**Abstract**

The parasitic larvae of *Philornis* downsi Dodge & Aitken (Diptera: Muscidae) were first discovered in Darwin's finch nests on the Galápagos Islands in 1997. Larvae of *P. downsi* consume the blood and tissue of developing birds, causing high in-nest mortality in their Galápagos hosts. The fly has been spreading across the archipelago and is considered the biggest threat to the survival of Galápagos land birds. Here, we review (1) *Philornis* systematics and taxonomy, (2) discuss shifts in feeding habits across *Philornis* species comparing basal to more recently evolved groups, (3) report on differences in the ontogeny of wild and captive *P. downsi* larvae, (4) describe what is known about adult *P. downsi* behaviour, and (5) discuss changes in *P. downsi* behaviour since its discovery on the Galápagos Islands. From 1997 to 2010, *P. downsi* larvae have been rarely detected in Darwin's finch nests with eggs. Since 2012, *P. downsi* larvae have regularly been found in the nests of incubating Darwin's finches. Exploring *P. downsi* ontogeny and behaviour in the larger context of taxonomic relationships provides clues about the breadth of behavioural flexibility that may facilitate successful colonisation.

**Keywords:** *Protocalliphora*, *Passeromyia*, *Philornis*, nest larvae, hematophagous, subcutaneous, Darwin's finches, Passeriformes
