ABSTRACT
Arthropods and insects as largest groups of organisms with respect to species numbers or biomass contain an incredible number of biologically active low and high molecular compounds (Pietra 2002, Gronquist and Schroeder 2010). When taken by humans these compounds may have medicinal, intoxicating (venoms, toxins), performance enhancing or many other effects and therefore are called drugs. A large fraction of commercially available drugs represent natural products or represent derivatives of natural products (Dettner 2011), wherefore the search for such compounds is very important (Cragg et al. 2012, Tringali 2012). Very often these chemicals are toxic or may deter other animals and are therefore called allomones (defensive compounds) which are advantageous for the sender and disadvantageous for the receiver. With respect to predator prey interactions there exist various antipredator mechanisms in insects. Primary defenses are active before predators perceive prey (passive defenses such as mimicry, crypsis), secondary defenses only work after the predator has discovered its prey. As shown by Witz (1990) secondary defensive mechanisms are more important as compared with primary defensive mechanisms. Among secondary defense mechanisms chemical
Chair of Animal Ecology II, BayCEER, University of Bayreuth, 95440 Bayreuth, Germany. Email: k.dettner@uni-bayreuth.de
defenses are most important as compared with mechanical defense, defensive stridulation or escape (Eisner 1970, Blum 1981, Eisner 2003, Eisner et al. 2005, Dossey 2010, Unkiewicz-Winiarczyk and GromyszKalkowska 2012). These insect natural products usually are produced by the arthropods themselves (Bradshaw 1985, Morgan 2004), in many cases however such compounds originate from dietary plants or animals or might be even produced by symbiontic microorganisms (Pankewitz and Hilker 2008). In many cases the functional signifi cance of these biologically active molecules within insects is unknown. In addition microorganisms isolated from insects may produce biologically active metabolites in the laboratory whereas these products can be never detected within the host insect. Table 1 compiles all insect orders according to Beutel et al. (2014) and their ways of chemical defense. However in this review the term insects includes also entognathous orders which are placed outside of insects by Beutel et al. (2014). Chemical defenses are obviously seldom in primitive and hemimetabolous insects, in contrast especially large holometabolous orders are characterized by a variety of very different chemical defensive mechanisms (Eisner 1970, 2003, Hilker and Meiners 2002, Eisner et al. 2005).
