Insects feed on a wide variety of plant, animal, and dead organic matter. Dietary requirements for all insects include carbohydrates; amino acids; cholesterol; B vitamins; and inorganic nutrients, such as P, K, Ca, Na, etc. (R. Chapman 2003, Rodriguez 1972, Sterner and Elser 2002). Insects lack the ability to produce their own cellulases to digest cellulose. Nutritional value of plant material often is limited further by deficiency in certain requirements, such as low content of N (Mattson 1980), Na (Seastedt and Crossley 1981b, Smedley and Eisner 1995), or linoleic acid (Fraenkel and Blewett 1946). Resources differ in ratios among essential nutrients, resulting in relative limitation of some nutrients and potentially toxic levels of others (Sterner and Elser 2002). High lignin content toughens foliage and other tissues and limits feeding by herbivores without reinforced mandibles. Toxins or feeding deterrents in food resources increase the cost, in terms of search time, energy, and nutrients, necessary to exploit nutritional value.
For particular arthropods, several factors influence food requirements. The most important of these are the size and maturity of the arthropod and the quality of food resources. Larger organisms require more food and consume more oxygen per unit time than do smaller organisms, although smaller organisms consume more food and oxygen per unit biomass (Reichle 1968). Insects require more food and often are able to digest a wider variety of resources as they mature. Holometabolous species must store sufficient resources during larval feeding to support pupal diapause and adult development and, for some species, to support dispersal and reproduction by nonfeeding adult stages.
Some species that exploit nutritionally poor resources require extended periods (several years to decades) of larval feeding in order to concentrate sufficient nutrients (especially N and P) to complete development. Arthropods that feed on nutrient-poor detrital resources usually have obligate associations with other organisms that provide, or increase access to, limiting nutrients. Microbes can be internal or external associates. For example, termites host mutualistic gut bacteria or protozoa that catabolize cellulose, fix nitrogen, and concentrate or synthesize other nutrients and vitamins needed by the insect. Termites and some other detritivores feed on feces (coprophagy) after sufficient incubation time for microbial digestion and enhancement of nutritive quality of egested material. If coprophagy is prevented, these organisms often compensate by increasing con sumption of detritus (McBrayer 1975). Aphids also may rely on endosymbiotic bacteria to provide requisite amino acids, vitamins, or proteins necessary for normal development and reproduction (Baumann et al. 1995).
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