The office of a member of the board of trustees shall become vacant immediately without declaration of any officer or any acceptance of the board of trustees or its members, upon the death of the incumbent, or his being adjudicated insane or being found to be mentally incompetent by the proper court; his resignation; his removal from office; his conviction of a felony; his election or appointment being declared void by a competent tribunal; his refusal or neglect to file his acceptance of office, or his refusal or neglect to take and subscribe to the constitutional oath of office and deposit the same in the manner and within the time prescribed by law; his ceasing to possess the legal qualifications for holding office including his residence qualification.