In the Next Life, Canto II, stanzas 18-21

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56
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So this is how it’s happened all along.
This is the story every mother tells.
I’ve seen it three times now; I can’t be wrong.
We walk ourselves to where we store the shells.

Thrice now I’ve followed groups of two at night—
a different pair each time I’ve gone that route.
All six have started out with that red light.
The five who have survived have put it out.

I’ve watched them walk together to the store.
I’ve watched them take a new shell to their ends,
attach the legs, and, when there is one more,
apply a number that by one ascends.

I’ve seen as well why some have not come back.
I’ve seen the hungry rodents lie in wait.
I’ve seen the loose connection, the attack,
and then the empty shell we find too late.

©2010 Louis A. Merrimac

By one ascends: The idea is that each carrier at “birth” is assigned a unique number so they can tell one another apart. The precise manner in which this is done is one of those pesky details that the author excuses himself from providing. He says novelists give details, and he is not a novelist.
Loose connection: It sure is. He’s trying to illustrate the risk of reproduction for the carriers. Presumably a failure to seal the hatches while docking would allow vermin to pass through to where the precious humans reside. I gather that the machines are vaguely aware that the insiders are necessary for their survival.