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1993.46:145-59

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INSECTS IN AMBER George O. Poinar, Jr. Department of Entomological Sciences, University of California, Berkeley, California

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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. The oldest amber deposits containing insects are from the Middle East and 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 Table 1

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Major world amber deposits containing fossil insectsa Approximate age

Amber deposits

Location

(million years)

Plant source

Tertiary Baltic

Northern Europe

40

Not known, possibly

Burmese

Burma

40 (some younger)

Not known

Claiborne

Arkansas

45

Dominican

Dominican Republic

Shorea (Dipterocarpaceae) Hymenaea protera and

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araucarian

25-40 (some younger, 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

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

States

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).

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 northern and eastern portions 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.

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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 (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

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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 25-40 million 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 related to an extant species found in East Africa (15). Such examples of disjunct distributions occur in other amber deposits as well, especially the Baltic and Canadian. Behavior Interpreted from Amber Insects The sudden death of insects that fall into resin often allows them to be preserved in behavioral acts. Thus mating insects are often preserved before they can disassociate. Mating pairs of scatopsid flies, as well as chironomids and gall flies (Figure 4) have been noted in Dominican amber. Figure 5 shows a rare find of mate guarding in a pair of water striders, Electrobates spinipes ( 1) from Dominican amber. In mate guarding, which

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AMBER INSECTS

Figure 1

149

A butterfly, probably of the family Riodinidae (Lepidoptera) in Dominican amber. Not

only are such fossils rare but they are quite expensive on the open market, selling for thousands of dollars (present location of specimen unknown-unless otherwise noted , all specimens in figures are in the Poinar collection maintained at the University of California, Berkeley).

Figure 2

A female mosquito (Culicidae: Diptera) that was recently discovered by Ted Pike

(University of Calgary, Alberta) in Canadian amber (photo courtesy of Ted Pike; specimen deposited in the Royal Tyrrell Museum of Paleontology).

Figure 3

the

of Topez-Penha collection, Smithsonian

A female fig wasp (Agaonidae: Hymenoptera) in Dominican amber indirectly tells

presence of fig trees (specimen

in the

Brodzinsky

collection).

Figure 4

A mating pair of gall midges (Cecidiomyiidae: Diptera) in Dominican amber.

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occurs in some extant species of water striders, the male insect remains in contact with its mate between periods of copulation, apparently to prevent other males from attempting to mate with the same female. The above case appears to be the first record of mate guarding involving insects in the fossil record. After extensively reviewing the fossil record for evidence of behavior and coevolution, Boucot (3) showed that the behavior of organisms remains constant and that one can assume that the behavior of extinct organisms, as determined by functional morphology, is similar to that of modem descen­ dants. Thus, the behavior, discussed in the following paragraphs, of the two insects fror.n Dominican and Mexican amber showing specialized morpholog­ ical features can be reconstructed on the basis of this principal of the behavioral fixity of fossil organisms. The first example is the ant bug, Praecoris dominicana (14), a member of the subfamily Holoptilinae of the family Reduviidae (Figure 6). With the exception of one specimen, all of the known ant bugs are Old World and have various modifications for preying on ants. One modification is a gland opening on the midline of the abdominal stemites. The secretions from this gland are both attractive and paralyzing to ants. When an ant approaches, the bug raises its body and exposes its abdominal stemites. Soon after imbibing the secretions, the ant becomes lethargic, and then the bug strikes, inserting its beak and withdrawing the prey's hemolymph. The long stiff hairs on the antennae and legs of the bug are for protection against biting ants not yet paralyzed. Another ant bug, Proptilocerus dolosus occurs in Baltic amber (39). Found with this latter fossil ant bug are two dried bodies of an ant, Dolichoderus tertiarius, providing additional evidence of behavioral fixity because the extant ant bug, Ptilocerus ochraceus feeds on members of the same ant genus, Dolichoderus bituberculatus. Fossils of Dolichoderus (genus now extinct in the West Indies) also have been described from Dominican amber. Termite bugs belong to the genus Termitaradus of the Termitaphidae (Hemiptera) and occur in the colonies of termites belonging to the family Rhinotermitidae. These forms are highly adaptive and their entire body is strongly flattened dorsoventrally, thus concealing their head and body appendages. In addition, termite bugs have lost their compound eyes, ocelli, and wings, clearly as further modifications for surviving in termite colonies. A termite bug (Termitaradus protera) in Mexican amber (21) clearly has the morphological modification typical of the extant forms (Figure 7). These morphological adaptations, together with the adjacent remains of two termites, strongly supports the contention that the fossil species also lived in termite colonies. Functional morphology also can be used as evidence for caterpillar-ant symbiosis. A caterpillar containing clusters of bladderlike setae projecting

151

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AMBER INSECTS

Figure 5

A pair of water striders (Gerridae: Hemiptera) in Dominican amber, with the male

(arrow) showing typical mate-guarding behavior. Figure 6

An ant bug, Praecoris dominicana (Reduviidae: Hemiptera), in Dominican amber.

Note the heavy setae on the appendages for protection from ant bites.

Figure 7

A termite bug, Termitaradus proterus (Termitaphididae: Hemiptera) in Mexican amber.

Note a flattening of the body appendages as an adaptation for living in tennite colonies.

Figure 8

Caterpillar, possibly of the family Riodinidae (Lepidoptera) in Dominican amber. Insert

shows paired glands on eighth abdominal segment (arrows) that could be attractive to ants.

152

POINAR

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forward above its head could be a metalmark butterfly larva (Riodinidae: Papilionoidea: Lepidoptera) (Figure 8). It possesses a pair of conspicuous glands on the eighth abdominal segment, inconspicuous dorsal setae but a fringe of long lateral setae, and a pair of vibratory papillae on the thoracic shield. These characters are typically found in tropical riodinids associated with ants that obtain secretions from the paired abdominal glands and protect the caterpillars from enemies. This larva may also have been associated with ants in its original habitat.

Mimicry Fossil evidence of mimicry is rare; however, an example of ant mimicry (myrmecomorphy) in Dominican amber involves a female of the cerambycid beetle genus Tilloclytus. Members of this genus tend to be diurnally active and are modified in structure, coloration, and behavior to mimic ants (11). Typically the mimics bear a pair of white markings on either side of the elytra, thus creating the illusion of the petiole of an ant. The legs and rest of the body resemble those of an ant. The Dominican amber Tilloclytus sp. in Figure 9 was entrapped next to an ant of the genus Camponotus and may have been associated with a group of such ants at the time it was entombed.

Fossil Evidence of Phoresy Phoretic associations in the fossil record are rare but do occur in amber (13). Mites have been found on several insects in amber, including a drosophilid fly (22) in Dominican amber. Figure 10 shows a number of mites on the underside of a leiodid beetle (Leiodidae: Coleoptera), also in Dominican amber. Horseshoe beetles (Trichopseninae: Staphylinidae: Coleoptera) together with termites in Dominican amber illustrate the age of this association (Figure 11).

Fossil Evidence of Disease and Parasitism The delicate nature of the capture and fossilization of amber insects sometimes preserves evidence of insect pathogens and parasites. Several pathogenic fungi have been described from amber insects ( 12, 25). These are usually minute forms growing on the body surface that require high resolution for identifi­ cation. In Figure 12, a rare fungal pathogen dwarfs the host insect. The host is a member of the bark louse genus Troctopsocopsis (Troctopsocidae: Psocoptera) in Dominican amber. The fungus has produced an elongate synnema similar to those found in present-day representatives of the genus Hirsutella. One can also find fossil evidence of nematode parasitism of insects in amber

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AMBER INSECI'S

153

10

II Figure 9

Right: A Tilloclytus ant-mimic longhorn beetle (Cerambycidae: Coleoptera) (determi­ J. Chemsak) that could have been mimicking the Camponatus ant (Formicidae: Hymenoptera) (determination by E. O. Wilson) (left). Arrow shows white marking that mimics nation by

an ant petiole . (Dominican amber: specimen in the American Museum of Natural History.)

Figure 10

A leiodid beetle (Leiodidae: Coleoptera) (determination by

Q. Wheeler) containing

a number of mites on its undersurface demonstrating a case of phoresy in Dominican amber.

Figure 11 ican amber.

A horseshoe beetle (Staphylinoidea: Coleoptera) associated with tennites in Domin­

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(18, 19). Figure 13 shows two mermithid nematodes (Mermithidae: Mer­ mithida) exiting from the abdomen of an adult midge (Chironomidae: Diptera) in Dominican amber. The first fossil of an allantonematid nematode parasite (Allantonematidae: Tylenchida) of insects was reported in a drosophilid fly (Drosophilidae: Diptera) from Dominican amber (18). A second drosophilid fly parasitized by allantonematid nematodes in Dominican amber is seen in Figure 14. Parasitic mites still attached to their insect hosts also appear in Dominican amber. One case involved two species of moths as hosts (26) and the other a chironomid midge (17) (Figure 15). Such cases clearly show that certain parasitic groups with life cycles apparently similar to extant forms were present 25-40 million years ago.

Paleoenvironmental Reconstruction Insect fossils in amber can also be used to reconstruct the physical and biological conditions that prevailed in the original forest. Fossils in Dominican amber suggest that the original ecosystem represented a tropical, evergreen angiosperm forest not unlike virgin areas found today in South America. More detailed information on specific insect genera can provide further ecological data. Thus, the presence of the psocid Psyllipsocus sp. in Dominican amber suggests the presence of large buttress-based trees, and the presence of representatives of the psocid genera Rhyopsocus, Isthmopsocus, and Echmep­ teryx in the same deposits suggests extensive stands of palms in forest openings (E. Mochford, personal communication). As previously noted, the presence of fig wasps in Dominican amber provides indirect evidence of fig trees. All of the information obtained from the insect remains agrees with that gleaned from plant macrofossils found in the same deposits, which indicates an early to mid-Tertiary tropical evergreen rain forest habitat. Figure 16 illustrates a partial reconstruction of a bark scene in Dominican amber. All components are based on actual fossils recovered from the amber, including the tree upon which the insects are resting. Reconstructing the Baltic amber paleoenvironment is much more challeng­ ing because the amber preserves a mixture of warm temperate and subtropical groups (10). The presence of pines, palms, and oak fossils in Baltic amber plus a range of invertebrate types, some of which occur today in warm temperate and others in tropical climates, has resulted in several theories on the nature of the original forest. It is generally accepted that the majority of the Baltic amber flora and fauna suggest subtropical conditions. Those temperate elements present could have originated from areas of higher elevation, but then the Baltic amber tree, if a single species, would have had to be fairly adaptable in regard to growing conditions. Another suggestion is that the climate cooled rapidly as the Baltic amber formed, which would result

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AMBER INSECTS

Figure 12

A fungal synnema arising from the body of a Troc/opsocopsis sp. (Troctopsocidae:

Psocoptera) (determination by

Figure 13

155

E. Mockford) in Dominican amber.

Two mermilhid nematodes emerging from the body of a midge (Chironomidae:

Diptera) in Dominican amber.

Figure 14

A drosophilid f ly (Drosophilidae: Diplera) (determination by D. Grimaldi) parasitized

by allantonematid nematodes (Allantonematidae: Tylenchida) (arrows) in Dominican amber.

Figure 15

Parasitic mites (arrow) attached to the abdomen of a midge (Chironomidae: Diptera)

in Dominican amber.

in the earlier deposits containing subtropical elements and the later ones temperate forms (10).

Tissue Preservation Researchers often thought that only the external surfaces of amber insects were preserved, the internal tissues having been degraded by autolysis or

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Figure 16

Reconstruction of a bark scene from fossils in Dominican amber. From top to

bottom: a woodgnat (Valeseguya; Anisopodidae, Diptera) alights on the bark of the leguminous tree, Hymenaea protera. On right in background is a Mastotermes sp. (Mastotermitidae: Isop­ tera). Dolichoderus ants (Formicidae: Hymenoptera) walk across the tree bark. An ant bug, Praecoris dominicana (Reduviidae: Hemiptera) observes the ants. Trigonid bees, Proplebeia

dominicana, are visiting a source of fresh resin for hive construction; one has become entombed in the sticky material. A resin bug (Apiomerinae: Reduviidae: Hemiptera) waits to catch one of the bees. All of these insects are now extinct at the specific or generic level and none are represented at the generic level in Hispaniola today. (Drawing by Christina Jordan.)

AMBER INSECTS

157

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microbial agents that then perished inside the fossil's body cavity. Then

electron-microscope studies on a Baltic amber midge showed epidermal and muscle cells in various degrees of preservation (23, 24). Nuclei containing electron-dense nucleoplasm possibly corresponding to chromatin were de­ tected. What appeared to be nuclear envelopes surrounded the denser nuclei and the cytoplasm contained membranous profiles, often oriented in curvilin­ ear arrays. Mitochondria, lipid droplets, and ribosomes were also observed. Trachea and tracheoles, some still containing their plasma membrane and inner lipoprotein cuticulin layer also were present. Such tissue preservation is largely the result of inert dehydration resulting from the replacement of water in the tissues. This type of physical preservation can be obtained today when fresh insect tissues are treated with ethylene glycol. The result is an extreme form of mummification, involving the preservation of tissues by a natural embalming process. Attempts to extract ancient DNA from insects embedded in amber have been successful with the isolation and partial characterization of DNA from the Domincan amber bee, Proplebeia dominic­ ana (3a).

Biodiversity Much attention has recently been given to the diversity of life forms on this planet, especially in the New World tropics. Concern over the destruction of the natural environment in the Neotropics is widespread, and annual reductions in the diversity of tropical forest insects have been estimated (40). An estimate of Neotropical insect biodiversity (number of insect species and their distributions) before human intervention can be obtained from the study of tropical American amber (Mexican and Dominican deposits). The general picture from these amber studies is that in the Tertiary, both insect species diversity and species and genera distributions were much greater than in the corresponding land areas today. There are two problems in attempting to relate what is found in the amber deposits with what exists today. First, we don't know all of the extant spe cies of insects, especially in the New World tropics, and in comparative studies, it is desirable that the total extant fauna of particular groups be known. This can be accomplished in isolated areas such as small islands or with specific insect groups but is rarely accomplished on large continental areas, as in Central and South America. Second, even if the extant fauna were fully known, how can we distinguish between natural extinctions and extinctions that result from human activity? One could make some type of computer projection to decide this issue if a number of fossil sites in the same geographical area were spaced approxi­

mately 10 million years apart throughout the Tertiary, ending with a deposit formed just before humans appeared on the scene. Unfortunately, we do not have this ideal sequence of fossil deposits; nevertheless, further studies on this issue should prove very rewarding.

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Summary We can extract a range of information about the past insect faunas (as well as plants and other animals) of particular global areas from amber fossils. Such fossils are normally preserved with such clarity that they can be compared in minute detail with extant representatives and offer opportunities to study both micro- and macroevolution. Amber fossils provide the earliest known records of some insect orders and families and numerous genera and species and are important in reconstructing

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insect phylogenetic lines. Much effort has been made in attempting to determine primitive and derived characters based on an interpretation of extant forms. Fossils show irrefutably what characters were present at a particular time in the earth's history. As demonstrated here, amber fossils can also tell us about the behavior of extinct insects, the earlier structure of their biological communities, and symbiotic associations in the past. In essence, they provide us with an opportunity to discover many details of the past world of small organisms that are unavailable through other types of fossils.

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Andersen, N. M., Poinar, G. O. Jr. 1992. Phylogeny and classification of an extinct water strider genus (Hem­ iptera, Gerridae) from Dominican amber, with evidence of mate guarding in a fossil insect. Z. Zool. Syst. Evol­ utionsforsch. In press Baroni Urbani, C. 1980. The first fossil species of the Australian ant genus Leptomyrmex in amber from the Dominican Republic. Stuttg. Beitr. Naturkd. Ser. B 62:1-8 Boucot, A. J. 1989. Evolutionary Pale­

obiology of Behavior and Coevolution.

Amsterdam: Elsevier. 725 pp. Cano, R. J., Poinar, H., Poinar, G. O. Jr. 1992. Isolation and partial char­ acterization of DNA from the bee Proplebeia dominicana (Apidae: Hy­ menoptera) in 25-40 million year old amber. Med. Sci. Rev. 20:249--51 Grimaldi, D. A. 1991. Mycetobiine woodgnats (Diptera: Anisopodidae) from the Oligo-Miocene amber of the Dominican Republic, and Old World affinities. Am. Mus. Novit. 3014:1-24 Keilbach, R. 1982. Bibliographie und Liste der Arten tierischer Einschliisse in fossilen Harzen sowie ihrer Au­ fbewahrungsorte. Deut. Entomol. Z. 29:129--286; 301-491

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Krishna, K., Emerson, A. E. 1983. A new fossil species of termite from Mexican amber, Mastotermes elec­ tromexicus (Isoptera, Mastotermitidae). Am. Mus. Novit. 2767:1-8 7. Krishna, K . , Grimaldi, D. 1991. A new fossil species from Dominican amber of the living Australian termite genus M astotermes (Isoptera: Mastoter­ mitidae). Am. Mus. Novit. 3021:1-10 8. Kulicka, R. 1978. Mengea tertiaria Menge (Strepsiptera) from the Baltic amber. Pro Muz. Ziemi 29:141-45 9. Lambert, J. B., Frye, J. S., Poinar, G. O. Jr. 1985. Amber from the Dominican Republic: analysis by nu­ clear magnetic resonance spectroscopy. Archaeometry 27;43-51 10. Larsson, S. G. 1978. Baltic Amber-a Palaeobiological Study. Entomono­ graph, Vol. I. Denmark: Klampenborg. 192 pp. 11. Linsley, E. G. 1959. Mimetic form and coloration in the Cerambycidae (Coleoptera). Ann. Entomol. Soc. Am. 52:125-31 12. Michner, C. D., Grimaldi, D. 1988. A Trigona from late Cretaceous amber of New Jersey (Hymenoptera: Apidae: Meliponinae). Am. Mus. Novit. 2917: 1-10

AMBER INSECTS 13.

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Annu. Rev. Entomol. 1993.38:145-159. Downloaded from www.annualreviews.org Access provided by Oregon State University on 12/22/16. For personal use only.