  We show that a method presented in [S.L. Trubatch and A. Franco, Canonical
Procedures for Population Dynamics, J. Theor. Biol. 48 (1974), 299-324] and
later in [G.H. Paine, The development of Lagrangians for biological models,
Bull. Math. Biol. 44 (1982) 749-760] for finding Lagrangians of classic models
in biology, is actually based on finding the Jacobi Last Multiplier of such
models. Using known properties of Jacobi Last Multiplier we show how to obtain
linear Lagrangians of those first-order systems and nonlinear Lagrangian of the
corresponding single second-order equations that can be derived from them, even
in the case where those authors failed such as the host-parasite model.
