I found something interesting about some of the lines in the song:

Run, run, we've been moving all night, rivers to the left.
If your mother don't love you then the riverbed might
Gone, gone, the water's all gone
Mummy come back 'cause the water's all gone

I'm guessing it's saying since the spiders can't deal with the absence of love from a mother, they would kill themselves by drowning in the river. However, there's no water to drown in since the river is dried up, so they must endure the pain of loneliness.

Is this referring to Bowie's own mother? I remember reading how she was very proud of his fame, but didn't like how he never visited her, and was hard on him about that. Perhaps it could be dealing with guilty feelings he had about that? She felt like he left her, but he still cared for her but was too busy to come visit her (or was afraid to after his brother's death, since he wasn't there for his brother's funeral).

Interesting how he would relate the story to spiders, since his band during the Ziggy era was the Spiders from Mars...

BTW, I got the info on his family from a book titled "Alias David Bowie".

From now on, ending a sentence with a preposition is something up with which I will not put.
- Sir Winston Churchill