The AqE gene encodes a sulfolactate dehydrogenase-like enzyme of the LDH2/MDG2 oxidoreductase family. The gene is found in bacteria and fungi, as well as in animals and plants whose lifestyles are associated with aquatic environments. The AqE gene is present in arthropods and, in particular, insects that are predominantly terrestrial. The distribution and structure of AqE was studied in insects in order to trace its evolutionary fate. The AqE gene was found to be absent from certain insect orders and suborders, being apparently lost. AqE duplication or multiplication was observed in some orders. AqE was found to vary both in length and intron-exon structure, from intronless to multi-intron. An ancient nature was demonstrated for AqE multiplication in insects, while younger duplications were also detected. It was assumed that a new function might be acquired by the gene with the formation of paralogs.