A simple device was used to quantify changes in the mechanical properties of the cuticle of Rhodnius prolixus and Triatoma infestans that take place when these insects feed, i.e., plasticization. Different stimuli were presented for 1min to test for their ability to trigger plasticization. These were: a blood meal, a Ringer solution meal, contact with a warm surface and thermal stimulation without such contact. Our results supplant any previous hypotheses that have supposed that the presence of food in the alimentary tract is necessary to evoke plasticization. We find that mere contact of the proboscis with a warm surface (without any food intake) is sufficient to trigger plasticization indistinguishable from that produced by a blood meal. Thermal stimulation alone, i.e., without physical contact, was not effective.