Phenotypic plasticity of wing size and shape has been evaluated in Aphidius ervi developing in its host, Acyrthosiphon pisum, parasitized at seven different ages. The parasitoid wing size was used as an estimator of both whole body size and its cellular composition. No size difference was observed in A. ervi adults emerged from aphids 1, 2 or 3 days old at parasitization. Body size then increased in A. ervi emerged from hosts older at parasitization. Body size values as related to host age at parasitization were achieved by adjusting developmental time, developmental rate or both. Parasitoids of similar size, but developed in hosts parasitized at different ages, had different wing cellular composition, while the increase of parasitoid body size was related to a general increase in both cell area and cell number. These results seem to suggest a trade-off between adult size and developmental time, at least for parasitoids developed at the two extremes of host ages at parasitization, and that A. ervi can reach the same adult size via different trajectories, adapting its ontogenetic processes. Wing shape was typical for all the different parasitoid classes considered and differed strongly between males and females, independent of their size. Parasitoid males (haploids) and females (diploids) did not differ in either cell area or cell number, suggesting a possible sex-determined dosage compensation in somatic tissue endoreplication.