Unraveling the Sexual Dimorphism of First Instar Nymphs of the Giant Stick Insect, Cladomorphus phyllinus Gray, 1835, from the Atlantic Forest, Brazil

SIMPLE SUMMARY: Members of the order Phasmida are popularly known as walking stick insects. They have remarkable camouflage, resembling moss, sticks, and leaves. Cladomorphus phyllinus is a giant stick insect from the Atlantic Forest of Brazil. Described in 1835 by George Robert Gray, the species is widely distributed in the wild, but several aspects of its biology and ecology remain to be studied. An inedited description of the first instar nymphs of C. phyllinus is presented, showing clear sexual dimorphism in the thorax and distinct abdominal sternites. A sex-specific suture was identified in the metanotum of males but was absent in females. Differentiation of the last three abdominal segments, which are already shaped to form the copulatory apparatus in both sexes, was also recorded and illustrated. Such sexual dimorphism is rare in first instar insect nymphs. Therefore, this study uncovered new characteristics that allow more precise data generation from samples collected in the field for conducting experiments, recording specimens in entomological collections, and improving the species concept and knowledge of sexual dimorphism in C. phyllinus. ABSTRACT: The first instar nymphs, both male and female, of the giant stick insect Cladomorphus phyllinus Gray, 1835 were carefully described and measured, revealing a remarkable sexual dimorphism that is considered rare among insects and is poorly explored in the order Phasmida. The studied F1 nymphs originated in captivity from eggs laid by a coupled female specimen collected in the Atlantic Forest in the vicinity of Petrópolis city, state of Rio de Janeiro, Brazil. The first instar nymphs of C. phyllinus were measured and illustrated in high-resolution photographs to show the general aspects and details of sexually dimorphic traits, making clear the phenotypic differences in the sexes. A total of 100 nymphs were kept alive until morphological sexual dimorphism was confirmed and quantified. All recently hatched first instar nymphs were separated based on the presumed male and female characteristics, i.e., the presence and absence of the suture in the metanotum in the males and females, respectively, had their sexes confirmed in 100% of the specimens as previously assigned. These results confirm this new morphological trait, which here is named “alar suture” as sex-specific in the first instar nymphs, a novelty in this stage of development of sexual differentiation. In addition, the distinct conformations of the last three abdominal sternites of both sexes were recorded.