Rhopalosyrphus is defined (synapomorphy) by its 1) abdominal structure and 2) pilose meropleuron. Other diagnostic characters are 3) antenna elongate, longer than face, usually about twice as long; 4) scape and basoflagellomere elongate; 5) face produced ventrally; 6) occiput greatly developed on dorsal 1/3; 7) metasternum developed, not reduced; and 8) metatibia flared apically (Weems et al. 2003).
Rhopalosyrphus guentherii has 3rd tergum elongate, as long as the 2nd tergum; 2nd tergum not greatly elongate posteriorly; eye without enlarged ommatidia; alula bare basomedially; cell R completely bare behind spurious vein; metasternum with long, distinct pile, not reduced; and face and anepisternum entirely white pilose. R. ramulorum Weems & Deyrup differs by having alula completely microtrichose; cell R extensively microtrichose, bare only on basoposterior 1/4 or less; metasternum appearing bare, with pile greatly reduced; and face and anepisternum partially black pilose (Weems et al. 2003).
From original description (Lynch-Arribalzaga 1891) in Latin.
Nigro-fusca, opaca, subtiliter crebreque punctata, tenuiter albido-sericeo-villosa. Antennae articulo primo nitidulo, basi rufo-piceo, apice nigro-piceo, reliquis duobus nigro-fuscis, opacis, griseo-pruinosis, chaeta nigra. Alae hyalinae, iridescentes, dimidio antico longitrorsum fusco, cellula basali secunda disco vix infuscata at obscurius fuscano-limbata. Pedes antici et medii castanei s. fuscano-ferruginei, tarsi autem fusco-picei ubique albido-sericeo-villosi, remora postica picea basi apiceque ferruginea, tibiae posticae basi late diluteque flavidae apice fusco-piceae, tarsi postiei fusci infra flavido-rufesco-tomentosi. Abdomen supril nigro-fuscum, densins (praesertim ad apicem) liavido-sericeo-villosum, segmento tertio basi antrorsum tubulosa utrinque obsolete ferruginea, segmentis apicalibus obtectis ventreque piceis.
From the redescription (Weems et al. 2003).
Head: brownish black, yellowish white pilose; occiput grayish white pollinose on ventral 2/3, shiny dorsally; eye with ommatidia of more or less equal size; antenna brownish black except orange basal 1/3 of scape, about twice as long as face; antennal ratio 5: 1: 8.
Thorax: brownish black; pleuron silvery-white pilose; scutum brown pilose medially, silvery white pilose anteriorly, along transverse suture, laterally and posteriorly; scutellum silvery white pilose; calypter white, with brown margin and fringe; halter orange; wing brown anteriorly, hyaline posteriorly, microtrichose except bare cell R & BM, anterobasal 1/2 of cell CuP and basomedially on alula; legs reddish brown except yellow basal 1/2 of metafemur; pro- and mesofemora black pilose anteriorly, yellow pilose posteriorly; pro- and mesotibia yellow pilose; tarsi black pilose; metafemur black pilose with a few yellow pili intermixed; metatibia yellow pilose basally, black pilose apically.
Abdomen: brownish black except yellow basal 1/3 of 3rd tergum and reddish apically on 2nd tergum; 1st tergum white pilose; 2nd tergum constricted on apical 1/3, yellowish white pilose; 3rd tergum about twice as broad apically as basally, yellow pilose; 4th tergum brown pilose basomedial 2/3, yellow-white pilose apically, about as long as 2nd tergum; 5th tergum yellowish-white pilose.
The species looks like the genus Tripoxylon (Hymenoptera, Crabronidae).
Body length: 12 mm. Wing length: 8.8 mm (male) - 10.5 mm (female) (Weems et al. 2003).
Rhopalosyrphus belongs to the subfamily Microdontinae and is the sister group of Ceriomicrodon Hull, together these taxa are the sister group of Microdon Meigen, sensu lato. Rhopalosyrphus shares with the Microdon clade the bifid aedeagus and appears closely related to Ceriomicrodon. The very distinctive shape of the puparium of Rhopalosyrphus separates it from that of Microdon: it has a curved ventral surface that is broader than the narrow, flat dorsal surface (Weems et al. 2003).
A wide ranging species; R. guentherii is found from southwestern United States to northern Argentina (Weems et al. 2003).