossed by a river with a gravel-based riverbed, likely providing favourable nesting sites. The species might be primitively eusocial, but this trait has to be confirmed (Pesenko et al. 2000). The female is polylectic with a preference for species within the families Asteraceae (Cichorium and Centaurea) and Dipsacaceae (Knautia, Succisa, Scabiosa) (Pauly and Vereecken 2018, Westrich 2018). While three of the specimens collected were indeed foraging on Asteraceae plants, one of them was sampled on Digitalis purpurea (Plantaginaceae). Females emerge from diapause in April, while males fly from June onwards (Westrich 2018). Diagnosis A medium-sized Halictus (10-11 mm) with dark legs and a densely- punctuated scutum (Fig. 12). Females cannot be distinguished from the closely related Halictus compressus and H. langobardicus, based on morphology. The lateral faces of the propodeum are conspicuously punctuated. The apical strips of hairs on tergites 1 and 2 are interrupted medially. Males feature concave genae and unmodified, thin mandibles (Pauly 2015 a).