def fetch(self, is_dl_forced=False):
        """
        Override Source.fetch()
        Fetches resources from String

        We also fetch ensembl to determine if protein pairs are from
        the same species
        Args:
            :param is_dl_forced (bool): Force download
        Returns:
            :return None
        """

        file_paths = self._get_file_paths(self.tax_ids, 'protein_links')
        self.get_files(is_dl_forced, file_paths)
        self.get_files(is_dl_forced, self.id_map_files)