Mark Jervis was one of the world's foremost experts on life history and development strategies in parasitoids. He held a special place for the nutritional ecology of immature and adult parasitoids; his work on adult nutritional ecology and reproduction is considered seminal. Here, we discuss aspects of parasitoid growth and development, focusing on species that attack feeding, growing hosts (so-called koinobiont parasitoids). We provide a simple graphical model illustrating how selection pressures can alter host usage strategies, employing growth trajectories as a useful comparative tool. Furthermore, we discuss how the recent evolution of the hemolymph-feeding strategy in koinobionts has enabled these parasitoids to greatly expand the suitable size range of hosts, as well as providing additional adaptive functions such as gregarious development, resource sharing, and the use of the dying host as a ‘bodyguard’ against predators and hyperparasitoids. Overall, we conclude that koinobionts exhibit a wide array of strategies in exploiting and utilizing variably sized hosts in response to a suite of quite divergent selection pressures that influence their growth, development, and survival.