Fig. 1. The true setae and a setae field (mirror) of O.  lunifer: (A) a seta is sharp at the proximal end facilitating penetration of the skin and mucous membranes of animals, and backward-pointing barbs allow forward only migration through the tissue as an animal moves; (B) the proximal ends of true setae are embedded into pits in the cuticle but not attached to the integumen

Fig. 2. Schematic maps of setae fields on typical examples of 3rd–8th instars of O. lunifer caterpillars. The pattern on 7th instar larvae is variable. Head capsule widths are approximate and typical for the given instar.

Fig. 3. Two size classes of the commonly-described true setae are found in each setae field of O. lunifer: A, compound micrograph of a clump of true setae removed from a setae field; (B) the proportion of short setae is greater on older instars

True setae borne on the abdominal tergites of Ochrogaster lunifer Herrich-Schӓffer caterpillars are the agents of an irritating contact dermatitis, osteomyelitis, ophthalmia, and severe allergic reactions in humans, and are the cause of Equine Amnionitis and Fetal Loss in Australia. The setae are detached and readily dislodge from the integument whereby they disperse throughout the environment. To better understand the true setae of O. lunifer as agents of medical and veterinary concern, we studied their characteristics and distance dispersed. Whereas members of the European Thaumetopoeinae have been widely studied, their southern-hemisphere counterparts such as O. lunifer are not well known despite their harmfulness and known medical and veterinary importance. The caterpillar’s investment in true setae increased with age and size, and two distinct size classes co-occurred in setae fields. A previously undescribed morphological type of true seta was found on the first abdominal segment. All true setae were calculated to travel long distances in the air even under light breeze conditions. Our results show there is a high risk of exposure to airborne urticating setae within 100 m of elevated caterpillar activity, and a likely risk of exposure for some kilometers in the direction of the prevailing breeze. This information should be used to inform management strategies in areas where urticating processionary caterpillars are active, and especially during periods of an outbreak.

Figure 4. Venom components of different caterpillar species and their role on human body targets in the pathophysiology. Caterpillar venoms contain pharmacologically active components that are able to interfere with targets in the normal human cellular physiology. Some components are responsible for the local effects such as inflammation, erythema, edema, intense itch, tissue damage, pain and may exert an allergic reaction. Others affect the hemostasis by acting on the coagulation cascade or on the fibrinolytic pathway. Venom components colored in green activate a step in the cascade, while the components colored in red are able to inhibit a step. In some cases, this can lead to the development of an acute kidney injury (AKI). The venom-induced AKI cascade in the figure is copied with permission from [50].