40 ILCS 5/11-220

Whenever territory is annexed to the city, any person then employed as a laborer by such annexed territory, who shall be employed by the city on the date of annexation shall automatically come under this Article, and any service rendered for annexed territory shall be considered, for the purpose of this Article, as service rendered to the city. Such laborer shall be treated, as of the date such annexation comes into effect, as a present employee of the city on the effective date.