Superhydrophobic cuticle with a “pinning effect” in the larvae of the iris sawfly, Rhadinoceraea micans (Hymenoptera, Tenthredinidae)

The integument of some sawfly larvae can be easily damaged causing haemolymph bleeding (easy bleeding phenomenon). In the present study, we investigated intact cuticle, cuticle without wax coverage and cuticle replicas of sawfly larvae by using cryo-scanning electron microscopy and contact angle (CA) measurements. The easy bleeder Rhadinoceraea micans was compared to the non-easy bleeder Nematus pavidus (both Tenthredinidae), and the examination showed that the surface structures of R. micans were hierarchically arranged, whereas the cuticle surface of N. pavidus appeared much smoother. The intact cuticle proved to be superhydrophobic in R. micans (CAs: 156° dorsally, 120° ventrally), but not in N. pavidus (CAs: 67° dorsally, 47° ventrally). The wettability of R. micans increased significantly after the waxes had been peeled off. Replicas of the intact cuticle surface were hardly wettable in both species. It is suggested that wax crystals in combination with cuticle microsculptures are the most important features for rendering the integument of the easy bleeder species superhydrophobic. The wax-free tips of the cuticle microsculptures in R. micans are sites where haemolymph droplets can adhere. The integument surface of easy bleeders is assumed to be adapted to their particular defence strategy, simultaneously contributing to the maintenance and “reuse” of released haemolymph droplets on the superhydrophobic surface due to this “pinning effect”.