 How does the darkness affect the robots' mobility?
The robots were built to serve Man; to do his work, see to his comforts, make smooth his way. At midday some few of the blue sun's rays filtered through to the jungle floor, but now, late afternoon on the planet, the shadows were long and gloomy. In the movement, his elbow struck the top of a small earthy mound and he was instantly engulfed in a swarm of locust-like insects that beat disgustingly against his eyes and mouth. The planet's double moon had risen and he could see a robot rolling slowly across the clearing in his general direction, blasting indiscriminately at whatever mind impulses came within its pickup range, birds, insects, anything.
 Their mobility is not affected, but it is harder for them to aim their blasters.
 It makes it harder for them to differentiate people from other animals.  Their tracking of animals is unimpeded, but they still have plans to contend with.  Their mobility is not affected, but it is harder for them to aim their blasters.  It makes smaller signals from insects more distracting.