Recently, it was determined that the mitogenic effect of a Manduca sexta fat body extract on the midgut stem cells of Heliothis virescens, was due to the presence of monomeric α-arylphorin. Here we report that in primary midgut cell cultures, arylphorin stimulates stem cell proliferation in the lepidopterans M. sexta and Spodoptera littoralis, and in the beetle Leptinotarsa decemlineata. Results for S. littoralis confirm the results of the earlier study, i.e., the mitogenic effect is due to free α-arylphorin subunits. In addition, feeding artificial diets containing arylphorin increased the growth rates of several insect species, but not the sweet potato whitefly, Bemisia tabaci. When tested against several continuous cell lines, including some with midgut and fat body origins, arylphorin had no effect; however, a cell line derived from Lymantria dispar fat body did grow more rapidly in medium containing a chymotryptic digest of arylphorin.