Phenotypic plasticity is defined as the ability of an organism to produce different phenotypes in response to changes in internal or external environmental conditions. Experimental modification of animal diets has been widely used to study phenotypic plasticity in physiological and life-history traits. Here we compared the data from a previous experiment, conducted in a harvestman species, that was aimed to evaluate the effect of diet quality on maintenance costs (standard metabolic rate), growth rate, internal organ size (visceral mass) and fecundity (clutch mass and egg number), with new data obtained from field (mark—recapture) specimens. We assumed that of the two experimental diets tested before, animals in the field would probably consume the one of intermediate quality (i.e. have a generalist omnivorous diet), and then, we predicted that field animals should exhibit intermediate values for both physiological and life-history traits. We found that field animals and animals consuming a high quality diet showed a greater growth rate than animals consuming a poor quality diet. In addition, animals consuming a high quality diet showed a larger clutch mass than both field animals and animals consuming a poor quality diet, which, in turn, was related to higher maintenance costs. Our results illustrate how animals adopt different life history strategies according to the quality of the diet that is available, which is correlated with phenotypic adjustments at the anatomical and physiological levels.