Microlenses are important components of optofluidics systems and their lab-on-a-chip applications. We report on liquid microlenses that can be in situ formed in microchannels in a single batch via pneumatic control. These microlenses have optical axes that are parallel to the substrate, and their focal length can be pneumatically tuned separately and independently. In addition, the microlenses can also be pneumatically removed individually and reformed on demand. The parameters affecting the profiles and optical properties of the microlenses, such as the pressure difference and gravity, were investigated by simulation. We then demonstrated the enhancement of fluorescence emission in a microfuidic channel using our microlenses to focus attenuated excitation laser beams into regions of interest in the channel. With the microlenses, the region with visible fluorescence response was enlarged by up to 13 times and the intensity of fluorescence emission was enhanced by up to 38 times, compared to the cases without the lenses.