The teratogenic role of two short-chain unsaturated fatty acids, octinoic acid and undecylenic acid on the hemimetabolic metamorphosis of the insect Rhodnius prolixus (Hemipter) is studied. The acids penetrate through the cuticle of the abdomen and tarsi, independently of the amount of distention. The effects are registered equally in satiated or hungry insects, in those treated topically or in those where the treatment was applied to the support paper. The acids apparently do not affect the formation of the cuticle, melanization, nor the metamorphic process. The damage induced by these acids are manifested at random in the locomotor as well as the cephalic appendices, a displacement of the proboscide being observed as the dosage is increased. Octinoic acid acts as a teratogen at doses of undecylenic acid which are lethal for the insect. The malformations of the proboscide include the labium the most dramatically damaged, as well as other bucal appendages, separately or accompanying the damage to the labium. Damage in the locomotor appendages is frequently displaced to the second and third pair of legs, while the first pair is the least affected.