An incredible diversity of organisms, from bacteria and social insects to bird flocks and fish schools, self-organize to achieve collective tasks. They have inspired the development of decentralized algorithms and robotic swarms. While numerous models have been proposed to understand the self-organization of animal groups, the role of mechanics was rarely considered. In this talk, I will demonstrate experiments and models of the collective behaviors of two insect species, red imported fire ants (Solenopsis invicta) and black soldier fly larvae (Hermetia illucens), in both static and moving fluids. Particular attention will be paid to how their collective behaviors are affected by fluid forces. My results suggest a timescale associated with the limit of animal perception and locomotion. Collective phenomena shorter than such a timescale are driven by mechanical forces and random movements of the individuals. To conclude, I will extend the framework to consider the hydrodynamics of fish schools and discuss how I use computer vision and biomimetic robots to seek insights into this complex system.