Because of the sudden death of captured organisms in amber, symbiotic associations may be preserved in a manner unlikely to occur with other types of preservation. Also, the fine details of preservation may reveal morphological features characteristic
of symbiotic associations. Cases of paleosymbiosis in amber include inquilinism, commensalism, mutualism, and parasitism.
Paleoinquilinism involves two or more extinct organisms living in the same niche but neither benefiting nor harming each other. Numerous insects form inquilinistic associations under tree bark, and many pieces of amber contain flies and beetles common to this habitat.
Phoresis (one organism transported on the body of another organism) is probably the most typical type of paleocom-mensalism in amber. This usually involves mites and pseudoscorpions being carried by insects. The arachnid benefits by being conveyed to a new environment, where the food supply is likely to be better than the last one. The carrier generally is not harmed and only serves as a transporting agent. An example of this category in Dominican amber consists of pseudoscorpions being carried by platypodid beetles (Coleoptera: Platypodidae) (Fig. 3). The method of attachment of the pseudoscorpion to the beetle was the same then as it is today. In fact, these ancient records lead scientists to believe that such behavior is mandatory for the survival of the pseudoscorpions that live in beetle tunnels and require effective dispersal mechanisms for survival.
In paleomutualism, both organisms benefit and neither is harmed. Amber bees carrying pollen provide evidence of insect—plant mutualism in which the bee obtains a food supply and the plant is pollinated. An example of insect—insect mutualism is demonstrated by a rare fossil riodinid butterfly larva in Dominican amber. Specialized morphological features of this Theope caterpillar indicative of a symbiotic association are balloon setae and vibratory papillae in the neck area, and tentacle nectary organ openings on the eighth abdominal tergite. Extant caterpillars in this genus have similar features and are associated with ants. The tentacle nectary organs provide nourishment for the ants, whereas the vibratory papillae (which beat against the head capsule and make an audible sound) and balloon setae (which emit a chemical signal) are used to attract ants when the caterpillar is threatened by an
invertebrate predator or parasite. This fascinating association between butterfly larvae and ants was established at least 20 mya.
Paleoparasitism is very difficult to verify in the fossil record. There are many records of amber insects (especially wasps and flies) whose descendants today are parasitic on a wide range of organisms, but to discover an actual host—parasitic association is quite rare.
Paleoectoparasitism is the most obvious of all parasitic associations found in amber. The ectoparasite is often still attached to its host, and systematic studies can be conducted on both organisms. In amber, ectoparasites are usually parasitic mites, such as the larvae of Thrombididae attached to the mouthparts of a fly in Baltic amber (Fig. 4). These larval mites were feeding on the host's hemolymph, and their mouthparts are still in place. After molting to the nymphal stage, the parasites would leave the fly and become free-living predators. Large infestations could kill the host. These mites are not to be confused with phoretic ones, which are simply carried around by insects.
Paleoendoparasitism is extremely difficult to verify because internal parasites are rarely preserved as fossils. However, some parasites attempt to leave their hosts when they encounter resin. Mermithid nematodes (Mermithidae: Nematoda) and hairworms (Nematomorpha) that have nearly completed their development and are almost ready to emerge from their host will often reveal their presence (Fig. 5). Under normal conditions, they would enter soil or water and initiate a free-living existence.
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