The tokus compartment of Heliothis virescens (Fabricius) larvae contains aerating trachea and tracheole tufts supplied from the eighth spiracle. This tracheal tuft is known to act as a "lung" for haemocytes in Calpodes ethlius (Stoll) and a few other lepidopteran larvae. The goal of our research is to study the effects of this tracheal "lung" on parasitoid development inside the host larva, Heliothis virescens. Thus, our first objective has been to determine if the eggs of the parasitoid, Toxoneuron nigriceps (Viereck), in the host larva exploits the tokus area by being predominately present in that region compared to the other regions, irrespective of their oviposition site. Our second objective has been to study the effect of varying oxygen concentrations in vitro on various developmental stages of the egg. The results thus far suggest that most of the eggs are found in the tokus region irrespective of the oviposition site after several hours of oviposition, and that decreasing oxygen concentrations adversely affects the egg development of the parasitoid. Our next objective is to determine the oxygen concentrations in the tokus region of the host larvae and compare it to other regions of the body, which we are currently measuring using oxygen sensors in vivo. Additionally, we are determining the effect of the tokus "lung" on the development of the first instar larvae of the parastioid.