Delia pruinosa

Michelsen, Verner, 2012, Revision of the European Delia pruinosa species group (Diptera: Anthomyiidae) feeding as larvae in seed capsules of Silene L. (Caryophyllaceae), Zootaxa 3434, pp. 31-48 : 32-34

The Delia pruinosa species group

Diagnosis. The structure of the mouth parts ( Figs. 1–4 ) and antenna ( Fig. 5 ) may distinguish both males and females of the Delia pruinosa species group from all other Delia : proboscis with a large and bulky haustellum covered in grey dusting, and with black prestomal teeth in a single row notably strong and claw-like; antennal postpedicel short, apically rounded; arista also short, conspicuously thickened basally. Supplementary characters: fronto-parafacial angle projected beyond short face; genal setae in single row; prealar seta short, sometimes barely discernible from adjacent setulae; vein C without v-setulae; fore femur without a-setae; mid tibia without av- and v-setae; male sex: apical pv-seta on fore tibia simply pointed and fore- and mid tibiae with submedian setation very reduced, only consisting of 1–2 short p-setae.

Description. Male and female. Medium size anthomyiids, wing length 4.0–5.6mm.

Colour: Ground colour brownish black, except anterior parts of head and basal antennal segments often ochre brown to ochre yellow. All parts including prementum covered in light grey to brownish grey dusting; male mesonotum more or less covered in brown dusting; male abdomen in posterior view with a narrow dark vitta on tergites II–IV, but without distinct dark bands anteriorly on tergites; female abdomen without such dark pattern. Calypteres and basal part of wing more or less tinged light brown in males and ochre yellow in females.

Head ( Figs. 1–5 ): In profile with convex frons and rounded fronto-parafacial angle projecting distinctly in front of short face; male parafrontals on upper part narrow, contiguous to well separated by frontal vitta; parafacial narrower than width of postpedicel; gena short and broad with setae arranged in one irregular row. Antennal postpedicel short, not reaching lower facial margin; arista short and strongly thickened basally; aristal pubescence short, its length not exceeding greatest diameter of arista. Haustellum large and bulky, fully as long as fore tarsomere 1; prestomal teeth black, enlarged and robust, pointed and claw-like, arranged in a single row.

Thorax: Presutural acrostichal rows closer together than their distance to dorsocentral rows, middle pair of acrostichal setae moderately enlarged. Notopleuron bare apart from two usual setae. Prealar seta shorter and weaker than posterior notopleural seta, sometimes practically absent.

Wings: Vein C with anterior spinules very short, even at subcostal break, without setulae on d-, a- and vsurfaces.

Legs: Fore femur without series of fine a-setae; fore tibia with 0–1 ad- (present in female, absent in male) and 1–2 p-setae, apically only with d-seta and short pv-seta, the latter simply pointed in both sexes. Mid femur basally with 0–1 av-seta (present in female, normally absent in male), and 1–3 pv-setae (besides basal v-seta), all very short; mid tibia in male with 1–2 very short p-setae only, in female even with 1 ad- and 1 short pd-setae; male mid tarsus unremarkable. Hind femur with av-setae but these become very short and irregular in basal half, without pvsetae except near apex; hind tibia with 0-1 short av-, 2–3 ad- and 2–3 pd-setae and without apical pd-seta; male hind tibia without usual short p–pv setae in middle.

Abdomen: Tergites II–V in both sexes with weak hind marginal setae and without discal setae; male sternites III and IV rather broad with longish setae at lateral margins or basally on sternite III. Male terminalia typical for Delia , as in other members of the D. cardui section without specialized setae on lobes of sternite V ( Fig. 9 ) and with acrophallus ( Figs. 30, 31 ) retained as a small sclerite disc between paraphallic lobes. Oviscapt more variable than usually seen in species of Delia , ranging from rather short ( Fig. 15 ) to strikingly extended ( Fig. 43 ); cerci with freely projecting apical part shorter than normally seen in Delia .

Egg. Description based on uterine eggs from pregnant female of D. judicariae: Whitish , relatively slender (c. 1.5 x 0.3 mm), cylindrical, long axis barely curved in lateral view, rounded at posterior end, tapering and somewhat truncate at anterior (micropylar) end. Chorion ornamented with fine ridges forming hexagonal reticulations, dorsally without a hatching pleat delimited by raised ridges.

Biology. The first published biological observation on the Delia pruinosa species group was by Tiensuu (1936), who from 22–27 June 1934 found ‘ Hylemyia (Delia) flavidipennis ’ in numbers after sunset, allegedly pollen-feeding on flowers of Silene viscaria (L.) Jess. My examination of Tiensuu’s material has shown that his observation involved two species, Delia pruinosa and D. judicariae . It remains questionable, however, if the flies observed by Tiensuu were actually feeding on pollen rather than nectar, as I have never noticed pollen grains as gut contents when dissecting male and female flies for identification purposes. It is also stated in the literature (see below), that these flies play no role as pollinators of their host plants.

In Britain, Niblett (1951, 1955) bred ‘ Delia flavidipennis Stein’ from larvae found in seed capsules of Silene vulgaris (Moench) Garcke after they had overwintered in the pupal stage in soil. These breeding records refer to D. pruinosa as the only British species known from this species group (see below). Hennig (1974: 811, 1976: 958) also noted the association of ‘ Delia flavifrons ’ (= D. pruinosa species group) with Silene , but was mistaken in listing it as a larval leaf-miner.

Pettersson (1991) briefly mentioned that ‘ Delia flavifrons ’ on the Baltic island of Öland is a seed predator of Silene vulgaris . In a more detailed study of the same fly and its association with Silene vulgaris and S. uniflora ssp. petraea (Hartm.) Jonsell & H.C. Prent on Öland, Pettersson (1992) observed that the adults feed, mate and rest on these plants, and that the females lay eggs singly on the petal stalks of young flowers, preferably in the late afternoon when the flower buds open. He also noted that the flies play no role in the pollination of the flowers, which is rather carried out by noctuid moths of the genus Hadena Schrank. If successful, the newly hatched larva makes its way to the fruit body, where it feeds on developing seeds. In order to reduce competition on the larval food from stronger, likewise seed-feeding larvae of Hadena moths, the flies tend to lay eggs later in the flowering season than the moths. On request, Pettersson kindly sent me some male and female ‘ flavifrons ’ that all proved to belong to D. pruinosa . In subsequent papers on seed production and seed predation in Silene uniflora ssp. petraea by Pettersson (1994, 1997), it is stated that the flies, now recorded as ‘ Delia pruinosa ,’ play a minor role as seed predators compared to the noctuid moths, because they are less common and each larva consumes seeds from one capsule only. Ackland & Pont (1996) showed that D. pruinosa is the only species of the Delia pruinosa species group known from Britain thus far, and that some of the specimens examined according to the labels had been taken on or reared from Silene vulgaris .

My own observations suggest, in agreement with Pettersson (1992), that anthomyiids of the Delia pruinosa species group are closely associated with their host plants, more so than other species of the Delia cardui section that likewise feed as larvae on species of Caryophyllaceae . Therefore, the best ways to obtain material of these species are to look for the flies on their host plants during the flowering season or to collect maturing seed capsules for rearing of the larvae.

Figures

FIGURES 1 – 7. Delia spp.: 1. D. sileni sp. nov., female proboscis. 2. Same, male proboscis. 3. Same, female labella. 4. Same, male labella. 5. Same, male left antenna. 6. Same, detail of female left hind leg. 7. D. pruinosa, detail of female left hind leg. Scales: 0.25 mm.

FIGURES 8 – 14. Delia pruinosa, male: 8. Sternites III and IV. 9. Sternite V. 10. Hypopygium, caudal. 11. Hypopygium, lateral. 12. Pre- and postgonites. 13. Phallus, lateral. 14. Distiphallus, caudal. Scales: 0.5 mm for 8 and 9, 0.25 mm for 10 – 14.

FIGURES 26 – 31. Delia sileni sp. nov., male: 26. Sternites III – V. 27. Hypopygium, caudal. 28. Hypopygium, lateral. 29. Pre- and postgonites. 30. Phallus, lateral. 31. Distiphallus, caudal. Same scale: 0.25 mm.

FIGURES 15 – 16. Delia pruinosa, female: 15. Oviscapt, flat mounted. 16. Same, distal part only. Scales: 0.5 mm for 15, 0.25 mm for 16.

FIGURES 42 – 44. Delia spp., females: 42. D. capdellae sp. nov., distal part of oviscapt, flat mounted. 43. Delia sp. indet. A, oviscapt, flat mounted. 44. Same, distal part only. Scales: 0.25 mm for 42 and 44, 1.0 mm for 43.