County mutual fire insurers, upon becoming members of such reinsurer, shall give it their written obligation binding themselves and their successors to pay their pro rata share of the necessary expenses and loss sustained by any member of the reinsurer during the time for which their respective policies are written. They shall also at the time of effecting the reinsurance pay such percentages and such other charges as are required by law or by the rules and by-laws of the reinsurer.