Structures and functions of a mosquito's pump system were investigated experimentally to clarify the mechanistic aspects of this optimized bio-pump. The three-dimensional strucure of the pump that was reconstructed by 1500 slices of the mosquito's head was obtained using a histopathological method. The mosquito's pump motion is predictable, and the pump volume can be estimated by the three-dimensional structural observations. The pump volume of the pumps, an oral cavity pump and pharynx pump, were estimated as being 7.92×10 -4mm3, 63.3×10--4mm3 respectively. The frequency of the mosquito's pumps was predicted to be 2.09-16.7 Hz, and was estimated using the pump volume and the volumetric blood flow rate. The motion of the mosquito's pumps was visualized directly by an x-ray phase contrast imaging method using synchrotron radiation. It was found that the mosquito's pumps (i.e., the oral cavity pump and the pharynx pump) worked alternately with a 4.23 Hz pump frequency. Moreover, the phase difference of each pump was almost 0.5, suggesting that the mosquito's pumps worked in opposite phases. In addition, the mosquito's pumps were found to work as a inline pump, which is suitable for increasing the pressure head of pump.