Aposematic compounds

A prominent characteristic of most true bugs is their use of defensive chemicals produced in specialized scent glands, usually found in the abdomen in im-matures, and in the metathorax in adults. However, this pattern is not absolute species that feed on poisonous plants from which they sequester toxic chemical defenses tend to have reduced or modified glands [8,26-28]. Many of these species are also aposematic, vividly advertising their toxicity to would-be predators. The defensive chemistry of bugs has been the subject of a number of reviews [4,6,8,9,12,29,30] and will only be summarized here, with a focus on compounds with interesting or unusual chemistry. 

Defensive Compounds. The aposematically coloured Chauliognathusfallax which feed on Senecio brasiliensis (Asteraceae) sequester the four pyrrolizidine alkaloids senecionine (100 main compound), integerrimine (101 main compound), retrorsine 102, and usaramine 103 [203] (Scheme 11). Other Chauliognathus-species may contain either precoccinelline 104 and related alkaloids (C. pul-chelus) or Z-dihydromatricaria acid 105 (C. pennsylvanicus). 

Many aposematic lepidopteran insects are associated with poisonous plants and sequester the toxins from their host instead of, or in some cases in addition to, biosynthesizing their own defensive compounds. 

The aposematic beetle, Metriorrhynchus rhipidius, contains three pyrazines as warning odor components and two amides as bitter principles (Tables III, V, and VIII) (97). Of the three components with the beetlelike odor, the most characteristic is 2-methoxy-3-isopropylpyrazine (24b). The other two components are 2-methoxy-3-methylpyrazine (24a) and 2-methoxy-3-sec-butylpyrazine (24d). It would seem likely that these compounds will occur in the defensive systems of the aposematic beetles. The two amide components, detectable in the hemo-lymph exuded by adult beetles, are 3-phenylpropanamide (130) and l-methyl-2-quinolone (57), the latter being the major component. It seems likely that these bitter principles contribute to distastefulness to potential predators. 

Many of the studies reviewed in this chapter have focused on the meroplankton. However, little is known about ontogenetic shifts in concentrations and patterns of defense in marine invertebrate larval forms.40 Further work is needed to determine if, for a wider range of species, developing larvae are capable of secondary metabolite synthesis or if defensive compounds are derived directly from adults. While a number of studies have been conducted on chemical defenses in lecithotrophic larvae of benthic invertebrates, the database is still quite small for planktotrophic larvae. Additional carefully controlled studies of aposematism in marine invertebrate larvae are also needed to determine if there is indeed a general pattern of chemical defenses in conspicuously colored larvae. 

Because the butterfly carries warning coloration (aposematic labeling) that the bird learns to recognize quickly, only a fraction, about 50% of the insects, need to have the toxic compounds within their bodies for all of the insects to be protected (Brower, 1969). Both the plants and the insects differ in cardenolide content. Monarch butterflies do not accumulate each of the compounds in the plant equally well. Some cardenolides are absorbed intact, others are biochemically modified, and yet others are excreted. 

Many of the colors used for aposematic labeling by animals (bright reds, oranges, and yellows) are produced by carotenoid pigments (Rothschild, 1975, 1978). These compounds ultimately arise from plants (or from symbiotic bacteria ), as animals cannot synthesize carotenoids. 

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