Colobaea bifasciella (Fallen, 1820)

Bratt, Albertus D., Knutson, Lloyd V., Murphy, William L. & Daniels, Anthony A., 2020, Biology, immature stages, and systematics of snail-killing flies of the genus Colobaea (Diptera: Sciomyzidae), with overviews of aspects of the tribe Sciomyzini, Zootaxa 4840 (1), pp. 1-64 : 39-40

Colobaea bifasciella

Egg ( Figs 2–3 , 7 ) (based on three specimens, from Hillerød, Denmark, and Brattby, Sweden). Length 0.65–1.0 mm; greatest width 0.1–0.2 mm. Slender; posterior end enlarged, bulbous; ventral surface slightly more convex than dorsal surface in lateral view. Chorion very granulose and punctate, sculptured into fine reticulate pattern.

First-instar larva (based on three specimens, ex egg from Brattby, Sweden). Length 0.65–1.00 mm; greatest width 0.16–0.30 mm. Basal ring of sensory papillae darkly pigmented; spinules of post-oral spinule patch large, numerous, darkly pigmented. Three large, unicuspid spinules on each lateral surface of mouth opening, contiguous to paired sclerites below mouthhooks. Mouthhooks long, slender; cephalopharyngeal skeleton light brown, 0.17–0.20 mm long. Indentation index 87.

Posterior end with small ventral and ventrolateral lobes. Spiracular tubes 0.02 mm long, protrudent, robustly cylindrical. Hyaline float hairs short.

Third-instar larva ( Fig. 21 ) (based on 10 specimens from Hillerød , Denmark, and Brattby , Sweden). Length 3.0– 6.5 mm; greatest width 1.0– 2.5 mm. Body form dorsoventrally compressed on posterior half; lateral welts apparent, obscuring lateral pads. Anterior spiracles large ( Fig. 50 ), 14–20 marginal papillae, integument closely enclosing each papilla; disc of spiracle with scattered prismatic spots ( Fig. 35 ) .

Cephalopharyngeal skeleton ( Fig. 42 ) 0.50 mm long. Indentation index 90. Ventral cornu large, with irregularly pigmented posterior margin; dorsal cornu with elongate, incomplete window; dorsal bridge small. Hypostomal sclerite small. Mouthhooks large, anteriorly directed; ventral arch small, reduced to lightly pigmented strap, with 20–22 long, slender teeth.

Spinules more numerous and more darkly pigmented than in other species of Colobaea . Segment III without ventral spinule patch, segment IV with single ventral spinule patch, segments V and XII with divided patches, segments VI–XI with three-part patches. No spinules on lateral pads. Posterior third of body with fine scales dorsally and laterally; segments XI and XII with dense scales and spinules within dorsal pigmented areas, integument in those areas thickened and tanned ( Fig. 22 ).

Posterior spiracles strongly protrudent from small posterior spiracular disc. Two pairs of peripheral lobes (ventral and ventrolateral) on disc, both small, not acute, appearing as welts; no mammillae at bases of ventral lobes ( Fig. 16 ); 3–4 rows of small, dark spinules on lobes and adjacent areas.

Puparium ( Figs 4 , 24, 27 ) (based on 10 specimens from Hillerød , Denmark). Length 4.3–5.0 mm; greatest width 2.0– 2.5 mm. Shaped to fit inside snail shell, dorsal part of first six segments occluding aperture of shell, posterior segments twisted. Robustly spindle shaped in lateral aspect, first six segments strongly convex with longitudinal groove formed by pressure from the columella of the snail shell, segments XI and XII upturned .

Anterior half dark reddish brown, integument strongly sclerotized; posterior half yellowish brown, integument thin. Integument subshiny, with fine punctations and wrinkles. Segmentation conspicuous; lateral intersegmental pads obscure; lateral longitudinal welts apparent. Spinule patches obscure.

Anterior spiracles large, projecting dorsally from angles of cephalic cap; posterior spiracles protruding weakly. Lobes on posterior end reduced.

Figures

FIGURE 50. Colobaea bifasciella (Fallén), third instar, anterior segments, dorsal view. Measurement in mm.

FIGURES 2–4. Eggs and puparium of Colobaea bifasciella (Fallén): 2, 3, eggs on shell of Stagnicola palustris; 4, puparium of C. bifasciella in shell of Stagnicola sp.

FIGURES 5–7. Eggs, lateral outline: 5, Colobaea pectoralis (Zetterstedt); 6, C. americana Steyskal; 7, C. bifasciella (Fallén). Measurement in mm.

FIGURES 21–22. Colobaea bifasciella (Fallén), third-instar larvae: 21, lateral view; 22, dorsal view. Abbreviations: ASp, anterior spiracle; CpS, cephalopharyngeal sclerite; LP, lateral pad; IDSP, intrasegmental dorsal spinule patch; PaP, peri-anal pad; PIF, primary integumentary fold; PSp, posterior spiracle; SIF, secondary integumentary fold; VL, ventral lobe; VIL, ventrolateral lobe. Measurement in mm.

FIGURES 32–35. Anterior spiracles, third-instar larvae: 32, Colobaea pectoralis (Zetterstedt); 33, C. deemingi Knutson & Bratt; 34, C. punctata (Lundbeck); 35, C. bifasciella (Fallén). Abbreviations: PrS, prismatic spot; StS, stigmatic scar. Measurement in mm.

FIGURES 40–43. Cephalopharyngeal skeletons of third-instar larvae: 40, C. punctata, lateral view; 41, C. pectoralis, lateral view; 42, C. bifasciella (Fallén), lateral view; 43. C. deemingi Knutson & Bratt, lateral view. Abbreviations: AvP, anteroventral process of pharyngeal sclerite, DB, dorsal bridge, DC, dorsal cornu, DW, dorsal window, ES, epistomal sclerite, HS, hypostomal sclerite, LS, ligulate sclerite, MH, mouthhook, PB, parastomal bars, PS, pharyngeal sclerite, VA, ventral arch, VC, ventral cornu, VW, ventral window. Measurement in mm.

FIGURES 15–20. Posterior ends of third-instar larvae: 15, Colobaea americana Steyskal, caudal view; 16, C. bifasciella (Fallén), caudal view; 17, C. pectoralis (Zetterstedt), caudal view; 18, C. punctata (Lundbeck), caudal view; 19, C. deemingi Knutson & Bratt, caudal view; 20, C. punctata, lateral view.Abbreviations: DL, dorsal lobe, LP, lateral pad, M, mammillae, PaP, peri-anal pad, SpP, spiracular plate, SpT, spiracular tube, VL, ventral lobe, VlL, ventrolateral lobe. Measurement in mm.

FIGURES 23–27. Puparia: 23, Colobaea pectoralis (Zetterstedt), lateral view; 24, C. bifasciella (Fallén), lateral view; 25, C. americana Steyskal, dorsal view; 26, C. pectoralis, dorsal view; 27, C. bifasciella, dorsal view. Measurement in mm.