Toxic associations: A review of the predatory behaviors of millipede assassin bugs (Hemiptera: Reduviidae: Ectrichodiinae)

Millipedes (Diplopoda) are a diverse group of arthropods that include 16 orders represented by approximately 12,000 species in 145 families (Sierwald & Bond, 2007). In 11 of these orders, millipedes are protected from predators by chemical defenses produced in glands that vary in number among orders and are located laterally or mid-dorsally in the diplosegments (Eisner et al., 1978; Hopkin & Read, 1992). The major defense components of these secretions seem to exhibit a phylogenetic pattern: Julida, Spirostreptida, and Spirobolida (Superorder Juliformia) produce benzoquinones; Callipodida and Chordeumatida produce phenols; Glomerida and Polyzoniida produce alkaloids and quinazolinones; Polydesmida produce cyanogenic compounds; and Platydesmida, Siphonocryptida, and Siphonophorida produce terpenoids (Eisner et al., 1978; Hopkin & Read, 1992; Sierwald & Bond, 2007). Despite such defenses, many vertebrates prey on millipedes (Sierwald & Bond, 2007). In addition, some invertebrates are specialized predators or parasitoids on juliform millipedes (Banks, 1911; Picard, 1930; Lawrence, 1984; Eisner et al., 1998; Dejean et al., 2001; Brunke et al., 2009; Larsen et al., 2009). None of these groups of millipede specialists are particularly speciose. This is dramatically different in the Ectrichodiinae, the fifth largest subfamily of Reduviidae (Hemiptera: Heteroptera), or assassin bugs, that appears to be specialized on millipedes (Green, 1925; Haridass & Ananthakrishnan, 1980; Haridass, 1985). The circumtropical millipede assassin bugs comprise >660 species in 123 genera (Maldonado, 1990; Dougherty, 1995; Carpintero & Maldonado, 1996; Weirauch et al., 2009; Chlond, 2010). Species have been collected while feeding on millipedes, but details of the predatorprey relationships, including prey specificity and point of mouthpart insertion are largely undocumented. Thus, it is unknown if Ectrichodiinae target the millipede’s nervous system to maximize the impact of their toxic saliva. It is also unknown if Ectrichodiinae target the body regions that possess defensive glands to sequester their own defensive compounds, or if they avoid the millipedes’ defense glands. In addition, predation in Ectrichodiinae may be a communal activity (Haridass & Ananthakrishnan, 1980; Haridass, 1985) rather than a solitary behavior as usually observed in Reduviidae (Readio, 1927; McMahan, 1983; Li et al., 2010), but details on this unusual subsocial behavior are scarce. Here, we compile published data and evaluate images available on the Internet for prey specificity, point of stylet insertion, and communal predation. In addition, data for Rhiginia cinctiventris (Stål, 1872) are presented based on observations in the field and laboratory at one site in Costa Rica; feeding observations have never been reported for this genus before. Literature and original data were analyzed in response to three questions: (1) Does the literature data suggest a pattern of Ectrichodiinae prey preference for species in particular millipedes orders? (2) Assuming that Ectrichodiinae either target or circumvent the millipede’s defense glands, where do they insert their mandibular and maxillary stylets to inject toxic saliva and digestive enzymes? (3) Are communal predation strategies commonly observed in nymphs and adults? REVIEW Eur. J. Entomol. 109: 147–153, 2012 http://www.eje.cz/scripts/viewabstract.php?abstract=1691 ISSN 1210-5759 (print), 1802-8829 (online)