Fly-killing climber?

Have others encountered this? With the approach of winter the greenhouse windows are now being closed at night. I entered mine this morning to open the windows and was very surprised at the sight on my 'Thunbergia coccinea var. lutea' climber as was covered with dozens of common house flies that had died on the leaves. No chemicals are used. My initial impression was they had dropped exhausted from the glazing above but this had not happened. The corpses are at all angles on the vertical leaves as though they had begun drawing nutrient from the leaf and promptly curled their toes up!

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