Chemical defense of an opilionid (Acanthopachylus aculeatus)

Abstract

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.

Chemical Defense in Harvestmen (Arachnida, Opiliones): Do Benzoquinone Secretions Deter Invertebrate and Vertebrate Predators?

“…The occurrence of alkylated 1,4-benzoquinones in G. longipes is chemotaxonomically in agreement with the constitution of the defensive secretions previously reported for other Gonyleptidae (Estable et al, 1955;Fieser and Ardao, 1956;Roach et al, 1980;Acosta et al, 1993;Eisner et al, 2004), including congeneric species, such as Goniosoma spelaeum (Gnaspini and Cavalheiro, 1998). The derivative 2,3-dimethyl-1,4-benzoquinone has been already identified in other harvestman species belonging to the families Cosmetidae, Gonyleptidae, and Manaosbiidae (Estable et al, 1955;Fieser and Ardao, 1956;Eisner et al, 1977Eisner et al, , 2004Roach et al, 1980;Acosta et al, 1993), but this is the first report of 2-ethyl-3-methyl-1,4-benzoquinone as a natural product. Moreover, the use of bidimensional nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry is reported for the first time for chemical characterization of substances from harvestmen, providing additional protocols for studies of scent secretions in the group.…”

“…The caustic, irritating, and odoriferous properties of these substances may cause invertebrate predators to desist from the attack and to perform vigorous cleaning activities (Eisner and Meinwald, 1966). Recently, Eisner et al (2004) showed that the secretion of the gonyleptid harvestman A. aculeatus (Pachylinae) was an effective deterrent against the ant Formica exsectoides. Using a different protocol, we here demonstrate that secretions containing the 1,4-benzoquinones produced by the gonyleptid G. longipes (Goniosomatinae) effectively repel five species of ants belonging to three subfamilies.…”

“…The ctenid Ctenus fasciatus is an important predator of gonyleptids in the wild (Gnaspini, 1996;Machado et al, 2000), and, therefore, it seems that the 1,4-benzoquinone secretions released by their prey have no effect on this spider. In captivity, individuals of the wolf spider Lycosa ceratiola (Lycosidae) were minimally affected by the effluent of the harvestman A. aculeatus, showing little response when the effluent was applied to their mouthparts as they fed on mealworms (Eisner et al, 2004).…”

“…However, the main function attributed to the scent secretion is defense against predators (review in Holmberg, 1986). Laboratory observations have shown that ants are repelled by the secretion released by the cosmetid Vonones sayi (Eisner et al, 1971), the leiobunine Leiobunum formosum (Blum and Edgar, 1981), and the gonyleptid Acanthopachylus aculeatus (Eisner et al, 2004). Anecdotal observations indicate that some spiders may be also deterred by harvestman secretions (e.g., Cloudsley-Thompson, 1958;Juberthie, 1976), but others are not (e.g., Bristowe, 1949;Immel, 1954).…”

“…Anecdotal observations indicate that some spiders may be also deterred by harvestman secretions (e.g., Cloudsley-Thompson, 1958;Juberthie, 1976), but others are not (e.g., Bristowe, 1949;Immel, 1954). Recently, Eisner et al (2004) demonstrated that the scent gland secretion of the harvestman A. aculeatus was unable to deter the attack of a lycosid spider. The effectiveness of the scent gland secretion against vertebrates is also controversial because some species of frog seem to present a strong aversive response (e.g., Edgar, 1971), whereas some mammals feast on harvestmen (e.g., Pelegatti-Franco and Gnaspini, 1996).…”