Enlarged hind femora as active defensive weapons in Cretaceous stick insects

Background Due to the scarcity or incomplete preservation of structures, fossils that can be compared with extant insects and directly reflect the evolution of survival strategies are extremely rare. Extant stick and leaf insects are renowned for their remarkable plant mimicry (mimesis), yet their active defensive behavior is rare and its origin and early evolutionary history remain unclear. Results We report two new stick insect species, Coniphasma inflatum sp. nov. and C. breve sp. nov. of the family Susumaniidae, from the Early Cretaceous Jehol Biota (~ 125 Ma) of China. Both species have enlarged hind femora that are significantly larger than those of other known species in this group, indicating the function for active defense. Moreover, besides the irregular color markings on their well-developed wings and enlarged femora that aid in camouflage, the legs of these two species are armed with prominent longitudinal carinae bearing rows of denticulate notches, suggesting the enhancement of active defensive strategy employed after camouflage has failed. Conclusions These fossils demonstrate that the origin of hind femora enlargement probably predates wing regression in Phasmatodea. This discovery suggested that the highly diverse morphological characters observed in living stick insects were already established by the Mesozoic, and these features have evolved independently multiple times within Phasmatodea.