Toxomerus Macquart, 1855 belongs to Syrphinae (Syrphidae) and it is the only genus included in Tribe Toxomerini, with approximately 150 species described in the world. It occurs mainly in the Neotropical Region with 143 species, followed by the Neartic Region with 16 species (9 endemic) (Borges and Couri 2009).
The genus Toxomerus is characterized by : eye with distinct triangular emargination on posterior margin; gena narrow; arista bare; postpronotum bare; meron bare; anatergum bare; katatergum bare; posterior spiracle with short pile on margin; metasternum bare; metaepisternum pilose; metaepimeron bare; male genitalia well sclerotized, very short to long postanal process arising from fused surstylar apodemes and projecting caudad between bases of surstyli (Borges and Couri 2009).
Larvae of Toxomerus apegiensis feed on pollen of Olyra obliquifolia Steudel, a bambusoid grass (Poaceae) that grows in the understorey of tropical rainforests. T. apegiensis and T. politus (Say, 1823), which feeds on pollen of corn (Zea mays), are the only known phytophages within the otherwise predatory genus Toxomerus (Reemer and Rotheray 2009).
Toxomerus apegiensis (Harbach, 1974).
Harbach, R.E. (1974) A new Neotropical syrphid fly, Mesograpta apegiensis (Diptera: Syrphidae). Proceedings of the Entomological Society of Washington 76, 31-34.
Adapted from original description (Harbach 1974).
MALE.
Head: Face shining brownish black, pale yellow at sides with whitish pubescence; front shining brownish black, pale yellow at sides and above; gena shining brownish black; occiput appearing pollinose, white or greyish pilose below, pale yellow pilose at sides, golden yellow pilose above and behind vertex; posterior half of vertex, behind ocelli, shining deep bluish purple and anterior fourth, in front of ocelli, grey pubescent; antennae brownish yellow, scape and pedicel browner.
Thorax: Scutum shining with median metallic blue vitta margined on each side by 6 others: a dark brown to black vitta outside of which lies a golden coppery vitta, next 2 respectively the same followed by a metallic bluish-black vitta, and margin pale yellow and confluent with postpronotum and margin of scutellum of same color; disc of scutellum dark brown; pleura shining metallic black, yellow vitta on posterior half of anepisternum, yellow macula on dorsal portion of katepisternum. Legs pale yellow; metafemur with brownish-black subapical ring; metatibia with similar but broader ring subbasal; tarsi golden to brownish yellow. Wings brownish, stigma darker brown; microtrichia absent from basal portion of cell R1, narrow area along basal portion of posterior vein of cell R2+3, all but scattered areas along spurious vein and apical end of cell R, cell BM but wedge of microtrichia extending from apical end, narrow areas along anterior veins at apical ends of cells R4+5, DM and CuA1, anterior half of cell CuP, and minute area at basal end of anal lobe.
Abdomen: More or less parallel sided, dark brown to black markings on yellow to reddish-yellow background; first segment largely yellow, narrowly black distally; second segment consisting of 5 fasciae of nearly equal width, anterior fascia brownish yellow, posterior fascia brownish red, center fascia yellow and margined on either side by dark brown or black fasciae that produce a very thin linear vitta that divides center fascia; third segment with posterior brownish-red fascia which does not quite reach lateral margins of tire segment and an anterior yellow or reddish-yellow fascia which divides the central dark brown or black of the segment by producing a narrow median vitta, 2 submedial lunulate maculae, and triangular areas in the corners of the segment; segment 4 same as 3 except dark brown or black of segment may reach base of segment 3; fifth segment with medial dark brown or black diamond-shaped macula with crescentic maculae of same color on either side; hypopygium with dark brown macula on right side. Male genitalia: Superior lobes somewhat rectangular with small lobes projecting at disto-dorsal edge, rounded lamellae projecting from base with hairs, vestiture of erect bristles along ventral margin; cerci covered with dense short pile and scattered long hairs; surstyli compressed more dorsoventrally than laterally, somewhat quadrangular in dorsal view, covered with scattered hairs with long hairs concentrated on disto-medial margin of dorsal side; triangular process slender and narrowly rounded at tip, half as long as surstyli; ejaculatory hood prolonged, narrowed, and almost pointed distad; sustentacular apodeme abruptly enlarged basad, dorsal keel nearly straight from apex to base, ventral keel somewhat rounded; ejaculatory apodeme laminate, flattened laterally, apex slightly flared; ejaculatory sac bilobed, lobes projecting latero-dorsad and flattened, upper surfaces with a depression; penis sheath twice as large as epandrium in lateral view; cereal emargination roughly V-shaped in ventral view with one-half diameter of entire penis sheath, dorsal tip of V narrowly rounded.
FEMALE.
Thorax and abdomen as in male; head with black of vertex confluent with black of front and sides of front yellow.
It is related to Toxomerus croesus (Hull), with only slight differences in the abdominal pattern and coloration of the scutum and head (Harbach 1974).
Body length: 8 mm (Harbach 1974).
Pupae can be parasitized by a number of tiny Chalcidoidea (Hymenoptera) (Reemer and Rotheray 2009).
Host plant (from Reemer and Rotheray 2009).
Olyra obliquifolia Steudel (Poaceae) is a large, somewhat bamboo-like grass of 50– 100 cm high. This grass occurs from Suriname and French Guyana to Brazil (Pará and Maranhão districts) and is locally dominant in moist forest groundstorey at elevations up to 780 m. It flowers throughout the year, with a peak in July–October. The inflorescences are arranged on bisexual panicles, with the male spikelets on the basal parts and the female spikelets on the apical parts of the panicle. The inflorescences contain oils, which probably attract dispersing ants, and they are frequently visited by Trigona bees (Meliponini) (Görts-van Rijn 1990). In the study areas by Reemer and Rotheray (2009), the plant was only found on the plateaus, not on the surrounding slopes. It grew along forest trails in half-shady conditions.
Neotropical species described from Belem, Pará State, Brazil. Also known from Suriname.
Reemer and Rotheray (2009) reported larvae and adults of T. apegiensis collected in the Brownsberg nature reserve (04°56'N, 55°10'W) and the Nassau Mountains (04°48'N, 54°36'W), located in northeastern Suriname. Both areas are more or less isolated mountain plateaus at an altitude of approximately 500 m, surrounded by forested lowland areas. The plateaus are mainly covered with primary forest. Annual rainfall is 2250–2500 mm (Haverschmidt and Mees 1994). Additionally studied adult specimens of T. apegiensis are from Sipaliwini in the south of Suriname. Apart from the altitude (400–500 m), nothing is known about the collection site of these specimens (Reemer and Rotheray 2009).
Description of third stage larva (from Reemer and Rotheray 2009)
Length 6.5 mm; width approximately 2 mm (n51). Subcylindrical in cross-section, widest in the middle part. In lateral view with highest point in the middle (1.5 mm). Body colour yellowish white, somewhat translucent. After emptying gut contents the colour changed to pale green.
Antennomaxillary organs on fleshy projections and separated by more than their length with a groove between them (presumably acting as a guide for the retractile apex of the head skeleton); lateral lips fleshy, not sclerotized. Mandibles long and slender, projecting clearly beyond labrum in lateral view, extended anteriorly into the fleshy lobes and apparently used to operate these. Head skeleton with labrum and labium strongly sclerotized and with sharp apices, which are curved respectively dorsad and ventrad. Dorsal and ventral bridges translucent, not sclerotized. Posterior breathing tubes on a slightly projecting fleshy bar and not joined by sclerotization; spiracular plates oval, opening 1 less tall than basally wide, openings 2 and 3 separated from opening 1 and diverging slightly from inner ends; cuticular scar next to opening 1.
Description of puparium (from Reemer and Rotheray 2009).
Length 5.5 mm (n52). Convexly inflated anteriorly, dorsoventrally flattened posteriorly. Colour brownish, with irregular pattern of darker and paler maculae.
From Reemer and Rotheray (2009).
Adults of T. apegiensis were observed on 4 March, 5 March and 2 April 2006 on the plateau of the Brownsberg nature reserve, and on 24 April 2006 in the Nassau Mountains. Males and females were seen flying around stands of O. obliquifolia (Poaceae). Both males and females were feeding on the male inflorescences. Some males were hovering nearby at a height of 50–100 cm. Some females were observed ovipositing on the underside of the inflorescences. The adult flies were only found at sites where the host plant occurred in numbers, not in places with only solitary plants. No other species of Syrphidae were seen near these plants.
Larvae of T. apegiensis were found on 4 March 2006 in the Brownsberg nature reserve and on 24 April 2006 in the Nassau Mountains. Most of the larvae had first been noticed after taking the inflorescences of O. obliquifolia and placing them in plastic containers. Apparently the larvae had been hiding among the male spikelets, relying on their cryptic colouration. Later, presumably when food became scarce, they started crawling around in the boxes. Despite searching, no Homoptera or other invertebrates could be found on the inflorescences. Nevertheless, the larvae were growing. Although active feeding of the larvae was not observed with certainty, microscopic examination of their yellow gut contents revealed that the larvae fed on pollen. Unfortunately, the grass panicles started moulding within a few days, which was probably the reason why most of the larvae did not reach maturity.
Only two larvae made it to the pupal stage. A third instar larva found on 4 March pupated on 8 March but for unclear reasons never hatched. Another third instar larva was found on 24 April. This was kept in a plastic box with some panicles of O. obliquifolia. After 1 day it emptied its gut contents and its colour changed from whitish yellow to pale green. Perhaps this is a form of camouflage, because when its colour changed, the larva moved from the whitish yellow inflorescences to the underside of the green stem of the panicle, where it became inactive. After another day it had pupated.
unless otherwise stated