Abstract
Chrysomya megacephala (F.) (Diptera: Calliphoridae) is a medically important blow fly species in many parts of the world. In this study, we examined the ovary of C. megacephala at the ultrastructural level using light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Dissection of the ovaries revealed eight stages of ovarian development, thus providing a means of estimation of the age of females based on this organ. In the initial stage I (day 0-2), freshly-emerged flies up to three days old each display piriform germaria while the follicles are not yet well differentiated. In stage II (day 3), each follicle is nearly spherical and sepa- rating from the germarium. During stage III (day 4), each follicle is now easily distinguished from the germarium and is only connected by a thin interfollicular stalk. The nurse cells are completely surrounded by the cuboidal epithelial follicular cells. In stage IV (day 5-6), each folli- cle is spherical and considerably enlarged; whereas, the nurse cells within are more differentiated into individual round inclusions and grouped in the follicle chamber. Stage V (day 7) marks appearance of the visible oocyte at the posterior pole of the follicle. During stage VI (day 8), each follicle displays oocyte expansion anteriorly to occupyfrom one-third to one-half of the total length. In stage VII (day 9) the oo- cyte characteristically fills the posterior two-thirds of the total length of the follicle and in the final stage VIII (day 10), each follicle is com- pletely filled by an oocyte and nurse cells have disappeared. The micropyle and narrow, elongate median area are clearly seen in this final stage. SEM images reveal that the ovary is covered by an ovarian envelope, which is tough and thickened in 3-day-old adult ovarioles; whereas, it becomes thin and fragile in 9-day-old adult ovarioles. TEM analysis of ovarioles of 3-day-old adult flies shows that cuboidal follic- ular cells containing large nuclei with visible fibrils of muscle simply encircle the large compartments of nurse cells; whereas, ovarioles of 7- day-old adult flies displayed a more completely developed enclosure for the developing oocyte. This was observed by the presence of the outermost follicular cell layer with underlying eggshell, vitelline envelope, trabecular layer, and layer of granular material that surrounded the innermost oocyte.
Highlights
The Oriental latrine fly, Chrysomya megacephala (Fabricius), is a blow fly species of medical importance worldwide, whose wide distribution ranges from the Oriental, Australasian, and Oceania regions through Africa, America, Europe, and the Mediterranean [1,2,3]
Results of the current study indicate that female C. megacephala complete egg development in eight stages under fluctuating ambient temperature (18-27°C)
Stage VI in C. megacephala demonstrated that the developing oocyte fills one-third to one-half of the total follicle length; whereas, this condition is related with stages VI and VII in C. bezziana [13] or stage V in C. hominivorax [14]
Summary
The Oriental latrine fly, Chrysomya megacephala (Fabricius), is a blow fly species of medical importance worldwide, whose wide distribution ranges from the Oriental, Australasian, and Oceania regions through Africa, America, Europe, and the Mediterranean [1,2,3]. In Thailand, C. megacephala was the most common species collected from several land use types, from the lowland of human habitations (cities, villages, paddy fields, disturbed mixed deciduous forests, and mixed orchards) to the mixed deciduous forested areas, which implies that it has a broad distribution concerning ecological localities [4] This fly species is one of public health concern based on the adults’ potential as a mechanical carrier of various pathogens (bacteria, viruses, protozoa and helminth eggs) involved in human diseases [5]; while larvae are known myiasis-producing agents [6]. In order to fill the gap of information regarding ultrastructure and/or development of this organ in this species, the aim of this study was to investigate the ovary of this fly species using LM, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) This should give insight into ovarian morphology at the ultrastructural level and into chronological ovarian development. Availability of such data should be useful as baseline biological information on C. megacephala which may be used toward fly control efforts and/or forensic entomology applications
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