Fannia pusio (Wiedemann) is originally from tropical and subtropical regions of the Americas, where it has repeatedly been collected from both animal carrion and human cadavers. This species is expanding its distributional range, and it has been introduced to Africa, Asia, Australia, Oceania, and Europe. Newly introduced species may be confused with native species of local arthropod necrophagous assemblages, which from a forensic entomology perspective may impact the accuracy of post-mortem interval (PMI) estimations based on insect evidence. In this work we aim to raise awareness among forensic entomologists of the potential benefits and risks associated with the expanding range of F. pusio. Morphology of all larval instars is documented with a combination of light, confocal laser scanning and scanning electron microscopy. Characters allowing identification from other forensically important Fanniidae are listed. Thermal requirements for the development of immature stages of F. pusio were examined under nine ambient temperatures. Models describing changes in larval body length over time were produced for eight different temperatures. The thermal summation constant (k) and developmental zero (T_min) were calculated for six developmental events: hatching, first ecdysis, second ecdysis, wandering, pupariation, and eclosion.