Toxomerus politus (Say)
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).
Toxomerus politus has face 1/3 as long as eye width, yellow; frontal triangle yellow; proepimeron yellow; scutum black except broadly yellow laterally; scutellum yellow, black pilose; anepimeron anterior and dorsomedial yellow, anepimeron posterior black. Femora and tibiae yellow. Wing extensively microtrichose; alula broader than cell BM appically, microtrichose. Abdomen with 3rd and 4th terga yellow on basal 2/3, with narrow black subbasal fascia connected to medial vitta to form L-shaped macula not touching the lateral margin. Surstylus almost all with the same width; postanal process 1/3 as long as surstylus; aedeagus enlarged, proeminent dorsally and ventrally; superior lobe somewhat rectangular, acuminated dorsally, with hairs in distad anteroventral margin and scattered hairs, with small lobe ventrally with hairs. Female genitalia: 9th tergum with somewhat trapezoid black macula concave posteriorly; epiproct slender; cercus slender; hypoproct as a triangular arch; two elliptical spermatecae similar in size (Borges and Couri 2009).
From Metz and Thompson (2001).
MALE.
Head: Face yellow, white pollinose and white pilose laterally, bare and shiny medially; gena yellow; frontal lunule yellow, with reddish-brown macula medially; frontal triangle yellow, white pilose; eyes narrowly holoptic anteriorly; vertical triangle black, bronze pollinose anterior to ocellar triangle, except shiny black posterior to ocellar triangle, black pilose; occiput black except narrowly yellow ventrally, silver pollinose, white pilose except dorsal 1/4 black pilose anteriorly; scape and pedicel yellow; basoflagellomere orange, yellow pilose; labium black; labellum yellow.
Thorax: Generally black, yellow as follows: postpronotum, scutum laterally and continuously from postpronotum to scutellum, dorsad of procoxa (i.e., supraprocoxal macula), posterior half of posterior anepistemum, dorsolateral macula on katepistemum, anterodorsal macula on anepimeron, ventrolateral macula on metasternum, metapleuron; silver-gray pollinose; white pilose ventrally and laterally except posterior anepisternumyellow pilose, yellow pilose dorsally. Scutum greenish-bronze pollinose with narrow medial shiny vitta and sublateral shiny vittae; scutellum brown with yellow margin, black pilose; subscutellar fringe white; halter yellow; calypter yellow except brown on dorsal margin; plumula yellow. Legs: pro- and mesocoxae black except yellow on apical 1/4, metacoxa yellow, yellow pilose; femora yellow, yellow pilose except meso- and metafemora apical 1/3 black pilose; tibiae yellow, yellow pilose except metatibia black pilose; tarsi yellow except distal metatarsi black dorsally, yellow pilose except distal metatarsi black pilose dorsally. Wing: Hyaline except for dark stigma, extensively microtrichose, bare as figured.
Abdomen: Sterna yellow, yellow pilose; terga mostly black pilose except yellow pilose laterally on first and basal 1/4 of second and with intermixed yellow pile; tergum 1 black except yellow anteriorly and laterally; tergum 2 black with a yellow, medial fascia; terga 3 and 4 yellow with sub-medial black vittae continuous to apical border and narrow, black sub-medial fascia discontinuous medially and black apical fascia; tergum 5 yellow, brown apically with medial black vittate macula which may be divided into submedial vittae and sub-lateral black fascia which are connected to medial or submedial vitta; male genitalia as figured.
FEMALE.
Similar to male except normal sexual dimorphism and face brown medially; frontal lunule completely yellow; frons broadly black medially except yellow maculae above antennae, black pilose anterolaterally; vertex completely bronze pollinose. Scape and pedicel with black pile; basoflagellomere brown. All metatarsi brown-black dorsally, black pilose dorsally.
GenBank accession number for this species are: protein-coding COI gene (EU241755), rRNA 28S gene (EU241803) and 18S gene (EU241863).
Toxomerus politus is readily recognized by its abdominal pattern with the narrow submedial vittae bent into and continued as elongate fasciate maculae subbasally. The only other species with such a pattern is aeolus Hull, 1942 (Mexico, Costa Rica and Cuba). Toxomerus aeolus is readily separated from politus by its entirely black anepimeron, partially yellow in politus. Both these species are typically less than 9 mm.
Length: body, 8.1-9.5 (8.8); wing, 6.3-7.3 (6.8) mm.
Toxomerus is the single genus of the tribe Toxomerini. Recently, Mengual et al. (2008) included several species of Toxomerus in their phylogenetic analysis and Toxomerini was resolved as a monophyletic group within the clade of the genus Ocyptamus.
The subfamily Syrphinae constitutes the largest predatory radiation inside the family, with approximately 1,800 described species. Syrphine larvae are predacious feeding frequently on soft-bodied Hemiptera, but also on the larvae of Neuroptera, Lepidoptera, Coleoptera, other Diptera, Acari, and Thysanoptera (for a review see Rojo et al. 2003). However, some Neotropical species are secondarily phytophagous. The larvae of Toxomerus politus (Say, 1823) also known as “the corn-feeding syrphid fly”, feed on pollen and sap from the saccharine cells of corn (Zea mays L.) (Marín A.1969; Smith 1974) and immature stages of Toxomerus apegiensis (Harbach, 1974) feed on pollen of a bambusoid grass (Reemer and Rotheray 2009).
Threre are doubtful records of larvae of Toxomerus politus feeding on aphids [Fluke (1929) and Wildermuth & Walker (1932)].
Widespread species known from Southern Canada to Argentina, including West Indies.
Description of the larva (from Heiss 1938).
The mature larva is 9-12 mm long, 2 mm wide, and 2 mm high. It is almost cylindrical in the mid-region but tapers cephalad and ends caudally in a blunt, flattened, emarginate anal segment. Each true segment is divided into 4 or 5 subequal annulations, which gives the maggot apparently 40 regular earthwormlike rings. The integument is finely papillose and so translucent that many internal structures are readily visible through it.
The color of the larva varies with different ages and individuals. All first instar larvae are clear pollen-yellow without markings, except where the visceral contents show as a brownish mass. Second instar larvae are translucent yellow with two bright rose (often purple) stripes, immediately on either side of the dorsal blood vessel, which pulsate with the contractions of the heart. The third instar larvae are colorless, pale straw-yellow or translucent pale-green with much softer rose or rosy-tan dorsal stripes or, more rarely, dorsal stripes completely lacking.
The anterior spiracles are small brown knobs on the second visible anterior segment. From these the main tracheal trunks run back half way down the sides as slightly undulating silver tubes, clearly seen through the integument, and end in the posterior respiratory processes. In the anterior third, under the rose stripes, is a large, shining white fat-body. The posterior respiratory process is sessile or nearly so, and it is unique among the syrphid larvae which the writer has examined in having the spiracular plates completely separated from one another. They appear as two light brown spots on the upper surface of the flattened, emarginate anal segment. The arrangement of the spiracles is striking; the upper spiracle is only half as long as the others and isolated by more than twice its length from the other two. The latter are crowded down to the ventral edge but not reduced in size. Rectal gills appear to be absent.
Puparium. The puparium is 7-8 mm. long, 2.4 mm. wide at the widest point, and 2 mm. high. It is club-shaped, rounded anteriorly, and tapers gradually to the anal segment, which is broader and flatter than that of the larva. The regular annulations of the larva have disappeared, and a carina is present immediately anterior to the posterior spiracles. The puparium is at first lucid yellow or greenish and retains the rose-colored stripes of the larva; it then changes to a clear opaque tan, variously pencilled with dark lines, which have no relation, apparently, to the color markings of the larva. When fully marked there is: (1) a median dorsal longitudinal line which may extend from the apex of the puparium to the posterior respiratory process, (2) a well defined longitudinal lateral line halfway down each side for about three-fourths the total body length, (3) a broken, undulating, sub-dorsal line on each side, midway between the last two, and (4) a sub-lateral, broken line on each side below the lateral line. The presence of the lateral lines alone is the commonest condition, but all may be absent. The posterior processes differ from those of the larva in that they are more or less united by a brownish hardening of the integument between them, and the spiracles are heavily ringed with black.
Description of third stage larva (from Reemer and Rotheray 2009).
Length 7–9 mm; width 1.5–2.5 mm. Subcyclindrical in cross-section; tapered anteriorly, truncate posteriorly. Body colour pale yellowish green. Antennomaxillary organs on fleshy projections and separated by more than their length with a groove between them acting as a guide for the retractile apex of the head skeleton; lateral lips fleshy, not sclerotized. Mandibles slender, not projecting beyond labrum in lateral view. Head skeleton with apex of labrum projecting anteriorly more than labium and more heavily sclerotized; dorsal and ventral bridges transluscent, not sclerotized.
Ventral surface of prothorax with stippled rows dorsally and laterally, vestiture, i.e. microsculpturing of the integument, consisting of 0.015 mm tall domes, as tall as they were basally broad and separated by about twice the basal width; ventral surface mostly matt with some shiny areas; anterior spiracles tapering, about 0.04 mm tall and as tall as broad, flat tipped with two spiracular openings.
Sensilla 9–11 aligned on abdominal locomotory prominences; segmental sensilla on slight fleshy projections with a single accompanying seta, basal projection about 0.03 mm tall and same length as seta; apex of anal segment with a pair of slight fleshy projections with sensilla. Posterior breathing tubes on a slightly projecting fleshy bar and not joined by sclerotization; spiracular plates rounded, opening 1 about as tall as basally wide, openings 2 and 3 separated from opening 1 and diverging slightly from inner ends; cuticular scar next to opening 1.
The first instar larvae were found in quiescent masses of a hundred or more lying closely packed and parallel to one another under the upper leaf sheaths of sweet corn. On being disturbed, they become very active, moving rapidly about with a looping motion. When at rest, they seek the moist situations about the plant, probably because of their delicate skins. At this stage they are engorged with yellow pollen and are rich yellow. In the second instar they are exactly the same size and of much the same shape as a single corn anther; the groove of the anther is mimicked by the double dorsal stripe. Thus, when seen out on the leaf, lying quiet, they resemble a fallen anther among the pollen on the leaf surface. They are scattered farther down the stalk than the first instar and probably feed on juices of the plant as well as pollen.
Adult larvae were observed later beneath every leaf sheath from the top to the base of the corn stalk, in small groups of three or four, scraping away industriously at the succulent tissues of the leaf sheath and leaving wet patches of broken cells. That they also feed on pollen is shown by golden yellow, or brownish, visceral contents, but some show only clear translucent bodies, indicating that they feed on exuding sap alone. The mature larvae, with their soft green or straw-colored bodies and purplish dorsal lines, mimic the purple-veined glumes of the corn tassel. Some are found here as well as under the leaf sheaths and on the blades. Their habit ofpupariating head downward among the florets of the tassel and their shape and coloration makes them almost indistinguishable from the true florets. Under the leaf sheaths and in the crevices of the ear husks they also pupariate head downward, often in such pools of sap and dew that only the posterior spiracles, now uppermost, protrude from the liquid (Heiss 1938).