Singh-Pruthi (1925) defined the ejaculatory reservoir as “an expansion of the ejaculatory duct at the base of the vesica [=aedeagus] present in Heteroptera.” After carefully examining the external male genitalia of Dinidoridae, Scutelleridae, Tessaratomidae, and Cydnidae, we found that Singh-Pruthi’s definition could not be satisfactorily applied to the above-mentioned families. Therefore, it is necessary to redefine the ejaculatory reservoir in the Pentatomomorpha. We found that the structure protrudes from the dorsobasal portion of the expansion of the ejaculatory duct within the apical one-half of the aedeagus. We redefine that part of the structure outside the base of the aedeagus as the “ejaculatory reservoir” and the other part within the aedeagus, including the expanded ejaculatory duct, as the “reservoir lumen.” There are three differences between our definition and that of Singh-Pruthi for the ejaculatory reservoir: 1) the position (an evolutionary event) within the middle of the aedeagus versus at the base of the aedeagus, 2) the nature of the structure protruding from the expanded ejaculatory duct versus an expansion of the ejaculatory duct, and 3) the presence in only Pentatomomorpha versus in Heteroptera.