Megaselia spiracularis Schmitz (Diptera: Phoridae) is a scuttle fly species of forensic importance and has been recorded colonizing human corpses in Malaysia, Korea and China. The necrophagous activity of M. spiracularis larvae on human corpses can be referred to estimate minimum postmortem interval (PMI_min) in death investigation. However, developmental data of M. spiracularis derived from ambient fluctuating temperature is scarce and taxonomic descriptions to larval instars for species identification are still lacking. This research was conducted to address these limitations by studying the development of M. spiracularis at fluctuating ambient temperatures. From each sampling occasion, larval body lengths were obtained and samples were mounted on slides to determine larval instars based on taxonomic characteristics of the cephalopharyngeal skeleton, anterior spiracles and posterior spiracles. At fluctuating temperatures of 25–37 °C and relative humidity of 47–93%, developmental duration of egg stage was 7–9.5 h; first instar larva, 9.5–17 h; second instar, 6–12 h, feeding third instar, 18–36 h; post-feeding third instar, 6–12 h; and pupa, 168–204 h. Accumulated degree hours for each developmental stage based on fluctuating ambient temperatures were also determined. The size of M. spiracularis larvae varied from 0.64–0.78 mm after eclosion to 5.20–5.95 mm before entering post feeding phase. The first, second and third instar larva could be distinguished mainly by the morphology of cephalopharyngeal skeletons, anterior spiracles and posterior spiracles. Developmental durations of M. spiracularis larvae and the larval instars can be used as references to estimate PMI_min as this species is likely to be featured in future forensic cases.