Integrated morphological analyses of Cladomorphus phyllinus and transcriptomic analysis of Cladomorphus trimariensis provide insights into the cardiac morphophysiology of stick insects (Phasmida: Phasmatidae).

The heart of Phasmida (stick insects) remains poorly understood, particularly regarding its overall physiology and associated cells. Here, we integrate the morphology of the heart and associated cells in Cladomorphus phyllinus with transcriptomic profiling of corresponding structures in Cladomorphus trimariensis. The heart is a tubular structure composed of cardiomyocytes physically associated with alary muscles, with a location and structure similar to that described in other insects. Numerous pericardial cells are organized in cord-like aggregations around the heart, displaying abundant membrane invaginations and an extensive endocytic and lysosomal machinery, consistent with their role in hemolymph filtration. Given their abundance, these cells likely represent the main source of the numerous transcripts related to endocytosis, vesicular trafficking and lysosome-mediated digestion. Hemocytes are also present around the heart, often forming aggregations indicative of immune responses. The transcriptomic analysis indicates immune activity, including melanization and antibacterial defense, predominantly driven by the hemocytes, with the pericardial cells providing a supportive role. Our results confirm that heart and associated cells act not only in hemolymph propulsion, but also as a site involved in maintaining hemolymph homeostasis and immune defense. This work provides an integrated morphological and functional framework for the circulatory system of Phasmida, expanding comparative perspectives across insects.