Hi Ross
My first reaction is that f5.0 sounds a very wide aperture, considering you are using a flash to supplement the daylight. In this case though I think the daylight was strong enough (at f5.0 and 1/160th second) that the flash hasn't had much of an impact on the photo, leaving the insect movement to create slight blurring from the daylight component of the lighting. Flash will freeze an image when the flash is the major component because, although the camera might be set at a 160th the flash will give an effective shutter speed much much faster than that ... as long as no other 'permanently on' light source (eg. the sun) illuminates the subject.
My suggestion would be to use f11 or smaller when taking photos in daylight so that if the flash didn't fire the photo would be massively under exposed. Then when the flash does fire it will be the major lighting source and the effect will be to freeze the subject. You will get a much better depth of focus too, as a side effect, though your overall sharpness will (apparently) suffer
