Chemical defense is crucial for invertebrates such as beetles and lepidopteran larvae which are preferential prey of many higher organisms. Consequently, arthropods evolved a broad variety of defense compounds which serve them for passive and active defense.
Ground-living beetles usually cannot escape attack of predators even though many are able to fly. But beetles cannot fly right away because they need time to unfold their wings. Consequently, beetles have developed a variety of chemical defense mechanisms to repel attackers (Figure 2).
Many beetles make use of abdominal glands containing dischargeable defensive cocktails that can be sprayed highly aimed against predators. High concentrations of simple chemicals such as formic acid 2 or acetic acid 3 efficiently irritate the enemy and thus allow an escape.
Bombardier beetles defend themselves in a similar way. They discharge irritating 1,4-benzoquinone 4 under high pressure and at temperatures of around 100 °C. The beetle produces 1,4-benzoquinone in a specialized reaction chamber containing a catalase and a peroxidase. H2O2 and hydroquinone are supplied from a reservoir chamber. The catalase generates O2 from H2O2 and the peroxidase oxidizes the 1,4-hydroquinone to 1,4-benzoquinones in an exothermal reaction.
Like plants, some beetles use alkaloids as defense compounds. The alkaloids are either emitted upon danger by reflex bleeding or are stored in glandular hairs covering the body, so that the attackers are exposed to the deterrent chemicals when they touch the beetles. For example, the seven-spotted ladybird beetle, a Coccinellidae beetle, uses the tricyclic coccinelline 5 as a powerful antifeedant. Coccinelline 5 tastes bitter and efficiently repels both
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