It would be no exaggeration to state that the tremendous abundance of insects constitutes the primary food source for diverse vertebrate and invertebrate predators. For insects in a variety of orders, blunting the attacks of their omnipresent predators is identified either with the production of defensive compounds in exocrine glands or with the acquisition of these compounds from external sources. These deterrent allomones sometimes represent novel natural products that have a very limited distribution in the Insecta. In short, exocrine compounds, characteristic of species in orders or genera, have evolved to function as versatile agents of chemical defense.
It has been generally assumed that de novo biosynthesis characterizes the origins of insect defensive compounds. However, recent investigations suggest that novel insect defensive allomones, including the complex amide pederin from staphylinid beetles (Paederus spp.) and unique steroids from dytiscid beetles, are biosynthesized by endosymbiotes. These results raise the question of whether other novel insect allomones, including cantharidin and steroids in chrysomelids and lampyrids, may have microbial origins.
Often, however, the deterrent allomones constitute ingested allelochemicals such as cardenolides (milkweeds) and toxic pyrrolizidine alkaloids (asters, heliotrope). Furthermore, some of these plant natural products have been metabolized after ingestion into products that are suitable for sequestration and use as deterrents, as for ingested steroids from milkweeds by the monarch butterfly, Danaus plexippus. These compounds are also transferred to eggs to function as effective predator deterrents. In addition, these allelochem-icals may be added to the secretions of exocrine glands, further increasing the deterrent properties of these exudates. The dependence on ingested plant natural products of some insect species is further emphasized by the utilization of "stolen" defensive exudates that essentially represent mixtures of pure plant allelochemicals that have been appropriated, unchanged, from the host plants.
In some species, ingested allelochemicals are sexually transmitted by the male as a copulatory "bonus" for the female. For example, the sperm-rich spermatophore of ithomiine butterflies is accompanied by pyrrolizidine alkaloids that provide protection for the female and her eggs. Importantly, this very adaptive system is functional because the spermatozoa are resistant to the well-known toxic effect of these alkaloids.
Some allelochemicals also possess great selective value for insects as antibiotic agents. Alkaloids such as a-tomatine, a constituent of tomatoes, reduce the infectivity of bacteria and fungi for lepidopterous larvae. Other compounds reduce the activity of viruses and in some cases are highly toxic to insect parasitoids.
Insects have adapted for defensive functions a variety of glands not identified as defensive organs. For example, salivary glands have been converted into defensive structures that deliver deterrent compounds biosynthesized in these glands. Even respiratory structures have assumed the role of deterrent organs as further testimony to the insect emphasis on defensive adaptations. For a variety of insect species, chemical defense is clearly identified with survival.
The defensive value of insect allomones has been further enhanced by the ability of these arthropods to adapt a variety of these natural products to subserve a surprising variety of multiple functions. This phenomenon, semiochemical parsimony, has been particularly emphasized by insect species such as fire ants, whose alkaloidal venoms possess a dazzling variety of pharmacological activities. The same may be said of can-tharidin, the potent vesicant from blister beetles (Spanish fly).
Things are seldom what they seem. The sting-associated glands of bees and wasps are obvious candidates for the production of compounds with considerable deterrent activities. These glands have evolved as biosynthetic centers clearly dedicated to the biogenesis of pharmacologically active compounds that can be delivered by the sting in an unambiguous act of defense. On the other hand, some glands clearly identified with nondefensive functions have been adapted by a variety of insect species to function as defensive organs with varied functions. Furthermore, the deterrent efficiency of these secretions may be considerably enhanced by adding repellent plant natural products to the exudate. And insects have not neglected adapting enteric products to discourage their omnipresent predators. If all else fails, many insects eject blood, sometimes fortified with toxic allomones, at their adversaries with startling results. It is no exaggeration to state that for these species, bleeding has often provided an extraordinary means of deterring a variety of aggressive predators.
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