The adult Reduvius personatus is dark brown to black, with paler bases to the tibia (legs) and antennae. The adult is typically 16–18mm long.
This species is the only representative of its genus in the UK, but can be confused with other reduviids elsewhere in its global distribution.
Even by insect standards the taxonomic relationships of these bugs are poorly understood. However the traditional and still widely used classification places the assassin bug family (including R. personatus) in the Heteroptera, along with related groups such as the shield bugs.
Although Heteropterans may be mistaken for beetles (Coleoptera), they are readily distinguishable by their ‘rostrum’ - a beak-like mouth part that is used for feeding. R. personatus itself has a 3 segmented rostrum.
The assassin bugs have a distinctive groove under their body that the rostrum fits into. When a bug rubs the rostrum against the ridges in this groove, it produces sound that is used to intimidate potential predators.
Peak assassin bug diversity is centered around the tropics, as with many insect groups. Members of the Triatominae subfamily include vectors of Chagas disease - a parasitic infection that some speculate Charles Darwin may have suffered from.