Kissing bugs, which feed on vertebrate blood, are thought to have evolved from the so-called assassin bugs. The assassin bugs feed on and kill other bugs. Dr. Dorn is a contributing author of a recent article testing if kissing bugs can still survive feeding only on other bugs (specifically the hemolymph, the equivalent of blood in insects). Surprisingly, one of three species tested, the largest in the U.S. (Triatoma recurva), can survive to adulthood feeding just on cockroaches! Its development is delayed, and some of the T. recurva tested lived much longer that the ones that were blood-fed. So it appears that some species of kissing bugs can survive utilizing the ancestral behavior of feeding just on other insects. 

Schmidt, J., P.L. Dorn, S. Klotz. "Second-best is better than nothing: Cockroaches as a viable food source for the kissing bug Triatoma recurva (Hemiptera: Reduviidae)" J. Med. Entomol. (2018). To read the full article, click here.