Four African species of true water bugs (Nepomorpha: Hemiptera: Heteroptera) are studied by mass spectrometry and biological assays to gain information on the presence, structure and function of peptides from the adipokinetic hormone (AKH) family, which are produced in the corpora cardiaca (CC). The water scorpion Laccotrephes fabricii Stål (Nepidae) has the peptide code-named Peram-CAH-I with the sequence pGlu-Val-Asn-Phe-Ser-Pro-Asn-Trp amide, whereas Appasus grassei Poisson (Belostomatidae) produces Anaim-AKH, which is a Ser7 analogue of Peram-CAH-I (pGlu-Val-Asn-Phe-Ser-Pro-Ser-Trp amide). The giant water bug Hydrocyrius columbiae Spinola (Belostomatidae) has two adipokinetic hormone family members: Anaim-AKH and Letin-AKH, which again differ only at position 7 (Ser7 versus Tyr7). When the sequence data are compared with current molecular phylogenetic analyses of Nepomorpha, they are essentially in agreement with the newest ideas on phylogenetic relationships among the families. Functional investigation of these peptides reveals a mainly lipid-based energy metabolism in these insects, as demonstrated by a hyperlipaemic response after injecting crude CC extract or the appropriate peptide into the respective species. The carbohydrate concentration in the haemolymph is not affected by such injections, and the carbohydrate level in most cases is lower than that of the circulating lipids. During physical exercise, such as swimming for 1 h, carbohydrates may contribute to some extent to the provision of energy; the substantial increase in the concentration of lipids in the haemolymph, however, is a strong indicator that the peptides are released from the CC and act primarily as true adipokinetic hormones during this period of intense muscular activity.