Chemistry, ontogeny, and role of pygidial gland secretions of the vinegaroon Mastigoproctus giganteus (Arachnida: Uropygi)

Pygidial gland secretions of the vinegaroon, Mastigoproctus giganteus, consist of 26% water and 74% short-chained fatty acids, with acetic acid and octanoic acids the dominant acids. The organic composition of the secretion consists of 83% acetic acid, 15% octanoic acid, 1% (Z)-5-octenoic acid, 0.0002% of the corresponding (E)-isomer, 0.11% heptanoic acid, 0.02% hexanoic acid, 0.007% decanoic acid, and in some individuals a trace of 2-nonanone. Formic acid is absent from the secretion. Glandular composition, especially in the concentrations of the minor components, varies dramatically among individuals and shows no age pattern differences among the four free-living subadult instars, or sex differences between adult males and females. Each individual exhibits its own specific blend of components which is identical in the two separate and different sized glands. The secretions act solely as allomones against potential predators, and do not act as pheromones, or elicit a communicative or behavioral response. Behavioral tests using live animals or a synthetic blend of vinegaroon secretion revealed that the secretion is maximally effective on sensitive respiratory, visual, or sensory systems of potential predators and is ineffective, or nearly so, when contact is restricted to inside the predatorʹs mouth. Thus, the effectiveness of the secretion apparently results from its delivery as a spray of fine droplets, rather than as an exudate or vapor.