The fly

My mother spotted this large fly outside a window:

The fly creeped her out, as it was huge compared to regular house flies; however, what fascinated me and my brother about the fly was its behaviour. While we were unsure exactly what it was doing, it appeared to be cleaning itself, as it was scraping its forward and back legs over its body as though it were removing bits of detritus. It seemed uninterested in anything else and only flew away when it finished.