 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. Frowning, worrying about the sounds, Alan momentarily forgot to watch his step until his foot suddenly plunged into an ant hill, throwing him to the jungle floor. 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. But the back pressure of the cat's body overloaded the discharge circuits. The robot started to shake, then clicked sharply as an overload relay snapped and shorted the blaster cells. The metal killers, tuned to blast any living flesh, would murder them the instant they stepped from the ship! 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. Apparently the robot hadn't sensed him yet, but Alan didn't know what the effective range of its pickup devices was. Powerful as a small tank, the killer robot was equipped to crush, slash, and burn its way through undergrowth. The twin moons cast pale, deceptive shadows that wavered and danced across the jungle floor, hiding debris that tripped him and often sent him sprawling into the dark.
 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.