The importance of being heterogeneous: the complex phase behaviour of insect cuticular hydrocarbons

Global warming dramatically increases desiccation risk for insects. To avoid water loss, terrestrial insects are covered with a thin, chemically complex layer of cuticular hydrocarbons (CHCs), which also serve as communication signals in many species. Effective protection against desiccation depends on the phase behaviour of this layer. However, little is known about CHC phase behaviour, how it changes with temperature and how insects adjust it to cope with changing temperatures. We addressed these questions, studying the viscosity of CHCs from two European ant species using microrheology. Here, the CHC layers consisted of a gel phase and a liquid phase. The liquid phase itself was heterogeneous, characterized by a bimodal distribution of local viscosities. The higher viscosity mode had a viscosity similar to honey (600-1000 mPa s), while the lower viscosity mode resembled olive oil (100-150 mPa s). The bimodality suggests that each mode may fulfil a distinct function, thereby enabling the simultaneous execution of otherwise conflicting functions. Furthermore, ants kept under warm conditions showed a significantly more viscous CHC layer, tentatively suggesting that CHC composition is adjusted to maintain a homeostatic viscosity and minimize water loss. These insights contribute to our understanding of the physical mechanisms that underlie the biological functions of the CHC layer.