Section 8A. At any hearing which the commissioner is authorized by law to hold, he may by summons require the attendance and testimony of witnesses and the production of books, records and papers touching upon the matters in question at such hearing, and may administer oaths to such witnesses. The fees of such witnesses for attendance and travel shall be the same as for witnesses before the superior court in civil cases and shall be paid by the commonwealth upon the certificate of the commissioner filed with the comptroller. Fees of such witnesses need not be paid or tendered to them prior to their attendance and testimony. This section shall not affect any powers conferred upon the commissioner by section four.