length thereof to exclude flies a removable secondary fly trap is suitably positioned in the top casing roller cassette 40 of the roller comprising a plastic moulding 41 that hangs from the roof of the cassette 40 and extends the length of the roller the moulding 41 having a brush 42 running along its length that rests on the screen roll 1 u2033 and accordingly remains continuously in contact with the screen roll 1 u2033 as the screen is extended or retracted and as the roll 1 u2033 correspondingly decreases or increases in diameter the upright tracks 30 on either side of the roller screen 1 u2033 each have a bristle brush 32 along one longitudinal edge and a stabiliser panel wing 33 along the opposing longitudinal edge each of the wings 33 is adjustably mounted to its track 30 on hinges 34 the hinges 34 take the form of bent pins being elongate and mounting in slots 35 in the wings 33 so that the wings 33 may be deployed by pulling laterally across and pivoting outwardly to release the edges of the roller blind screen 1 u2033 to allow it to be pulled down the wings 33 are then pivoted back and pushed back into the tracks 30 so that their curved rear faces push into and thereby to grip the fly screen against the brushes 32 in the tracks 30 once the screen is fully drawn down and deployed a strip of velcro u2122 or similar is suitably provided on the rear face of each wing 33 preferably extending substantially along its length to enhance its grip this gripping of the fly screen once the screen is deployed holds it firmly in place against any gusts of wind in the absence of a bottom track.