In a series of notes and papers published in the last few years, the writer has attempted to apply simple mathematical methods to the study of the numerical increase of entomophagous parasites at the expense of their hosts. This work was undertaken mainly in the hope of bringing to light the general laws underlying parasitic interrelations, considered simply in their quantitative aspect. But since the publication of these papers the writer has frequently been questioned by entomologists as to the utility of the mathematical method in parasitological operations of the practical order.