Scaeva pyrastri (Linnaeus)
Scaeva pyrastri (Linnaeus, 1758).
Linnaeus, C. (1758) Systema naturae... Ed. 10, Vol. 1. 824 pp. L. Salvii, Holmiae [= Stockholm].
Vockeroth (1969) provided an excellent diagnosis of this genus.
Medium to large, moderately slender to robust species with very clear wings, oblique and often lunulate abdominal maculae, and usually with strongly swollen frons in male. Eye densely haired although as much as posterior half sometimes almost bare in female. Dorsal 2/3 of eye of male with an extensive area of almost uniformly enlarged facets, this area anteriorly reaching the margin of the eye above and progressively farther from it below, posteriorly narrowly separated from margin throughout; facets much less enlarged in occidentalis Shannon than in other species. In all species but occidentalis front of male greatly broadened, strongly swollen, and with abundant erect black hairs, eyes meeting anteriorly at an angle of 135°-145°. In occidentalis front of male only slightly broadened and swollen but with moderately conspicuous black hair, eyes meeting anteriorly at an angle of 120°. Front of female only slightly swollen but very broad, at vertex from 0.28 to 0.32 head width. Face in both sexes moderately to strongly swollen, sometimes more strongly produced below, with moderately large and sometimes slightly compressed tubercle; face at its widest point from 0.50 to 0.66 times as broad as head. Face bright to pale yellow, usually with narrow brown to black median vitta which always ends just above the tubercle. Basoflagelloemere from 1.4 to 1.9 times as long as broad. Scutum shining black, always pale pilose, usually very obscurely yellowish laterally, rarely with well-defined, entire, pale yellow, lateral margin. Scutellum with the disc or rarely the entire surface translucent, dull yellow to dark yellow-brown usually with a narrow to broad posterior margin opaque and slightly paler. Pleura black, densely pale pilose, subshining to moderately pollinose, sometimes with posterodorsal corner of anepisternum distinctly yellowish. Dorsal and ventral katepisternal pile patches narrowly to broadly joined posteriorly and rarely narrowly joined anteriorly, otherwise narrowly separated; the ventral patch extending much farther dorsad than usual, anteriorly extending broadly anterodorsad to dorsoanterior corner of katepisternum. Metasternum bare. Vein R4+5 usually very broadly and shallowly dipped into cell R4+5 more rarely nearly straight. Microtrichia greatly reduced, absent from at least basal half of wing and usually from almost entire wing; usually at least a few short, weak, erect, scattered microtrichia present near posterior wing margin, rarely such microtrichia sparsely present on outer half of wing. Abdomen narrowly to broadly oval, flattened, strongly margined from just beyond base of tergite 2 to apex of tergite 5. Tergites 2 to 5 each with a pair of slightly to strongly oblique, narrow to broad, whitish-yellow to bright yellow maculae which usually are clearly separated from the lateral margin; maculae of tergites 3 to 4 at least slightly, and usually strongly, lunulate. Sternites very variable in colour; usually yellow, with or without transverse black median bands or longitudinal lateral and median stripes, rarely black with broad yellowish-white incisures. In melanostoma (Macq.) only, the yellow maculae of the tergites, and the entire surface of the sternites, when viewed obliquely from in front, show transverse striae of very fine pale pollen (Vockeroth 1969).
Adapted from Dusek and Laska (1985).
Head: Eye densely hairy, with distinct area of suddenly enlarged facets in upper part. Anterior angle of approximation of eyes wide, 128°-147°. Frons inflated, face broad occupying 55-61% of head width. Face not produced in profile.
Thorax: Scutum black with lateral margins at most vaguely brownish or yellowish. Wing membrane almost without microtrichia. Vein R4+5 strongly dipped, point of meeting of vein R2+3 and margin of wing situated distinctly more apically than point of meeting of vein R4+5 and vein M1. Wing membrane beyond marginal cross-veins rather regularly lengthwise undulated and relatively broad. Ratio of width of membrane beyond vein M1 to length of this vein 1:2.2-3,2. Profemur with pale and black pile. Metatibia usually with less distinct dark macula in middle than in S. selenitica.
Abdomen: White or yellow maculae on terga 3 and 4 oblique with their inner end situated much more closely to base of tergite than outer end. Anterior margin of these macuale distinctly concave and maculae not reaching side margin of terga. Male terminalia: Epandrium broader than high (width 0.65-0.75 mm, height 0.55-0.60 mm). Cerci remarkably longer than bluntly ended short paralobi. Hypandrium in front view rather angular, only slightly higher than broad (width about 0.50 mm, height 0,49-0.53 mm). In lateral view slender and pointed lingula and group of pointed nodules on posterior side of hypandrieum apparent. Base of aedeagus rather slender and long terminated by two approximated teeth of similar size. Surstyli with one rather small tooth. Tubus of aedeagus only moderately funnel-like broadened at end with not very distinct spines on lowerside of opening.
Face broad occupying 51-60% of head width, vertex occupying 27-33% of head width. Exceptionally whole abdomen black (this form was described by Curtis as S. unicolor). The cause of this melanism is unknown but is probably not temperature related during development.
Geographic variation. In the boundary area between the Palaearctic and Oriental regions the form occurs which has the body generally longer haired, more black hairs present in the lower half of face and larger black maculae on the sterna than the typical form. Maculae on terga 3 and 4 strikingly narrowed in middle, face usually broader.
Body length: 10-16 mm (Dusek and Laska 1985).
Ecology and Distribution
Very widespread species ranging from Fennoscandia south to Iberia, the Mediterranean, Canary Isles and North of Africa; from Ireland east through much of Europe and Asia Minor into European Russia; through Siberia from the Urals to the Pacific coast (Kuril Isles); India; China. In North America from Alaska to Alberta, south to California and New Mexico. This is an extremely migratory species, and occasional records from offshore islands of northern Europe, such as the Faroes (Jensen 2001) are almost certainly derived from seasonal immigration, rather than a resident population (Speight 2010).
Adults inhabitat clearings, tracksides, hedgerows, gardens etc. etc.; fast flying, usually within 3 m of the ground; frequently encountered flying round bushes and shrubs, in a slow and purposeful manner, only to speed away after 30 seconds or so (Speight 2010).
Flowers visited by adults: Umbelliferae; Calluna, Campanula rapunculoides, Cirsium, Convolvulus, Eschscholzia californica, Euphorbia, Hamamelis, Leontodon, Ligustrum, Lycium chinense, Parnassia, Pulicaria disenterica, Rubus fruticosus, R. idaeus, Senecio, Solidago virgaurea, Tripleurospermum inodorum, Ulmus (see extended list in de Buck, 1990) (Speight 2010).
Larave of S. pyrastri have been reported feeding on Adelgidae, Aphididae, Coccidae, Psyllidae and Thysanoptera (Rojo et al. 2003). Bloomfield (1900) reported them feeding on caterpillars of Plusia gamma (Lepidoptera), but Rojo et al. (2003) questioned its validity.
Flight period of adults is from February to November in most of continental Europe. Females are known to overwinter in central Europe, but there is little evidence for this in Atlantic parts of Western Europe, where the annual occurrence of S.pyrastri may well be totally dependent upon annual immigration from elsewhere and records of the adult insect are almost entirely confined to the period June/September (Speight 2010).
This species is to a significant extent anthropophilic, occurring in arable crops, hedgerows, orchards, gardens and conifer plantations (Speight 2010).
Larva of S. pyrastri has been described and figured by various authors, e.g. Bhatia (1939), who also describes the egg. Diagnostic features of the last instar larva and puparium are given by Laska et al (2006), who also provide a key separating both larvae and puparia of this species from those of the other Scaeva species known from Europe (Speight 2010).
Larva (from Dixon 1960).
Length 18-19 mm., width 4 mm., height 3-5 mm.; light green with a white median line of adipose tissue superimposed upon a broad median dark brown band, the white line interrupted by five transverse rows of orange-brown adipose tissue between dorsal segmental hairs on abdominal segments, aggregations of white adipose tissue forming flecks especially concentrated in dorsolateral prominences; subcylindrical, tapering anteriorly, cuspidate posteriorly; serrate in outline; ventral sole with seven segmentally arranged pairs of two prominences; integumentel vestiture of dense dark brown spinules; segmental ornamentation of long hairs mounted on individual papillae on slight prominences in ten rows of 12, 18, 16, 12, 12, 12, 12, 12, 12, and 2 respectively. Posterior respiratory process: recessed in a triangular depression where integumental vestiture is finer and denser; twice as broad as long; constricted at the base; median groove slightly less than half as deep as length of respiratory process; circular plates elongate oval, displaced towards the median groove and anteriorly; dorsal spurs prominent, pointed, and contiguous with inner and part of posterior border of circular plates; interspiracular ornamentation of small nodules; spiracles straight, equidistant and on slightly raised carinae; spiracles III parallel to median groove; spiracles with an inner border of hairs.
Larva (from Laska et al. 2006).
Previously described by Martelli (1911), Jones (1922), Krüger (1926), Fluke (1929), Heiss (1938), Bhatia (1939), Scott (1939), Brauns (1953), Láska (1959), Dušek & Láska (1959), Dixon (1960), Goeldlin de Tiefenau (1974). Sharma & Bhalla (1988) dealt with larval biology on laboratory breeding but did not describe immature stages. Barkemeyer (1994) provides a comprehensive literature review of what is known on the biology of this species. Larvae are predatory on a wide range of aphids and also Coccidae, Psyllidae and Thysanoptera; prey records listed by Rojo et al. (2003, p. 192–202).
Diagnostic characters. The last fold anteriorly on posterior respiratory process (PRP) rounded, without conical fleshy projection at median point. Segmental spines well developed, about 0.15–0.25 mm long similar as in the other Scaeva species. Integument on dorsum covered with distinct microtrichia about 0.02–0.035 mm long, of cuticle colour, some of them furcate around PRP. Posterior folds coated with microtrichia with a wide base that becomes progressively fine to apex. Puparium usually without brown cuticle segmental patterns and without dried fleshy projections under segmental spines. PRP: (width: 0.5–0.59 mm, height: 0.28–0.36; n = 10). Almost sessile, lustrous, sclerotized and pale brown in colour. Ecdysial scars slightly anterior to centre of each spiracular plate. Orificia I and II aligned or with angle between them a little less than 180°; orificia II and III almost parallel.
Evolution and Systematics
Nomenclature and Synonymy
Laska et al. (2006) were the last authors to study in deep the taxonomy of this genus. They concluded to divide Scaeva into two subgenera, Semiscaeva and Scaeva, based on morphological characters of the immature stages. One group (S. dignota, S. selenitica, S. mecogramma) has a distinct angle of about 90° between orificia II and III, as is typical for other related genera of Syrphini. A second group (S. pyrastri, S. albomaculata, S. latimaculata) has the orificia II and III (on spiracular plate) parallel, with insertion of orificium III beneath the level of insertion of orificium II (probably apomorphic situation).
Due to the impossibility to assign any different species with unknown larva to either group, I do not find the subgenera helpful for the taxonomy of the genus Scaeva, and at this moment both groups can be referred as species groups.
Musca pyrastri Linnaeus, 1758: 594.
Scaeva affinis Say, 1823: 93.
Scaeva unicolor Curtis, 1834: 509.
Musca mellina Harris, 1780: 30.
Musca rosae De Geer, 1776: 108.
Syrphus flavoscutellatus Girschner, 1884: 197.
Scaeva corrusca Gravenhorst, 1807: 375.
Dusek and Laska (1967) placed Scaeva, in their scheme, as sister group of Scaevosyrphus [=Eupeodes (Metasyrphus)], Lapposyrphus, Posthosyrphus [=Eupeodes (Metasyrphus)] and Metasyrphus [=Eupeodes (Metasyrphus)]; in their tribe Syrphini.
Rotheray and Gilbert (1989) recovered Scaeva as sister group of Eupodes, and Eriozona as sister group of these two genera. In 1999, Scaeva was placed in a tritomy with Eupeodes and Ischiodon (Rotheray and Gilbert 1999).
Mengual et al. (2008) using molecular characters recovered Scaeva as monophyletic and sister group of Ischiodon (although they used the synonymy with Simosyrphus suggested by Laska et al. in 2006). Pseudodoros (Dioprosopa) was resolved as their sister group.