Translation:It is the mother of six hundred and sixty-two little spiders.
To echo Renardo's comment - Yes, I think this is the reason we would use the definite article here. Indefinite seems to imply that it's countable and one of many potential mothers. Definite article means that it's unique and there's only one. English grammar books do not do a good job of stressing how this often takes precedence over the rule for using the definite article when we've already been introduced to something.
It must be a nightmare translating (creating) articles when translating from Czech to English.