ABSTRACT: Chemical communication is the oldest form of communication, spreading across all forms of life. In insects, cuticular hydrocarbons (CHC) function as chemical cues for the recognition of mates, species, and nest-mates in social insects. Although much is known about the function of individual hydrocarbons and their biosynthesis, a phylogenetic overview is lacking. Here, we review the CHC profiles of 241 species of Hymenoptera, one of the largest and most important insect orders, which includes the Symphyta (sawflies), the polyphyletic Parasitica (parasitoid wasps), and the Aculeata (wasps, bees, and ants). We investigated whether these taxonomic groups differed in the presence and absence of CHC classes and whether the sociality of a species (solitarily vs. social) had an effect on CHC profile complexity. We found that the main CHC classes (i.e., n-alkanes, alkenes, and methylalkanes) were all present early in the evolutionary history of the Hymenoptera, as evidenced by their presence in ancient Symphyta and primitive Parasitica wasps. Throughout all groups within the Hymenoptera, the more complex a CHC the fewer species that produce it, which may reflect the Occam's razor principle that insects' only biosynthesize the most simple compound that fulfil its needs. Surprisingly, there was no difference in the complexity of CHC profiles between social and solitary species, with some of the most complex CHC profiles belonging to the Parasitica. This profile complexity has been maintained in the ants, but some specialization in biosynthetic pathways has led to a simplification of profiles in the aculeate wasps and bees. The absence of CHC classes in some taxa or species may be due to gene silencing or down-regulation rather than gene loss, as demonstrated by sister species having highly divergent CHC profiles, and cannot be predicted by their phylogenetic history. The presence of highly complex CHC profiles prior to the vast radiation of the social Hymenoptera indicates a 'spring-loaded' system where the diversity of CHC needed for the complex communication systems of social insects were already present for natural selection to act upon, rather than having evolved independently. This diversity may have aided the multiple independent evolution of sociality within the Aculeata.
