The growing world population is placing an increasing demand on food production. In addition, abuse and misuse of chemical insecticides has led to the evolution of resistance in insect pests as well as environmental damage. Together, these developments have created a demand for new insecticidal compounds to facilitate global food production. Arachnid venom peptides provide an environmentally-friendly alternative as potential bioinsecticides given their advantages of being fully biodegradable, highly potent, and phyletically selective. However, the use of arachnid venom peptides as bioinsecticides has been questioned due to their presumed lack of oral toxicity. Thus, the aim of this work was to develop screens for oral insecticidal activity. Based on the high susceptibility of dipterans to venom peptides, fruit flies (Drosophila melanogaster) and sheep blowflies (Lucilia cuprina) were selected for screening 56 arachnid venoms. 71.4% of these venoms caused 50% or higher mortality in Drosophila, whereas 30.4% were lethal to blowflies at oral doses of 1 or 30 μg/fly, respectively. We used these assays to compare the oral and injection activity of four well-known spider venom peptides (Hv1a, Hv1c, Dc1a and Ta1a). Hv1c and Ta1a only showed weak or no oral activity in both species, while Hv1a and Dc1a showed higher oral activity in blowflies than Drosophila. Overall, we have established screens for oral toxicity in two dipteran insects. Our results indicate that oral insecticidal activity is more widespread in arachnid venoms than expected, and that some arachnid venoms and venom peptides exhibit phyletic differences in oral toxicity.