Other Types

Harborne (Ecological Biochemistry, pp. 98-100, see Further Reading) lists 12 types of plant toxin sequestered by insects, including the cardiac glycosides, veratrine and pyrrolizidine alkaloids, cyanogenic glycosides, and glucosinolates already considered. The others are aristolocic acids, quinolizidine alkaloids, iridoids (Chapter 7), and phenols. Nishida {Annual Review of Entomology, see Further Reading) lists 17 types for Lepidoptera alone. Even so, insects have only harnessed a small fraction of the hundreds of types of plant toxins that they can encounter.

Another example of partial modification in the insect is found with the beetle Diabrotica speciosa. It feeds normally on plants of the Cucurbita-ceae (cucumbers) which contain bitter triterpenoids (Chapter 7) called cucurbitacins. D. speciosa when fed on [14C]-cucurbitacin B converts it by removing the C-25 acetate and reduces the side-chain double bond to give dihydrocucurbitacin D (Figure 10.17). Other species raised on diets free of cucurbitacins, when fed cucurbitacin D, were able to glycosylate it (at the 2-OH), hydrogenate, desaturate and acetylate it.

The water beetles of the Dytiscidae family convert dietary sterols {e.g. cholesterol, see Chapter 7) to mammalian hormones like oestrone and testosterone, but most interesting is the cortisone-type cortexone (Figure 7.10). A Dytiscus beetle can contain as much as 1 to 3 mg of cortexone, which it can discharge from glands if attacked by a predator.

A cantharid beetle, Cauliognathus lecontei, feeding on Compositae that contain acetylenic esters, has been found to contain dihydromatri-caria acid (Figure 10.18), the first acetylenic compound found in insects, and which the beetles use as a defensive secretion.

Figure 10.14 Males of the Oriental fruit fly Bactrocera dorsalis feeding on a flower of Fagraea berteriana to obtain dimethoxycinnamic alcohol and acetate (Photo R. Nishida. Reproduced from Fig. 1 in J. Chem Ecol., 1997, 23, p. 2277, 'Acquisition of female-attracting fragrance by males of oriental fruit fly from a Hawaiian Lei flower Fagrea berteriana' by Nishida, Shelley and Kaneshiro. By kind permission of Kluwer Academic Publishers)

Figure 10.14 Males of the Oriental fruit fly Bactrocera dorsalis feeding on a flower of Fagraea berteriana to obtain dimethoxycinnamic alcohol and acetate (Photo R. Nishida. Reproduced from Fig. 1 in J. Chem Ecol., 1997, 23, p. 2277, 'Acquisition of female-attracting fragrance by males of oriental fruit fly from a Hawaiian Lei flower Fagrea berteriana' by Nishida, Shelley and Kaneshiro. By kind permission of Kluwer Academic Publishers)

ch3o s-

ch3o

(£)-coniferyl alcohol

Figure 10.15 The production of the sex attractant coniferyl alcohol by fruit flies from dimethoxycinnamic alcohol and acetate from flowers

The lampyrid beetles (fireflies, Chapter 9) produce lucibufagins (Figure 10.18) to make themselves unpalatable to predators. These substances come from dietary sterols, and therefore represent a group of more highly metabolized compounds. They are related in structure to the cardiac glycosides and toad poisons.

Figure 10.16 A thin layer chromatogram, origin arrowed right, of a solvent extract of the Fagraea berteriana flowers was developed with benzene-ethyl acetate (2:1). The solvent front, arrowed, is on the left. Male B. dorsalis flies were then allowed to gather on the plate to show where the active substances were located

(Photo R. Nishida. Reproduced from the same source as Figure 10.14 with the kind permission of Kluwer Academic Publishers)

Figure 10.16 A thin layer chromatogram, origin arrowed right, of a solvent extract of the Fagraea berteriana flowers was developed with benzene-ethyl acetate (2:1). The solvent front, arrowed, is on the left. Male B. dorsalis flies were then allowed to gather on the plate to show where the active substances were located

(Photo R. Nishida. Reproduced from the same source as Figure 10.14 with the kind permission of Kluwer Academic Publishers)

cucurbitacin B

metabolized

dihydrocucurbitacin D

metabolized cucurbitacin B

dihydrocucurbitacin D

Figure 10.17 An example of a triterpenoid slightly modified and sequestered, dihydrocucurbitacin D in chrysomelid beetles

^cooh dihydromatricaria acid

dihydromatricaria acid

romallenone romallenone

Figure 10.17 An example of a triterpenoid slightly modified and sequestered, dihydrocucurbitacin D in chrysomelid beetles

lucibufagins

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