The author and date of Eristalis dimidiata should not have parenthese according to Nomina Insecta Nearctica and The BiSystematic Database of World Diptera, "BDWD Nomenclator: Details':
http://www.sel.barc.usda.gov:8080/FMPro?-db=names.fp5&-format=%2fdiptera%2fnames%2fdetailno.htm&-lay=www%20detail&-sortfield=unsorted&Name=dimidiatus&author=wiedemann&-max=10&-recid=12629822&-find=