Digestion in the arachnid Mischonyx squalidus as a probable source of lipids to synthesize opiliones defense and communication molecules

Opiliones (Arachnida) comprises approximately 7,000 species. Due to their lack of venom glands, their defense is mainly based on the secretion of scent glands, which also play roles in communication and antimicrobial activity. This odoriferous secretion has a diverse composition according to harvestmen species but frequently contains benzoquinones. Studies of benzoquinone synthesis intermediates suggest a pathway based on lipid metabolism from diet. This study provides the first transcriptomic and enzymatic analysis of the midgut of Mischonyx squalidus to understand the acquisition of nutrients. The enzymatic analysis tested 11 digestive enzymes and found high lipase activity and moderate and low activity for peptidases and carbohydrases, respectively. Transcriptome sequencing yielded 19,658 unigenes, predominating enzymes and binding proteins closely associated with Xiphosura and Scorpiones proteins; a third of which are hydrolases. A multigene lipase family is expressed. Cathepsin L peptidases were prominently abundant, indicating their relevance for protein digestion. Additionally, toxin-like proteins and lipid metabolism-related enzymes, such as phospholipase A₂ and NPC cholesterol transporters, were represented, indicating a sophisticated digestive and metabolic system for lipids. These findings suggest a link between lipid digestive metabolism and defensive and communicative molecule synthesis, underscoring Opiliones' unique evolutionary adaptations.