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hindgut. For example, termite hindguts contain a veritable fermenter comprising many bacteria, fungi, and protists, including flagellates, which assist in the degradation of the otherwise refractory dietary lignocel-lulose, and in the fixation of atmospheric nitrogen. The process involves generation of methane, and calculations suggest that tropical termites' symbiont-assisted cellulose digestion produces a significant proportion of the world's methane (a greenhouse gas) production.

Transmission of extracellular symbionts from an individual insect to another involves one of two main methods, depending upon where the symbionts are located within the insect. The first mode of transmission, by oral uptake by the offspring, is appropriate for insects with gut symbionts. Microorganisms may be acquired from the anus or the excreta of other individuals or eaten at a specific time, as in some bugs, in which the newly hatched young eat the contents of special symbiont-containing capsules deposited with the eggs.

Intracellular symbionts (endosymbionts) may occur in as many as 70% of all insect species. Endosym-bionts probably mostly have a mutualistic association with their host insect, but some are best referred to as "guest microbes" because they appear parasitic on their host. Examples of the latter include Wolbachia (section 5.10.4), Spiroplasma, and microsporidia. Endo-symbionts may be housed in the gut epithelium, as in lygaeid bugs and some weevils; however, most insects with intracellular microorganisms house them in sym-biont-containing cells called mycetocytes or bacteri-ocytes, according to the identity of the symbiont. These cells are in the body cavity, usually associated with the fat body or the gonads, and often in special aggregations of mycetocytes, forming an organ called a mycetome or bacteriome. In such insects, the symbionts are transferred to the ovary and then to the eggs or embryos prior to oviposition or parturition - a process referred to as vertical or transovarial transmission. Lacking evidence for lateral transfer (to an unrelated host), this method of transmission found in many Hemiptera and cockroaches indicates a very close association or coevolution of the insects and their microorganisms. Actual evidence of benefits of endo-symbionts to hosts is limited, but the provision of the otherwise dietarily scarce essential amino acids to aphids by their bacteriocyte-associated Buchnera symbiont is well substantiated. Of interest for further research is the suggestion that aphid biotypes with Buchnera bacteriocytes show enhanced ability to trans mit certain plant viruses of the genus Luteovirus relative to antibiotic-treated, symbiont-free individuals. The relationship between bacteriocyte endosymbionts and their phloem-feeding host insects is a very tight phylogenetic association (cf. Wolbachia infections, section 5.10.4), suggesting a very old association with co-diversification.

Some insects that maintain fungi essential to their diet cultivate them external to their body as a means of converting woody substances to an assimilable form. Examples are the fungus gardens of some ants (Formicidae) and termites (Termitidae) (sections 9.5.2 & 9.5.3) and the fungi transmitted by certain timber pests, namely, wood wasps (Hymenoptera: Siricidae) and ambrosia beetles (Coleoptera: Scolytinae).

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