The pattern of phosphorus (P) content across the ontogeny of aquatic macroinvertebrates is poorly known.  We propose a theoretical model with two patterns.  A U-shaped pattern in percent body P content is expected for taxa that have non-feeding adults, whereas a monotonic decline for taxa that have feeding adults.  We hypothesize that adult feeding status is a key determinant of body P content across developmental stages because taxa with non-feeding adults are dependent on immatures to acquire all materials necessary for adult survival and reproduction, particularly the formation of P-rich gametes.  To test these hypothesized patterns, 10 aquatic macroinvertebrate taxa were analyzed for P content across their ontogeny.  Selected taxa exhibited differences in adult feeding status as well as other life-history characteristics.  A response model between development stage and body P content was developed using a linear or polynomial least squares regression model.  Response models were used to quantitatively describe the functional relationship between developmental stage and body chemistry and to compare observed to hypothesized curve shapes based on life-history traits.  Nymphs of mayflies (non-feeding adults) showed the U-shaped response predicted in the model.  This is the first demonstration of a non-linear change in organism P content across its ontogeny.