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).
Rhopalosyrphus was described by Giglio-Tos for an unnamed species. Later Giglio-Tos (1892: 2) declared that his unnamed species was the same as one earlier described by Lynch Arribalzaga from Argentina. Rhopalosyrphus is a group of Microdon flies with a distinctive petiolate abdomen and pilose meropleuron.
Synonyms:
Holmbergia güntherii Lynch Arribálzaga 1891: 198.
Rhopalosyrphus carolae Capelle 1956: 174.
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.
Rhopalosyrphus guentherii (Lynch Arribalzaga, 1891).
Lynch-Arribalzaga, F. (1891-2) Dipterologia Argentina, Syrphidae. Anales de la Sociedad Cientifica Argentina 32, 80-99, 118-131, 194-202, 247-256, 307-314; 33, 51-58, 111-121, 189-199, 236-253; 34, 33-46, 173-192, 242-280.
unless otherwise stated