Eric Fisher's comment:
Tolmerus is for three small-sized "Machimus" spp. in the US: the eastern M. maneei (Hine), along with a likely synonym M. hubbelli (Bromley), and the western M. vescus (Hine). Tolmerus is widely used in Europe, and has been previously used in the Nearctic also. The generic status of these 'Asilini' groups is very confused -- especially in the Nearctic but also worldwide; they are badly in need of phylogenetic study and revision. The story involving the genera of Asilinae related to Asilus is very complex but (greatly simplified) begins with the influential work of Loew in the mid 1800's. He proposed a then quite reasonable method for dividing-up the Asilus' of Europe into a number of genera, which also included Machimus, Tolmerus, Cerdistus, Epitriptus, etc. Hine (early 1900s) incorporated these European genera into the Nearctic fauna -- substitutions for the 'dumping ground' of species previously listed as Asilus -- but the fit wasn't always very tight. No one has really made sense of these groups since -- though Hull (worldwide, 1962) and Martin (w. NA, 1975) tried. Lehr, the late Russian asilid specialist, and expert on Asilinae, believed that only several species comprised Asilus in the strict sense; however, the great problem of how to correctly partition the huge mess of "species formerly known as Asilus" remains.
Perhaps the simplest solution is to treat Tolmerus (in the U.S.) as a subgroup of Machimus.