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Communication via chemical signals and cues is a widespread modality in animals. Producing, transmitting, receiving and processing chemical compounds impose important challenges. Nonetheless, certain arthropods rely almost exclusively on this channel for intra- and inter-specific communication. Through a preliminary literature review, I summarize here the morphological, behavioral and evolutionary implications of chemical communication in harvestmen (the arachnid order Opiliones), with particular emphasis in one group: the Eupnoi or “Daddy Long-legs”. This group has a unique secretory gland that opens in an ozopore in their dorsum. While relying mostly on short-range olfaction and contact chemoreception using different setae, some harvestmen are known to use chemicals in defense, alarm, spatial marking, recruitment, or reproduction. I then propose future research direction on the mechanisms of production and the evolutionary history of these traits. Specific questions can include (1) are chemical signals used as alarm pheromones in Eupnoi aggregations? (2) Do harvestmen rely on chemicals to mark their traditional roosting sites? If so, what are the chemicals involved and how do those differ from the ones used as alarm signals? Also, (3) what are the specific functions of the chemical communication during in female-male communication and multi-specific roosting aggregations? And (4) to what extent the use of chemical defenses imposes a trade-off with other defense mechanisms such as ‘playing dead’ or voluntarily releasing legs. Overall, these harvestmen provide a unique opportunity to test comprehensive and interdisciplinary hypotheses to understand the evolution of chemical communication, as well as the importance of chemical ecology on species diversification in arthropods.