Annu. Rev. Entomol. 1993.46:145-59 Copyright © 1993 by Annual Reviews Inc. All righis reserved INSECTS IN AMBER George O. Poinar, Jr. Department of Entomological Sciences, University of California, Berkeley, California 94720 KEY WORDS: fossil insects, tissue preservation, paleoentomology, paleosymbiosis, paleoen­ vironments Introduction Aside from their beauty, insects in amber represent the finest fossil remains of the Insecta and offer numerous opportunities to study microevolution, biogeography, mimicry, behavior, environmental reconstruction, extinction, paleosymbiosis, and molecular phylogeny. Amberization (changes involved in the process of forming amber from fresh resin) is a gentle process inducive to the preservation of insects that are small, delicate, and soft-bodied. It is the most complete type of fossilization known for insects, and by preserving the three-dimensional form, color pattern, and minute details of the exoskeleton, such fossils can be easily compared with their extant descendants. The study of amber insects was initiated over 200 years ago with the fIrst investigation of the Baltic amber fauna and flora (10). Indeed, most of the descriptions of amber insects today pertain to Baltic deposits. However, in the past 50 years, workers have begun to explore additional amber deposits. These are listed in Table 1. Access provided by Oregon State University on 12/22/16. For personal use only. The oldest amber deposits containing insects are from the Middle East and Annu. Rev. Entomol. 1993.38:145-159. Downloaded from www.annualreviews.org are commonly known as Lebanese amber. They date from the Early Cretaceous and extend back 135 million years. In the past 20 years, attention has centered on the highly fossiliferous Tertiary amber deposits in the Dominican Republic, which are fairly extensive and provide a steady income for Dominican workers primarily through sale for use as jewelry. Because of the scientific interest of specimens from these deposits, their availability, and the range of insects represented [some 235 families at present (13)], this review emphasizes Dominican Republic (or Dominican) amber insects. Amber forgeries can suddenly appear and look very authentic. Forgeries involve the placement of present-day insects into plastic or semi-fossilized 145 0066-4170/93/0101-0145$02.00 146 POINAR Table 1 Major world amber deposits containing fossil insectsa Approximate age Amber deposits Location (million years) Plant source Tertiary Baltic Northern Europe 40 Not known, possibly araucarian Burmese Burma 40 (some younger) Not known Claiborne Arkansas 45 Shorea (Dipterocarpaceae) Dominican Dominican Republic 25-40 (some younger, Hymenaea protera and some possibly older) possibly some other Hymenaea spp. (Legum­ inoseae) Fushun China 40-53 Not known Mexican Chiapas, Mexico 22-26 Hymenaea (Leguminoseae) Romanian Romania 30-40 (some 70) Taxodiaceae? Sicilian Sicily 30 Not known Cretaceous Alaskan Alaskan Coastal Plain 80? Araucariaceae Atlantic Coastal Plain Northeastern United 75-124 Araucariaceae plus others States Canadian Cedar Lake, Manitoba 70- 80 Araucariaceae French Northern France 95-100 Not known Middle East Lebanon, Israel, Jordan 120-135 Araucariaceae Taimyr Soviet Arctic 78-115 Not known a Data taken from Ref. 13. resin (copal). Scientifically made forgeries will pass the standard tests employed by gem and mineral institutes, but specific tests that will. detect most forgeries are available (20). The present account of amber insects emphasizes various areas of paleobio­ logy. Generic and specific lists and accounts of amber insects are also available (5, 10, 13, 32-38). Access provided by Oregon State University on 12/22/16. For personal use only. Annu. Rev. Entomol. 1993.38:145-159. Downloaded from www.annualreviews.org Dating Amber Amber has traditionally been dated by examining index fossils (foraminifers, coccoliths) in the bedrock containing the amber. This method, of course, provides a minimum date because it does not take into account the time involved for the final deposition of the amber in the bedrock. All amber from a particular geographical site may not be identical in age. Nuclear magnetic resonance (NMR) spectra showed that amber from different mines in the Dominican Republic varied in age and other characteristics (9). Ages of amber in the northernand easternportions of the Dominican Republic were given as 25-40 million years, with amber from the well-established La AMBER INSECTS 147 Toca mine being the oldest (40 million years). These dates correlate fairly well with the coccolith age determinations presented by Cepek (29), who gave ages ranging from 30 to 45 million years for the La Toea mine deposits. First Occurrences of Insect Taxa in Amber It is important to know when insect groups first appeared in the fossil record, and amber deposits contain many first records of insect families and even some orders. Instances of the geologically earliest records of insect families occurring in amber are too numerous to mention here and the earliest records of genera are even more extensive. All of the three main groups of social insects (bees, ants, and termites) appear in Cretaceous amber (12, 13, 41); however, Rasnitsyn has expressed doubts about the assigning of Trigona prisca (the oldest known bee) to Upper Cretaceous amber, and further investigations into that fossil may be warranted (27). A reputed ant in Lower Cretaceous Lebanese amber would be the earliest known record of this group if indeed the specimen is determined to be an ant (A. Acra, personal communication). Tertiary amber contains the earliest known undisputed representatives of the orders Thysanura (30), Mantodea (13), Zoraptera (16), Embioptera (28), Siphonaptera (13), and Strepsiptera (8). Fragile insects that normally are not found in fossil form may sometimes tum up in amber. Butterflies are one example; a probable metalmark adult (Riodinidae: Lepidoptera) in Dominican amber (Figure 1) is preserved down to the finest detail, including scales and color patterns. Amber also contains the first appearance of many blood-sucking flies in the fossil record (13). A recent find by Ted Pike of a mosquito in Cretaceous Canadian amber is by far the oldest known fossil of the Culicidae (Figure 2). Indirect Evidence of Plant Occurrence Indirect evidence for the existence of certain plant groups derives from the appearance of specialized insects in amber. The Dominican amber fig wasp Access provided by Oregon State University on 12/22/16. For personal use only. Annu. Rev. Entomol. 1993.38:145-159. Downloaded from www.annualreviews.org (Agaonidae: Hymenoptera) shown in Figure 3 already has its adaptations for entering fig fruits, and its presence is evidence that Ficus trees thrived in the vicinity during that epoch. Biogeography Dominican amber contains interesting examples showing that the distribution of some insects was much more extensive in the past than at present. The ability to relate amber insects to modem-day forms allows us to make some interesting observations on the past distribution of families, genera, and species. The study of amber ants is one example. Representatives of the extinct ant genus Sphecomyrma of the extinct subfamily Sphecomyrminae (forms 148 POINAR intermediate between modem ants and nonsocial wasps) have been found in Cretaceous New Jersey and Canadian amber (41). These forms are now extinct at the subfamily level. Although members of the ant genus Leptomyrmex exist in Dominican amber, that genus is extinct from the New World and is now confined to the Australian regions (2). Members of the ant subfamily Dolichoderinae are represented by several genera (induding Azteca, Dolichoderus, HypoC/inea, and Monacis) in Dominican amber (42). However, no extant representatives of these genera occur in the Greater Antilles. Very few sawflies occur in the West Indies and none are known from Hispaniola. Dominican amber contains several species of Didymia (Argidae: Hymenoptera), which belong to a subfamily now distributed from Panama to southeastern Brazil (31). Other biogeographical anomalies include the discovery of termites of the genus Mastotermes in Dominican and Mexican amber (6, 7). Today, the one living representative of this genus in the world is confined to tropical forest areas in Northern Australia. Further disjunct distributions involving amber insects were demonstrated with representatives of woodgnats (Anisopodidae: Diptera) in Dominican amber (4). Of the three genera of woodgnats reported, one of them (Mycetobia) no longer exists in the Greater Antilles, another (Mesochria) is now found only in the Old World tropics, and the third (Valeseguya) is known from a single specimen from Australia. Clearly these and many more examples show how amber inclusions can be used to determine past differences in the distributions of insects. The above examples of Old World and Australian relationships with insects from Dominican amber provide further evidence of a faunal relationship between South America and Australia as well as other parts of the world and show how the range of certain insects has been drastically reduced in the past 25-40million years. This reduction has been established not only for insects, but also for some plants in Dominican amber. For example, the tree that produced much of the Dominican amber, Hymenaea protera, is most closely Access provided by Oregon State University on 12/22/16. For personal use only. Annu. Rev. Entomol. 1993.38:145-159. Downloaded from www.annualreviews.org