SUMMARY The opilionid Acanthopachylus aculeatus was shown to produce a defensive secretion containing quinones (2,3-dimethyl-1,4-benzoquinone,2,5-dimethyl-1,4-benzoquinone and 2,3,5-trimethyl-1,4-benzoquinone),confirming the findings reported nearly a half century ago in a classic study. The mechanism by which the opilionid puts the secretion to use is described. When disturbed, the animal regurgitates enteric fluid, which it conveys by intercoxal clefts to the anterolateral corners of the carapace, where the two gland openings are situated. It then injects some of its quinonoid secretion into the fluid, and conveys the mixed liquid along the length of its flanks by way of two special channels. Such a discharge mechanism may be widespread among opilionids of the family Gonyleptidae (suborder Laniatores), to which A. aculeatus belongs. In a bioassay based on a scratch reflex in decapitated cockroaches ( Periplaneta americana) the liquid effluent of A. aculeatus was shown to be potently irritating. Use of the effluent was demonstrated to protect the opilionid against ants ( Formica exsectoides ). Wolf spiders ( Lycosa ceratiola ) were shown to be minimally affected by the effluent (they showed little response when the fluid was added to their mouthparts as they fed on mealworms, their normal laboratory prey), although they proved to be aversive to mere contact with the opiliond itself, and to reject the animal without inducing it to discharge. A. aculeatus may therefore contain distasteful factors besides its glandular products.