Food Relations

Important to their ecological roles are the kinds of food and feeding habits exhibited by insects (Brues 1946, Cummins 1973). A majority, herbivores (or phytophages), act as primary reducers of plant life (d'Araujo y Silva et al. 1967-68, Guagliumi 1966, Martell 1974, Martorell 1976), including a relatively few saprophages that live only on dead plant material such as dead wood. Necrophages specialize on animal carrion, and coprophages utilize animal feces. Higher on the pyramid is another large and highly competitive group, the predators, which catch, kill, and consume other living animals, most often other insects (Clausen 1962) but sometimes even healthy vertebrates (Formanowicz et al. 1981, Hayes 1983). (Body toxins sometimes protect anurans from predation by spiders; the latter may even react negatively to apose-matic coloration of this potential prey [Szeli-stowski 1985].) Modifying the strategy of predation are the parasites, which rob food (including blood) from hosts on or within whose bodies they intimately live without causing their death, and parasitoids, which live like parasites but eventually kill the host. The latter two types are not always clearly separated from each other (Askew 1971, Price 1975). There are also a variety of other narrow feeding specialists on odd nutrient sources, such as hair follicle secretions (hair follicle mites), beeswax (wax moths), cultured fungi (gardening ants), and many others. Whole guilds may be tied to a particular vertebrate host, a good example being the arthropod associates of sloths (Waage and Best 1985). Klepto-parasites and social parasites steal unguarded prey from spider webs or wasp food caches. Many insects combine such habits and are considered omnivores.

Herbivory and saprophagy are vastly complex feeding practices because of the variety of plant types and anatomical parts available as food for insects, including leaves (Ernest 1989), stems, buds, seeds, and fruits (Strong et al. 1984). Species range from highly restricted in their choice of species or part (host specific) to catholic in their tastes (generalists). There are insects that graze on diatoms and algae from films on rocks in streams, lichen browsers, and fern feeders (Forno and Bourne 1984, Hendrix 1980), in addition to those that eat angiosperms generally, both in aquatic (Cummins 1973) and terrestrial (Zimmerman et al. 1979) environments.

Living, dead, and even decomposing (Winder 1977) matter is utilized as food. The greatest availability to herbivores is found in forests, especially tropical rain forests, where the number of plant species is prodigious. Some general studies in this venue are those comparing herbivore damage in riparian versus dry forests (Stanton 1975), the effects of seed predation in determining tree distribution (Janzen 1970), and host specificity (Pipkin et al. 1966). The importance of insect reducers of dead wood and its conversion to soil in forests cannot be overstated (Moron 1985).

The superabundance of plants for herbivores would seem to allow enormous explosions of insect numbers, but the latter are rare events that are often associated with human perturbations, such as misuse of insecticides and monoculture. Natural factors limiting expansions of plant-eating insects are parasitism, predation, and un-palatability (from both physical and chemical deterrents), leading to intraspecific competition. There is evidence that the latter may be increased by the plant in direct response to insect attack (Wratten et al. 1988).

A special category of herbivores are the gall makers. Many flies, wasps, and mites, including entire families in these orders, the Cecidomyiidae, Cynipidae, and Erio-phyidae, respectively, stimulate their plant hosts to form abnormal neoplastic growths called galls (Ananthakrishnan 1984). These take many forms and occur on all parts of the plant and are often harmful (Fernandes 1987). They serve the developing stage of the insect as a nutrient-rich, abundant, and reliable food supply. The mechanism of gall formation is not fully understood and varies among the groups involved. Very little is known of the galls of Latin America (Occhioni 1979).

Leaf miners (Hering 1951) form another specialized group of herbivores. It is the larvae of several families of Diptera (Agro-myzidae, Tephritidae, Anthomyiidae) and Lepidoptera (Nepticulidae, Tischeriidae, Lyonetiidae, etc.), in particular, that have adapted to the confined microhabitat between the upper and lower epidermal tissues of leaves. Understandably, their chief morphological characteristics are small size and a compressed shape. Appendages are reduced or absent, although the mouth-parts are well developed. The same is true of borers, in wood and other tissues, found among the Coleoptera (Cerambycidae, Buprestidae) and Lepidoptera (Pyralidae, Castniidae, Cossidae).

Necrophagous insects occur chiefly in the orders Coleoptera and Diptera (Jiron and Cartin 1981). Several beetle families feed entirely on dead vertebrate animals or their skins and hair, such as the carrion beetles (Silphidae) and dermestids (Der-mestidae). Important carrion fly groups are the blowflies (Calliphoridae) and flesh flies (Sarcophagidae). The succession of communities of such insects in human cadavers is the key to their use in determining time of death as sometimes employed in forensic medicine (see Valuable Insects, chap. 3). Dead insect carcasses also provide a form of carrion for necrophagous species (Young 1986). These insects are important in nutrient turnover and, from the standpoint of environmental hygiene, fortunately, are abundant and widespread in nature (Morón and Terrón 1984).

A major group of the ecologically important coprophages, insects that feed in one stage or another on the feces of other animals, are the Phanaeine and Coprine dung scarabs (Scarabaeidae) (Peck and Howden 1984). Some groups are highly specific, for instance, larval sloth moths (Pyralidae, see Sloth Moths, chap. 10) and the scarab beetle, Uroxys gorgon, on sloth dung (Young 1981).

A great many insects with sucking mouthparts survive entirely on, or frequently supplement their diets with, liquid foods. The variety and quality of nutrients available in solution is great. Carbohydrates, amino acids, minerals, vitamins, and even fats are dissolved in such diverse and unlikely fluids as blood and lymph from animals, sap, rainwater solutions, nectar, animal and plant secretions (sweat, milk, tears), honeydew, decomposition products, weeping wounds, decay juices, plant juices, fecal matter, and urine.

Whatever the food source, its nutrient content must be appropriate for each species (Dadd 1973). Insects generally require the same basic classes of dietary substances—minerals, calories, protein, carbohydrates, fats, vitamins—as vertebrates, but many peculiar or restricted factors are needed to fulfill the metabolic quirks of idiosyncratic species. Some are incapable even of digesting their food without the intervention of symbiotic microorganisms in their guts (e.g., primitive termites and some cockroaches), while others must cultivate their food, the best example of which are the gardening ants (Attinae).


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