The original hypothesis is proposed that the parasitism of trombiculid mite larvae (Trombiculidae) is a rather young historical phenomenon which developed after the quiescent stages (calyptostases) and had evolved in the ontogenesis of this group of trombidiform mites. The larvae being primarily predatory-entomophagous with biting-sucking mouthparts, they could easily pass to parasitism on vertebrate animals in pasture conditions, firstly on mammals, becoming primarily lymphophagous, a,d, due to wide polyphagy, they are similar to free-living blood-sucking insects e.g. Diptera. The problem of specificity and specialization in trombiculid mites and other related groups is discussed.