I looked at the scale of the tiny spider, and the relatively enormous distance it still had to go to reach the floor, and I decided that the scale of distance was too great in this case; it wouldn't be practical to use all that silk. But the spider can not see how far the distance is; its vision is good, but only at very close range. When starting out to make a web, the spider has to investigate a possibility like this by making an attempt, and finding out what happens.
The spider pays out its gossamer lifeline by squirting liquid silk through an array of nozzles, or spinnerets, on its abdomen, and the extruded liquid hardens almost instantly into a strong thread. The spider clings to this thread with the claws on its hind legs, walking itself down as the thread is paying out. Quite a little miracle in itself.
But decisions of economy have to be made, for the supply of silk isn't endless. I could see that the spider had probably miscalculated in this case, and the attempt to reach the floor with a strand was not going to be practical. As we watched, the tiny arachnid let itself down another couple of inches, then paused again, motionless on the nearly invisible line. There was still a vast gulf between it and the floor.
All at once, the spider's sensorium, or brain, reached the appropriate conclusion and it suddenly began to climb back up. The silk it had expended on its downward exploration was gathered back in as it climbed, to be ingested and reprocessed into more silk for later use. The spider ascended rapidly, hand over fist, and it didn't pause until it regained the ceiling light fixture, there, presumably, to formulate a new plan.
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