tree of coleopterans colored boxes indicate high taxonomic...

29

CupedidaeMicromalthidaeCrowsoniellidaeOmmatidaeJurodidaeSphaeriusidaeHydroscaphidaeTorridincolidaeLepiceridaeAspidytidaeAmphizoidaeDytiscidae (diving beetles)HygrobiidaeNoteridaeMeruidaeHaliplidaeGyrinidae (whirligigs)Carabidae (ground and tiger beetles)RhysodidaeTrachypachidae

EucinetidaeClambidaeScirtidae

Derodontidae

LeiodidaePtiliidaeHydraenidae

ScydmaenidaeSilphidae (burying beetles)StaphylinidaeHydrophilidaeHisteridaeCeratocanthidaeGeotrupidaeScarabaeidae (dung beetles)Lucanidae (stag beetles)TrogidaePassalidaeByrrhidaeBuprestidaeElmidae

DryopidaeHeteroceridaeLimnichidae

PsephenidaePtilodactylidaeCallirhipidaeChelonariidae

ElateridaeLampyridae (�re�ies)Cantharidae

DascillidaeAnobiidae (woodworms)BostrichidaeDermestidaeNitidulidaePhalacridaeCucujidaeErotylidaeCoccinellidae (ladybugs)CleridaeLymexylidaeMycetophagidaeMordellidaeTenebrionidae (�our beetles)ZopheridaeOedemeridaeAnthicidaeMeloidae (blister beetles)CerambycidaeMegalopodidaeChrysomelidae (leaf beetles)NemonychidaeAnthribidaeBelidae

AttelabidaeCurculionidae (weevils, wood borers)Brentidae

ARCHOSTEMATA

MYXOPHAGA

ADEPHAGA

POL

YPH

AG

A

Cuc

ujif

orm

ia5

c. 2

35 M

YA

1

2

34

6

78

9

10

9

8

7

6

5

4

3

2

1

10

c. 2

15 M

YA

c. 2

70 M

YA

c. 2

85 M

YA

Tree of Coleopterans showing the phylogenetic relationships among the most significant groups. Colored boxes indicate high taxonomic ranks. Branches with thick lines indicate robust clades, and branches with thin lines indicate less-supported clades. The number in the green circle indicates the chapter in which coleopterans are also included. Orange circles mark the principal nodes and their ages. Photographs illustrate principal clades; boxed numbers associate photographs with clades.

34_TOL1e_34.indd 408 5/8/14 11:11 AM

© 2014 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher.

Beetles (coleoptera) form the most species-rich group of the insects: two of every five species belong to this order, which accounts for approximately one-fourth of all known animals. This overwhelming di-versity justifies the famous quip attributed to Haldane on the “inordinate fondness for beetles” enjoyed by the hypothetical designer of nature’s plan. Beetles oc-cupy almost all ecosystems except the open sea, from caves and endogeic environments to altitudes above 5000 meters, and their morphological and ecological

diversity is extraordinary. For centuries, Coleoptera have been a favorite of professional and amateur en-tomologists, and some groups (along with butterflies and dragonflies) are well known from a taxonomic and faunistic point of view, especially in Western Europe and North America.

In all probability, the beetles appeared in the Paleo-zoic Era, during the Late Carboniferous (ca. 300 Mya), from a common ancestor with Strepsiptera and Neu-ropterida. The ancestors of present-day Coleoptera had

summary The Coleoptera (beetles) are the most diverse order among the Metazoa, with around 180 families, almost 400,000 described species, and many more to be dis-covered. They are found on all the continents except Antarctica (although they are found in the subantarctic islands), and in an enormous variety of environments and ecological niches. Coleoptera are divided into four main groups that are considered suborders: two are species-poor (Archostemata and Myxophaga, with fewer than 200 species between them), and two are highly diverse (Adephaga and Polyphaga). The relationships among suborders, and among the main lineages within the two most diverse ones, are still largely controversial, despite the effort invested in their study with morphological as well as with molecular data. Coleoptera are one of the oldest groups of Holometabola, with fossils from the Early Permian period (the extinct Protocoleoptera), more than 250 million years ago. All suborders (with the possible exception of Myxophaga) are documented in the Triassic, and the main lineages (superfamilies and families) of extant beetles were already present in the Jurassic, before the appearance and radiation of the angiosperms. The current enormous diversity of Coleoptera may safely be explained by the longevity and stability of the main lineages and by their ability to adapt to a wide range of habitats and food resources, in-cluding the coevolution of the phytophagous Coleoptera and the angiosperms.

ColeopteransBeetles

Ignacio Ribera and Rolf G. Beutel34

What is a coleopteran (beetle)?Coleoptera (from the Greek koleos [cover] and pter-on [wing]) belong to the group of holometabolous insects (with complete metamorphosis), with a pupal stage between the larva and adult. They constitute an order of insects with some 350,000–400,000 described species, no doubt with many more to be discovered. They include some of the most common and familiar insects, such as dung beetles and lady-bugs, but also some of the most harmful, such as the Colorado potato beetle, some weevils, and the June bugs. They typically have a compact body, with

sclerotized forewings (elytra) covering and protecting membranous hind wings. They have an unparalleled species richness and enormous ecological and mor-phological diversity, as they have colonized virtually every habitable environment on the planet except the open sea and the most inhospitable regions of the poles and the high mountains. Beetle fossils belong to the oldest in Holometabola, known from the early Permian, 290 million years ago. Coleoptera owe their great diversity today in part to this age and their evo-lutionary persistence.

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410 CHAPTEr 34 • COlEOPTErAns

a more compact, more sclerotized body with fewer membranous surfaces than their neuropterid relatives. These characters are considered to be an adaptation to narrow spaces and to facilitate penetration under tree bark or burrowing in soil substrate. Additionally, a more compact body reduces the risk of predation and desiccation, and hinders the attack of fungi and oth-er pathogens. The earliest beetles had largely sclero-tized forewings called elytra, to protect the membra-nous hind wings used for flight. These elytra, which are fully sclerotized in almost all extant groups, form a tightly sealed chamber with the abdomen, leaving a subelytral air storage space that, as a consequence, made invasions of the aquatic environment possible. Extant beetles are characterized by a number of unique characters (Box 34.1), the most important of which in-cludes the transformation of the first pair of wings into sclerotized elytra.

Characteristics of Coleopteran GenomesTraditionally, two species of Coleoptera have been used as model organisms for physiological and mo-lecular studies: Tribolium castaneum and Tenebrio molitor (mealworms, family Tenebrionidae). There is an enormous amount of genomic information on these species, including the complete genome of T. castaneum. The molecular phylogenetic analysis of the relationships between families or suborders was initially based on the complete sequence of the 18S ribosomal gene, for which we currently have data from more than 3000 species. The genes most used for studying relationships between genera or spe-cies are mitochondrial (especially the cox1 and rrnL genes), although contributions from many other gene sequences, both nuclear as well as mitochondrial, are constantly being added.

Recently, the publication of expressed sequence tag (EST) libraries from a selection of Coleoptera has placed a huge resource of genetic information at the

disposition of the scientific community. Despite the large volume of published sequences, some groups are still underrepresented, particularly the suborders Archostemata and Myxophaga. The number of known mitochondrial genomes in Coleoptera has increased greatly in recent years, although the phylogenetic re-sults are difficult to interpret due to bias in the nucleo-tide composition.

The mitochondrial gene order is generally the same in all known genomes and identical to what is considered ancestral in Holometabola, although some RNA translocation cases in certain clades are known. Cases of variations in some anticodons, or in the start-ing codon in some proteins (leucine or, less frequently, glutamine or arginine instead of the standard methio-nine), have also been described.

Phylogenetic Results Contrasted with Previous ClassificationsThe first classification of the Coleoptera was present-ed by Linnaeus in his Systema Naturae (1758 edition), which included 22 genera in the Ordo (Order) Coleop-tera based on general external characters. Later clas-sifications did not involve any fundamental change. They were limited to refining the hierarchy by intro-ducing intermediate categories, using more detailed morphological characters and describing (and subdi-viding) new groups. The major conceptual change in coleopteran classification was produced at the begin-ning of the twentieth century with the late incorpora-tion of evolutionary ideas, and acceptance of the gen-eral principle, proposed by Darwin in On the Origin of Species (1859), that a natural classification must be a reflection of the genealogy (phylogeny). The current classification of Coleoptera is fundamentally based on the work of Roy A. Crowson, who established the major lineages and their fundamental characters based on their morphology. The most recent general classification of the group is that of Lawrence and Newton (Table 34.1), updated by Beutel and Leschen (2005–2014), with four suborders, 16 superfamilies, and 179 families, including the recently discovered Aspidytidae and Meruidae as well as the family Ju-rodidae, with only one known extant species but with fossil representatives.

Two of the four suborders of Coleoptera con-tain a very small number of species: Archostemata (around 40 species) and Myxophaga (around 100 spe-cies). The other two are disproportionately richer: Adephaga (ground beetles, tiger beetles, diving and whirligig beetles, etc.) with around 41,000 species, and Polyphaga (dung beetles, leaf beetles, weevils, lady-bugs, blister beetles, etc.) with about 350,000 species. The monophyly of the suborders is now accepted, but the phylogenetic relationships among them are

Box 34.1 Morphological characters unique to coleopterans

•Transformation of the first pair of wings into sclerotized elytra with epipleura, which cover and protect the second pair of wings when at rest

•longitudinal and transverse folding of the membranous wings

•reduction of all exposed membranous areas

•specially modified venation of the membranous wings

•Characteristically simplified thoracic muscle system

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IGnACIO rIBErA • rOlf G. BEuTEl 411

still controversial: morphological characters appear to support a basal position of Archostemata and a clade Myxophaga + Polyphaga; nuclear sequence data sup-port a clade Polyphaga + Adephaga; and analyses of complete mitochondrial genomes support a monophy-letic unit Myxophaga + Adephaga (also supported by characters of the wing venation).

ArchostemataAlthough some Palaeozoic fossils were traditionally included among the Archostemata, currently all the Permian taxa and some from the Triassic are included in the stem group of the entire Coleoptera (e.g., †Pro-tocoleoptera, †Permocupedidae). Current forms are distributed mainly in the Southern Hemisphere, with a single European species (Crowsoniella relicta) known only from the type series collected in Italy and which has not been found again, despite the efforts of numer-ous coleopterologists. One species of North American origin (Micromalthus debilis, Micromalthidae) has ex-tended its distribution through transportation of dead wood upon which it feeds. This species has one of the most complex life cycles of the Coleoptera (and pos-sibly of all insects), combining pedogenesis with vari-ous types of larvae: some are viviparous and others ovoviviparous); some devour the mother; and others give rise to diploid females and haploid males. Some

living fossils are found among the Archostemata, such as the recently discovered Sikhotealinia zhiltzovae, the only extant species from a family (Jurodidae) known as fossils from the Jurassic (see Box 33.2).

MyxophagaThe Myxophaga include species of small size and in-conspicuous appearance, linked to freshwater envi-ronments on all continents except for Antarctica. Four families are currently recognized as living in various biogeographic regions: Lepiceridae, with a single ge-nus (Lepicerus, Neotropical), and sister group to the rest of the suborder; Torridincolidae, with seven genera (Neotropical, Ethiopian, and Eastern Palaearctic); Hy-droscaphidae, with three genera (Nearctic, Palaearc-tic, Neotropical, and Ethiopian); and its sister group Sphaeriusidae, with a single genus (Sphaerius, Nearc-tic, Neotropical, Palearctic, Ethiopian, and Australian). The molecular data that we have is very scarce, so the phylogeny presented is based primarily on morpho-logical evidence.

AdephagaThe Adephaga (tiger beetles, ground beetles, whirli-gigs, diving beetles, etc.) include a group of mostly aquatic predatory families. Only three families are ter-

Basic termsAedeagus: Copulatory part of the male genital tract.Arboreal: Tree-dwelling.Cave dwelling: Living in deep subterranean

environments (caves, caverns, or large interstices).Coprophage: Organism that feeds on excrement. Cryptonephridia: Malpighian tubules with the distal

ends attached to the rectum; generally used to recycle water and reduce its loss.

Elytra: Modified mesothoracic wings, sclerotized and rigid, covering the dorsal side of the pterothorax and abdomen.

Endogeic: species living in deep soil horizons. Floricolous: Feeding on flowers.Keratin: Protein that is the main element of the outer

layer of the epidermis of many vertebrates, and which may form associated external elements (hair, feathers, scales, horns, or hooves).

Mycophagous: Feeding on fungi.Myrmecophile: An organism living in association

with ants, either as a parasite or as a commensal.Necrophagous: Feeding on cadavers.Neotenic: Larval characters retained in the adult

stage.

Ovoviviparous: Reproducing by eggs that hatch when they are still in the female genital tract or immediately after being deposited.

Pedogenesis: Development of sexual organs and ability to reproduce in the larval stage.

Phytophagous: Feeding on living or dead plant matter (but not in an advanced state of decomposition).

Pleural sclerite: Chitinized part on the lateral region of the thorax.

Pupal cell: Cavity formed in the soil or constructed of plant material or detritus, in which the larva becomes the pupa.

Saprophage: An organism that feeds on decaying plant material.

Sclerotized: Cuticle hardened by forming connections between the proteins of its matrix.

Termitophile: An organism that lives in association with termites, either as a parasite or as a commensal.

Viviparous: Reproducing through live birth, as opposed to laying eggs.

Xylophages: Organisms that feed on wood.

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restrial (Trachypachidae, Rhysodidae, and Carabidae, the latter including the Cicindelinae), although the Ca-rabidae (ground beetles) have by far the greatest num-ber of species.

One of the main features of the Adephaga is the emergence of highly specialized lineages, very diver-gent from the ground-plan features of the suborder, whose phylogenetic relationships are difficult to estab-lish. Thus, among the aquatic families, the position of Gyrinidae (whirligigs) and Haliplidae is key to deter-mining whether transition to the aquatic environment has happened once or several times. The morphologi-cal characters generally support the second hypoth-esis, with at least two invasions (Gyrinidae and Dytis-coidea + Haliplidae), but the available molecular data supports a single origin of the aquatic lifestyle (“Hy-dradephaga”).

Four of the six families of Dytiscoidea have a single genus with few species, but often very disjunct distri-butions (Hygrobiidae, Amphizoidae, Aspidytidae, and Meruidae). Within the terrestrial families, the positions of the Paussinae (many of which are myrmecophiles) and Cicindelinae (tiger beetles) within Carabidae, and especially that of the Rhysodidae (which feed on slime molds in dead wood), are still disputed, and neither the molecular nor morphological data available pro-vide well-founded hypotheses.

PolyphagaPolyphaga (rove beetles, dung beetles, leaf beetles, ladybugs, blister beetles, weevils, etc.) is the most species-rich suborder and the one that has the great-est morphological and ecological diversity. It has been suggested that the ancestor may have been a small beetle (no more than 5 mm), terrestrial, probably in-habiting the leaf litter and plant debris of wet forest soil, and with saprophagous feeding habits. Tradi-tionally, the Polyphaga are separated into five major se-ries, each having several superfamilies (see Table 34.1). The relationships between some of the series, and even their monophyly, are far from being understood with a minimum of confidence. The most recent molecular data suggest that two superfamilies (Scirtoidea and Derodontoidea), which contain relatively few species, could be the sister group to the rest of Polyphaga, al-though the position of Derodontoidea is not supported by morphological characters (see Table 34.1). The five series are Staphyliniformia (burying and rove beetles, among others), Scarabaeiformia (dung beetles, etc.), Elateriformia (click beetles, fireflies, glow worms, wireworms, etc.), Bostrichiformia (woodworms), and Cucujiformia (weevils, leaf beetles, ladybugs, etc.). If Derodontoidea and Scirtoidea were considered isolat-ed lineages within Polyphaga, they would constitute a sixth and seventh series.

staphyliniformia The Staphyliniformia and Scara-baeiformia series might be related, perhaps the second subordinate to the first (related to Hydrophiloidea), although both the morphological and the molecular evidence remain ambiguous. Both appear to be sister groups to the rest of the Polyphaga series (Elaterifor-mia, Bostrichiformia, and Cucujiformia). If confirmed, this fundamental division of the Polyphaga (not coun-ting the position of Scirtoidea and Derodontoidea) would largely correspond to traditional authors’ clas-sification into Haplogastra and Symphiogastra based on the respective presence or absence of a differentia-ted pleural sclerite in the second abdominal segment. However, there is little molecular or morphological

TaBle 34.1 Classification of the order of Coleoptera, according to lawrence and newton, and updated by Beutel and leschena

Order Coleoptera (179 ff/380,000 spp)

Suborder Archostemata (5 ff/40 spp)

Suborder Myxophaga (4 ff/94 spp)

Suborder Adephaga (11 ff/41,000 spp)

Suborder Polyphaga (159 ff/350,000 spp)

Series Staphyliniformia (11 ff/67,000 spp)

Superfamily Hydrophiloidea (4 ff/7000 spp)

Superfamily Staphylinoidea (7 ff/60,000 spp)

Series Scarabaeiformia (14 ff/35,000 spp)

Superfamily Scarabaeoidea (14 ff/35,000 spp)

Series Elateriformia (37 ff/42,000 spp)

Superfamily Scirtoidea (4 ff/800 spp)

Superfamily Dascilloidea (2 ff/180 spp)

Superfamily Buprestoidea (1 ff/14,000 spp)

Superfamily Byrrhoidea (12 ff/3800 spp)

Superfamily Elateroidea (17 ff/23,000 spp)

Series Derodontiformia (3 ff/30 spp)

Superfamily Derodontoidea (3 ff/30 spp)

Series Bostrichiformia (4 ff/4500 spp)

Superfamily Bostrichoidea (4 ff/4500 spp)

Series Cucujiformia (90 ff/195,000 spp)

Superfamily Lymexyloidea (1 ff/50 spp)

Superfamily Cleroidea (11 ff/9900 spp)

Superfamily Cucujoidea (35 ff/20,000 spp)

Superfamily Tenebrionoidea (28 ff/35,000 spp)

Superfamily Chrysomeloidea (7 ff/58,000 spp)

Superfamily Curculionoidea (8 ff/70,000 spp)

aThe number of families (ff) and the estimated number of species (spp) indicated are according to Hunt et al. 2007.

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evidence supporting the monophyly of Staphylinifor-mia, or that of Staphyliniformia + Scarabaeiformia.

The Staphyliniformia include some of the most diverse families of Coleoptera (that is, of the entire animal kingdom), in particular Staphylinidae, with ap-proximately 40,000 species described and probably as many yet to be discovered. In general, they are small in size (including the smallest beetles known, some Ptiliidae, which are about 0.35 mm long), although there are large species among the moderately diverse Hydrophilidae (with some up to 45 mm) and Silph- idae (burying beetles). The majority of species inhabit leaf litter and the upper layers of soil in damp environ-ments, although the ecological diversity is enormous: they may be termitophile, arboreal, cave dwelling, aquatic, riparian, ectoparasites of mammals, etc. Char-acters common to the group are a particular configura-tion of mouthparts, the specific wing folding mecha-nism, and some larval characters. The two major di-visions within the group include the Hydrophilidae and Histeridae, and Hydraenidae and Staphylinidae (along with a series of less diverse families; see Tree of Coleopterans). Hydrophilid species are mostly aquatic, with predatory larvae and saprophagous or coprophagous adults, although they include what may be the only filter-feeding Coleoptera in the adult state, the Spercheinae. Many forms are secondarily ter-restrial, adapted to feeding in microenvironments with more or less liquid substrates (manure, decomposed vegetable substances). The Histeridae are terrestrial and generally predatory, some with myrmecophilous habits, with extreme morphological and physiological adaptations to avoid being attacked by the ants.

The composition of the Staphylinidae has yet to be well established, and in recent years, groups that have traditionally been considered separate families (such as pselaphids, scydmaenids, scaphidids, or even silphids) have been included as subordinate subfami-lies. In general, they are found in soil or leaf litter, and the species of the more derived lineages are predators. Many species have adapted to an underground envi-ronment, such as deeper soil horizons, interstitial en-vironments, or caves. Carrion beetles (Silphidae and Agyrtidae), whose relationship with Staphylinidae is not fully understood, deserve a separate special men-tion. They have necrophagous habits and present very sophisticated parental care and collaborative be-haviors between males and females.

The sister group to Staphylinidae (s.l.) appears to be a relatively small group of less diverse families, among which the saprophagous or necrophagous spe-cies dominate. Many of them have also developed ad-aptations to a subterranean environment, most notably the Leptodirini tribe of the family Leiodidae. They are mainly distributed in the Mediterranean region and include some of the most spectacular examples of ad-

aptations to cave environments. The family Hydraen-idae, the larvae and adults of which are mostly aquatic, is also in this group.

scarabaeiformia The Scarabaeiformia series inclu-des the Scarabaeidae and related families (dung beet-les, rose beetles, June bugs, stag beetles, etc.). Species are usually large, including some of the giants of the order—the Neotropical Dynastes and the African Go-liath beetles. Ancestral forms appear to have been de-tritophages or mycophages, from which the current forms were derived, with a greater trophic diversity (in many cases coprophages, but also necrophages, xylophages, saprophages, and floricolous beetles). There are forms presenting varying degrees of paren-tal care, and some Passalidae have semisocial family structures, with cooperation from adults in the rearing of the larvae and the construction of pupal cells. The-re are many species of commercial interest, especial-ly among the Melolonthinae (June bugs and related forms).

elateriformia With the exclusion of Scirtoidea, the series is comprised of three major lineages. The only group with a problematic placement in Elateriformia is Dascilloidea, a lineage that is small but with a great diversity in lifestyles. Its inclusion in this series is, how-ever, supported by recent molecular and morphologi-cal data. One of the two main groups of Elateriformia is formed by a number of families linked in varying de-grees to the aquatic environment, the traditional Dryop- oidea, plus two terrestrial families, Buprestidae (jewel beetles, Figure 34.1) and Byrrhidae. The monophyly of Dryopoidea is suggested by a unique rearrangement of mitochondrial genes, which would imply a single ori-gin of adaptations to the aquatic environment.

The second major lineage, the Elateroidea, in-cludes Elateridae (wireworms, click beetles, and re-lated groups), Lampyridae (fireflies), and Cantharidae as the principal families. One of the most interesting

Figure 34.1 some of the jewels of the animal world are found among the Coleoptera, with species spectacular in both in their extravagant forms and their coloring. shown here is Belionota sumptuosa (Buprestidae, 23 mm), a species from the Molucca Islands.

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aspects of this group is the occurrence of larviform females with neotenic characters in several families. Several lineages also have the ability to produce light, both in the larvae and in the adults of one or both sexes.

bostrichiformia Traditionally, the superfamily De-rodontoidea and the families Nosodendridae and Jac-obsoniidae were included in the series Bostrichiformia. Recent molecular and morphological data suggest that these groups, although still of uncertain phylogene-tic relationships, should be considered a sixth series within Polyphaga (Derodontiformia). This would leave Bostrichiformia composed solely of the super-family Bostrichoidea, including the families Bostri-chidae, Dermestidae, Anobidae, and Endecatomidae. They are either associated with wood or adapted to feeding on dead organic substances with a low degree of humidity (e.g., leather, keratin). The superfamily includes the woodworms (Anobidae) and some of the most common pests of stored products, entomologi-cal collections, or hides (Dermestidae). All subgroups have cryptonephridia, Malpighian tubules of a speci-fic type, and they very probably should be included as subordinate within Cucujiformia.

cucujiformia Cucujiformia is the most species-rich series of the Coleoptera, containing more than half of the described genera and species. This enormous diver-sity is due mainly to the inclusion of some of the most species-rich families, such as Tenebrionidae (darkling beetles, flour beetles), Cerambycidae (longhorns), Chrysomelidae (leaf beetles), and Curculionidae (wee-vils). The last three (together with related smaller fa-milies) are grouped under the term Phytophaga. The synapomorphies of the group suggest an origin closely linked to moving into environments with low humidi-ty, with anatomical and physiological changes to redu-ce and control water loss. Cucujiformia includes relati-vely few riparian and aquatic species (only a few minor groups of chrysomelids and weevils), and some of the forms are very well adapted to extreme arid environ-ments, such as the darkling beetles or some weevils.

It has been argued that the ancestral food of both larvae and adults could have been decomposing plant matter, fungi, or yeast. This type of feeding persists in some of the most plesiomorphic groups (Cucujidae and Tenebrionidae), but most have changed to my-cophagous or phytophagous feeding habits. Preda-tors are rare (Cleridae, Coccinellidae), and some have developed different types of parasitism (such as Melo-idae, or blister beetles).

The relationships between the basal groups of Cu-cujiformia are virtually unknown. It is one of the groups in which the composition of families or superfamilies is still unstable, particularly in Cucujoidea and Cleroi-dea. The composition of Lymexylidae (the only family in Lymexyloidea) is well defined, but their relation-ships are unclear, although recent molecular and mor-phological results seem to relate it to Tenebrionoidea. Cucujoidea forms a group of relatively small families, some with cryptic habits, and difficult to characterize. There are very few molecular data for most of them. Monophyly is more or less supported in some family groups, such as Cleroidea, or the erotylid or cerylonid series (including Coccinellidae or ladybugs); but the question is still open as to how they relate to each other and with the large number of minor families.

The superfamily Tenebrionoidea is better defined, with a similar aedeagus structure, the derived tarsal formula (5-5-4: five tarsomeres in the pro- and meso-tarsi, and four in the metatarsi), and several shared characteristics in the larvae. The largest family, Tenebri-onidae, currently includes groups that had been con-sidered as separate families (Allecullinae, Lagriinae). It is possible that, when the phylogeny is known in more detail, it will include other closely related families. The Tenebrionidae include some of the pest species of greatest economic impact, in addition to some model species for genetic and physiological research, such as Tenebrio molitor and Tribolium castaneum (flour beetle).

Box 34.2 Coleoptera by the numbers

•number of species described c. 380,000, estimated c. 1,000,000

•number of families: 179 (according to Beutel and leschen 1995–2014)

•families with the greatest number of species: staphylinidae (rove beetles) and Curculionidae (weevils and the like), with about 50,000 described species each, and an unknown (but very high) number to be discovered

•rarest family: Jurodidae. The only known specimen of the only extant species (Sikhotealinia zhiltzovae) was found in siberia in 1996 (other fossil species are known).

•Oldest fossils: †Protocoleoptera, from the early Permian Period (†Tshecardocoleidae and †Moravocoleidae families)

•largest species: about 20 cm, Titanus giganteus (Cerambycidae, neotropical)

•smallest species: 0.35 mm, Nanosella fungi (Ptiliidae, nearctic)

•longest life cycle: Buprestis aurulenta (Buprestidae). Adults can emerge from dry wood more than 50 years after eggs are laid.

•fastest species: some species of Rivacindela (Cicindelinae, Carabidae) from the Australian desert reach walking speeds of up to 2.5 m/s.

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Tribolium castaneum was the first beetle to have its com-plete genome sequenced.

In Tenebrionoidea there are two groups with para-sitic larvae: Meloidae (blister beetles and Spanish flies, with larvae parasitizing Hymenoptera) and Rhipiph-oridae (parasites of Hymenoptera and roaches). The Meloidae are further characterized (along with Anth-icidae, or ant-like beetles) by secreting cantharidin, a medical irritant and vesicant that had some notoriety until the eightheenth and nineteenth centuries for its supposed aphrodisiac properties.

The Phytophaga include two superfamilies, Chrysomeloidea and Curculionoidea, characterized by their extreme diversity and their phytophagous habits. The division into a smaller or larger number of families varies depending upon the author, but re-cently complete molecular and morphological phy-logenies of both superfamilies have been constructed, and the general relationships are well known. The leaf and longhorn beetles include some of the most color-ful species of Coleoptera, and also some of the most harmful to live plants (e.g., the Colorado potato beetle, Leptinotarsa decemlineata, of North American origin), stored products (e.g., Bruchinae, or bean weevils), and dead wood (some of the Cerambycidae). In many cas-es they have been used as a model of coevolution—or evolutionary arms race—with plants: the plant develops chemical defenses against the insect and the insect de-velops modes of detoxifying them.

Curculionoidea is traditionally divided into a greater number of families than Chrysomeloidea, per-haps reflecting a greater ecological and functional di-versity. There are groups adapted to all kinds of plant matter, both living and dead, and they are also eco-nomically significant, as in the case of the palm wee-vil and some species of wood borers (Scolytinae and Platypodinae), currently considered as subfamilies within Curculionidae.

Evolution of CharactersEvolutionary character transformations in Coleoptera are usually linked to ecological changes or a modified way of life, whether by transitions of habitat (from ter-restrial to aquatic, for example, or from the surface to a subterranean environment) or by a change in food resources. Changes associated with sexual selection, either in external morphological characters (shape, color) or in internal or external genital organs, are an-other possible source of modifications. In many groups of Coleoptera, the species are virtually indistinguish-able by their external features, and a reliable charac-terization and identification (confirmed in numerous cases by genetic data) is possible only by examining the aedeagus, which can present a staggering degree of

complexity. Under these circumstances, it is habitually assumed that there is a strong component of sexual se-lection, although there is little hard data to support this hypothesis.

Changes in habitat or lifestyle have been associ-ated with a series of morphological changes, often developed independently in numerous lineages. One of the most spectacular cases is the transition to a sub-terranean environment (Figure 34.2). Generally, this transition is accompanied by a series of morphologi-cal changes: reduction in size or even loss of eyes, re-duced pigmentation and sclerotization, and changes in the shape and proportion of appendages. Thus, species living in large cavities or interstices usually have streamlined bodies and elongated appendages (see Figure 34.2), while those inhabiting endogean en-vironments or very narrow interstices usually have a smaller body size. In such cases, the body structure also becomes more compact, with shorter and more robust appendages (legs and antennae). In the most extreme cases of adaptation, the changes extend to physiology and fat metabolism, as well as to pro-found life cycle variation. For instance, whereas in typical surface species, females lay many small eggs that hatch into active larvae that then go through three stages before pupation, females of some subterranean species lay a single large egg every time, from which hatches a larva that does not feed and goes directly to the pupal stage.

There are many other examples of concerted evolu-tion of what could be called evolutionary syndromes, such as changes produced in species inhabiting the

Figure 34.2 The specialized cave dwellers are among the most interesting groups of Coleoptera from an evolutionary, physiological, ecological, and biogeographical standpoint.Although they commonly have cryptic habits, the troglobite beetles have attracted the attention of entomologists and evo-lutionary biologists for almost two centuries. The photo shows Ildobates neboti (Carabidae, 8–9 mm), an emblematic cave-dwelling beetle species of the Iberian Peninsula, exclusive to four or five neighboring caves on the Mediterranean coast.

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shallow layers of soil and leaf litter in forests, subcorti-cal species, species in riparian environments, or mimetic wasp forms. For all these cases, there are examples of species from diverse families that have converged via a series of apparently adaptive common characters.

Evolutionary TendenciesThe enormous diversity of the Coleoptera, both taxo-nomic and ecological, makes it difficult to summarize the evolutionary tendencies of the group as a whole. Some tendencies are apparent through the specializa-tions that have originated independently on numerous occasions. Many of them are associated with a transi-tion or change, whether in habitat or in food resources (or both) (see previous section). Among the most fre-quent habitat transitions are the switch from a terres-trial to an aquatic environment and from the surface to

subterranean habits. Among trophic resource changes are switches from gymnosperms to angiosperms (see section Biogeography and Biodiversity) and the evolution of specialized habits such as myrmecophily or termi-tophily.

There are many families of Coleoptera in which most or all species are linked to the aquatic environ-ment at some stage of the life cycle (larval, adult, or both), but the exact number of transitions is still un-known. Of the four suborders, only Archostemata has no known aquatic species; all the families in Myxo-phaga are linked more or less to the aquatic environ-ment (normally in riparian or hygropetric habitats); and there are several lineages in the two most diverse suborders in which most species are aquatic. The monophyly of aquatic families is still under discussion in Adephaga, but there are at least six or seven inde-pendent transitions in Polyphaga: the Scirtoidea (with

Mimesis in beetlesThe term mimesis (also called mimicry) refers to the imitation by one organism (the mimetic) of another (the model) to confuse a third party (the dupe, or the target). Different combinations of these three elements result in different types of mimesis. So, when the organisms belong to three different species and a harmless mimetic imitates a dangerous model to avoid being preyed upon by the dupe, we are speaking of Batesian mimesis (in honor of Henry Bates, who first recognized it in several Amazonian insect groups). When both the mimetic and the model are dangerous, and they imitate each other to strengthen the signal to the dupe, it is called Müllerian mimesis. The relationship can also occur between only two species, as when the mimetic’s target poses as an individual of another species that would be both the model and the dupe; or even within a single species, if for example, a male (mimetic) poses as a female (the model) to deceive another male (the dupe) to have easier access to other females. The type of signal used to per-petrate the deception can take many forms, depending on the characteristics of the dupe’s recognition system: visual, acoustic, chemical, behavioral, and so on.

In Coleoptera, there are numerous examples of ev-ery imaginable type of mimesis, including some of the most sophisticated forms of deception in the animal world. Enacting the classic Batesian mimesis, there are species of cerambycids and elaterids that mimic the shape, color, and the flight of bees or wasps to the point of confusing experienced entomologists, who are able to recognize them only once caught. In many families there are examples of species that mimic the shape and movement of stinging ants to avoid predation by birds (as in Anthicidae), or that mimic other toxic species of Hemiptera and Coleoptera. It is also the case that other

groups mimic species of Coleoptera, either to appear more dangerous than they are (like an African lizard, Eremias lugubris, which in its juvenile phase mimics the color and gait of Anthia beetles, which are large and have toxic secretions); or to pose as harmless (like some spiders of the family Salticidae, which pretend to be in-nocuous beetles so as to not alert their victims). Among the most spectacular examples of Müllerian mimesis are those of some tropical Lycidae groups, which have dozens of toxic species with the same appearance and patterns of coloration.

The most sophisticated cases of mimesis occur among parasitic or commensal species of social insects. Some species of Staphylinidae are able to synthesize the same chemical compounds that termites use to rec-ognize each other. If a specimen is placed in a colony of another species it is killed immediately, but may be introduced into the model species’ colonies without interference and may plunder at will amongst the eggs and larvae. Other species that cannot synthesize these compounds simply take them from the species that par-asitize, so that the specimens can adopt different cam-ouflages depending upon who is their prey. The nitidulid beetle (Aethina tumida) is a parasite of the honeybee that adopts the typical behavior of the workers to beg for food, so even though the appearance of the Aethina is that of a typical beetle, it obtains honey from other individuals that apparently take it for just another bee in the colony. Finally, the case of the meloid beetle (Meloe franciscanus) is noteworthy: the larvae huddle at the end of the stalks of grass, appearing to be a female of the bee species of which they are parasites. When a male attempts to mate with this larval mass, the larvae climb upon it and are thus transported into the colony.

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aquatic larvae and terrestrial adults); Hydraenidae and Hydrophilidae (respectively monophyletic, but not re-lated) in Staphyliniformia; the families of Dryopoidea in Elateriformia; and two or three independent tran-sitions in different lineages of Phytophaga (in Curcu-lionidae and Chrysomelidae).

Transitions to the subterranean world are even more numerous and varied. There are species adapted to a subterranean environment in all suborders (the ex-act way of life of Crowsoniella, in Archostemata, is still uncertain but likely to be subterranean) and in most polyphagan series and superfamilies, whether to deep soil, interstitial environments, or even caves. In most cases, these transitions are accompanied by a whole se-ries of morphological modifications that are repeated independently in multiple lineages (see Evolution of Characters and Figure 34.2).

Transitions to a myrmecophilous or termitophi-lous lifestyle are less numerous but all the more inter-esting, due to the profound morphological, physio-logical, and ethological changes they require. In many beetle families, there are species that live as commen-sals or as predators (parasites) in the nests of ants and termites. This requires developing protective mecha-nisms against attacks by the hosts, at a minimum; but in many cases it has led to developing sophisticated camouflage systems: morphological, in which they im-itate the shape and the movement of ants; biochemical and physiological, in which they develop pheromones and chemicals very similar to those used by the ants, or attractants to capture them; and behavioral, in which specialized behaviors develop for entering the nest, or to avoid direct attack from the hosts (see Box Mi-mesis in beetles). The most spectacular examples are in the Histeridae and Staphylinidae, although there are examples in Carabidae, Scydmaenidae, Tenebrionidae, Ptiliidae, and other families.

Biogeography and BiodiversityThe exact number of described Coleoptera species is unknown. Linnaeus originally described 654 species; his disciple, Fabricius, another 4112 between 1775 and 1801; the catalogs from the last third of the nine-teenth century included about 77,000 species; and the latest attempt to catalog all species, the Coleopterorum Catalogus by Junk and Schenkling, published between 1910 and 1940, included about 221,500 species. Since then, the number of described species has not stopped growing; current estimates vary between 300,000 and 400,000, among which are an unknown number of undiscovered synonymies. In recent years, numerous initiatives to attempt to catalog all known species of Coleoptera have been developed (among which the Palearctic catalog stands out). If estimating the number of described species is difficult, estimating the number

of actual species (including those not yet described) is even more so: the figures vary from one million to over ten million. The vast majority of species are concentrat-ed in eight large families: Carabidae, Staphylinidae, Buprestidae, Scarabaeidae, Tenebrionidae, Chrysom-elidae, Cerambycidae, and Curculionidae.

Beetles inhabit all geographical zones except the open sea and regions with the most extreme climates at the poles and in high mountains. There are many intertidal species, which are active during periods of low tide and which survive being submerged at high tide; others are among the most resistant species to ex-tremes of climate, in the subantarctic islands as well as at heights of over 5000 meters in the Himalayas and in the driest deserts. Most families, and even most tribes, are distributed in more than one biogeographic region, due in part to their evolutionary age and to their ca-pacity for dispersion. Biogeographic patterns are es-pecially evident at lower taxonomic levels (genera, some tribes) or in poorly diversified groups that have remained confined to reduced geographical areas.

Differentiation and SpeciationMany hypotheses have been proposed to explain the ex-traordinary diversity of Coleoptera, especially the possi-bility of using angiosperms as food, with the subsequent proliferation and diversification of species occupying a multitude of available niches. This explanation does not apply to at least four of the eight major families (Carab-idae, Staphylinidae, Scarabaeidae, and Tenebrionidae), in which most species do not feed on plants. Recent molecular data support a more prosaic alternative: their age and their ability to survive explains the enormous diversity of Coleoptera. The rates of diversification in the major groups are comparable to or lower than those of other animal or plant groups; however, among the extant beetles there are many ancient lineages that have persisted since the Jurassic, steadily accumulating spe-cies. They predate the rise of angiosperms during the Cretaceous era and survived mostly unaffected by the mass extinctions at the end of the Mesozoic.

Principal Questions RemainingDespite the considerable volume of molecular and morphological information available, there are still many uncertainties in the phylogeny of Coleoptera. Briefly, these are some of the major issues that remain unsolved:• What are the relationships among the suborders?• What are the relationships of Gyrinidae and

Haliplidae?• What are the basal families of Polyphaga?• Are Staphyliniformia and Elateriformia

monophyletic?

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Basic Bibliography Beutel, R. G. and Leschen, R. A. B. (volume eds.).

2005. Coleoptera, beetles. Volume 1: Morphology and systematics (Archostemata, Adephaga, Myxophaga, Polyphaga partim). In, Handbook of Zoology. A Natural History of the Phyla of the Animal Kingdom, Volume IV. Arthropoda: Insecta. Part 38 (Kristensen, N. P. and Beutel, R. G., eds.). Berlin, Germany: Walter de Gruyter.

Hunt, T., Bergsten, J., Levkanicova, Z., Papadopoulou, A., St. John, O., Wild, R., Hammond, P. M., Ahrens, D., Balke, M., Caterino, M. S., et al. 2007. A comprehensive phylogeny of beetles reveals the evolutionary origins of a super-radiation. Science, 318: 1913–1916.

Leschen, R. A. B. and Beutel, R. G. (volume eds.) 2014. Arthropoda: Insecta: Coleoptera Volume 3:

Morphology and Systematics (Phytophaga). In, Handbook of Zoology. A Natural History of the Phyla of the Animal Kingdom, Volume IV. Arthropoda: Insecta. Part 38 (Kristensen, N. P. and Beutel, R. G., eds.). Berlin, Germany: Walter de Gruyter.

Leschen, R. A. B., Beutel, R. G., and Lawrence, J. F. (volume eds.) 2010. Coleoptera, beetles. Volume 2: Morphology and systematics (Elateroidea, Bostrichiformia, Cucujiformia partim). In, Handbook of Zoology. A Natural History of the Phyla of the Animal Kingdom, Volume IV. Arthropoda: Insecta. Part 38 (Kristensen, N. P. and Beutel, R. G., eds.). Berlin, Germany: Walter de Gruyter.

Löbl, I. and Smetana, S. 2003–2013. Catalogue of Palaearctic Coleoptera, Vols. 1–8. Steenstrup, Denmark: Apollo Books.

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tree of coleopterans colored boxes indicate high taxonomic...

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