Chemical defense of an opilionid ( Acanthopachylus aculeatus )

Naturalists have long known that chemical defenses are commonplace in arthropods. Most often these defenses take the form of dischargeable integumental sacs, laden with irritants or repellents. Knowledge of these glandular structures was initially restricted to their anatomy and histology. It was only in the past five decades that efforts focused on the function of the glands and on the chemical elucidation of their products. An early contribution to this area of research was the study of an opilionid, Acanthopachylus aculeatus, initiated by Clemente Estable and María Isabel Ardao in Uruguay, where the arachnid is native, and involving eventually collaboration with Louis Fieser of the Chemistry Department of Harvard. The study, which stands as a landmark in the history of arthropod exocrinology, led to the isolation of three benzoquinones: (1) 2,3-dimethyl-1,4-benzoquinone, (2) 2,5dimethyl-1,4-benzoquinone and (3) 2,3,5-trimethyl-1,4benzoquinone, not previously known from nature (Estable et al., 1955; Fieser and Ardao, 1956). While it was assumed all along that the secretion of A. aculeatus was defensive, no proof to that effect had been obtained, nor had the animal’s glandular discharge mechanism been described. We have recently had occasion to study live A. aculeatus in the laboratory. We were able to confirm the earlier claim that the secretion is quinonoid in nature, and in addition obtained detailed photographic documentation of the glandular discharge mechanism, and proof that the secretion has defensive potency. We also obtained evidence that the opilionid might produce additional, as yet unknown chemical factors that contribute to its unacceptability to spiders. A. aculeatus belongs to the suborder Laniatores of the Opiliones. Quinonoid secretions may be of common occurrence in this suborder, as one might conclude from a number of chemical investigations (for references, see Acosta et al., 1993). The discharge mechanisms of the glands are not precisely the same in the different quinone-producing Laniatores. However, the mechanisms have one feature in common, namely that the quinones, upon discharge from the glands, are mixed with a diluent, in the form of regurgitated enteric fluid. Such preparative mixing of a glandular defensive product with gut fluid appears to be without parallel in other animals. The first Laniatores species in which the glandular discharge mechanism was described is Vonones sayi (Eisner et al., 1971). When this animal is disturbed, it promptly emits a droplet of fluid from the mouth. The fluid does not remain in place, but seeps almost instantly to the margins of the body along two The Journal of Experimental Biology 207, 1313-1321 Published by The Company of Biologists 2004 doi:10.1242/jeb.00849