I believe it only fits to the insect (a fly). The verb "to fly", according to my dictionary, is "kuruka".
Both 'nzi' and 'inzi' are being used for a fly. There are many words in Swahili where letters change a bit going from one area to another, like 'chandarua' and 'chandalua' ('r' and 'l' often get changed), 'ghafla' and 'ghafula' (words from Arabic origine), etc. You just have to allow for them.