Ecological Specialization

One means of estimating the ground plan feeding habits of Coleoptera uses observations of extant taxa, interpreted in context of phylogenetic hypotheses for the various lineages making up the order. By this method, we would deduce that the most primitive beetles were either saprophagous wood borers as larvae, such as extant Archostemata, or that they were campodeiform predators, such as the Adephaga and the sister group to Coleoptera, the Neuroptera+Raphidioptera+ Neuroptera. Examining the fossil record of Coleoptera as well as suggestive damage to fossil plants of the Triassic and Jurassic formations containing the earliest beetle fossils provides a second means of making such an estimation. By this method, we find that archostematans and primitive weevils predate fossils of all other types, suggesting that the earliest feeding habits were either saprophagous or herbivorous. Of course, fossil evidence of predation is not likely to be preserved, nor interpretable as such if it were. These two viewpoints, phylogenetic and paleontological, represent the diversity of opinion about how the first beetles lived their lives.

The two viewpoints can be reconciled if we view fossil data drawn from the various periods in light of phylogenetic estimates based on a diversity of taxa and characters. To do this, we must assume that the lifestyles of recent taxa represent those of their related fossil relatives. By this reasoning, it is very apparent that herbivorous taxa have

FIGURE 38 The number of beetle genera of each of three trophic levels from Permian to recent epoch. Permian genera represent Protocoleoptera. [Redrawn with permission from Farrell, B. D. (1998). "Inordinate fondness" explained: Why are there so many beetles? Science 281, 555-559. © 1998 American Association for the Advancement of Science.]

FIGURE 38 The number of beetle genera of each of three trophic levels from Permian to recent epoch. Permian genera represent Protocoleoptera. [Redrawn with permission from Farrell, B. D. (1998). "Inordinate fondness" explained: Why are there so many beetles? Science 281, 555-559. © 1998 American Association for the Advancement of Science.]

constituted a majority of the major life-forms, measured by their recognition as genera, for coleopteran faunas from the Jurassic and Tertiary to recent times (Fig. 38). Even before the advent of the flowering plants, more than half the variety of beetle life-forms had evolved to focus their feeding attentions on plants. We can investigate the impact of the origin and diversification of angiosperms on beetle diversity by looking at beetle sister taxa in which one group is restricted to gymnospermous plants, whereas its sister is found on angiosperms. Brian Farrell examined lineages within the Chrysomeloidea and Curculionoidea. He found angio-sperm-feeding taxa to be far more rich in species today than their gymnosperm-feeding sister groups. Clearly, angiosperm feeding has enhanced the species-level diversification of beetles living on them.

In addition to internal feeding on stem tissue, and feeding on saprophagous growth in decaying cambium, the angio-sperms offer floral resources unavailable from gymnosperms. Adult beetles of many families characterized by phytophagous, saprophagous, or scavenging larvae may be found feeding in or on flowers, associated exudates, or pollen. Melolonthine and cetoniine scarab beetles, whose larvae are subterranean root feeders or rotten wood feeders, respectively, often feed on flowers. Dermestid beetle larvae scavenge dead animal matter, then move to flowers to feed on pollen after they have eclosed as adults. Once a female dermestid has fed, she becomes negatively phototactic and searches for cavities containing animal remains, where she will oviposit. Other families well represented among the pollen-feeding adults include Buprestidae, Lycidae, Nitidulidae, Mordellidae, Rhipiphoridae, Meloidae, Anthicidae, and Cerambycidae. Meloids and rhipiphorids not only feed at flowers but oviposit there, with their hatching triungulin larvae waiting in the flower to climb on passing bees and wasps, which they parasitize. Feeding on hard pollen grains is facilitated by possession of mandibles bearing a well-developed mola. Such mandibles are also associated with fungal feeding, and families such as the Nitidulidae, Tenebrionidae, and Oedemeridae contain species representing both adult feeding habits; individual oedemerid species have been reported to feed on both fungi and pollen.

Coleopteran relationships with fungi are widespread throughout the order and diverse in form. Approximately 25 extant families of beetles are primarily mycophagous. Greatly unappreciated, however, are the less obvious trophic relationships between fungi and many beetles that are ostensibly saprophagous or phytophagous. Many beetles eat plant tissue only after it has been partially broken down by fungi. Some harbor endosymbiotic fungi that allow digestion of plant tissue or provide essential nutrients. Others are thought to ingest and acquire fungal enzymes that are essential for their existence as herbivores. John Lawrence estimated that as many as half of all beetle families either are truly mycophagous or feed on plant matter that has been altered by fungal enzymes.

Ancient Greeks believed fungi were merely homes of insects. A rich insectan fauna often dwells in larger fungi, and much of it comprises mycophagous and predaceous beetles. Through evolutionary time few fungus taxa have escaped the interest of beetles. Mycophagous families seem to be especially concentrated in the polyphagan superfamilies Cucujoidea, Tenebrionoidea, and Staphylinoidea. However, fungivory arose repeatedly in various other lineages within the order as well.

Fungi are tremendously diverse physically, chemically, behaviorally, and ecologically. Mushrooms, woody conks, puffballs, truffles, yeasts, smuts, rusts, and molds present separate special challenges as food sources. In addition, a single fungus often represents a composite of resources. For example, a single polypore shelf on a log may provide a delicate layer of spore-bearing tissue on the underside, a hard, woody context, and an area where its hyphae penetrate decaying wood. Some mycophagous beetles have a broad range of acceptable hosts; however, many are more selective, feeding only on some portions of fruiting structures from a few species at a particular stage of development or decay. Host specificity tends to be narrower for immature stages. Specialization of beetles has occurred in response to the various resources and challenges that fungi present.

Woody polypore shelves offer large, persistent sources of food for mycovores. There are many different strategies for the use of the soft spore-bearing tissue of wood polypore fungi. Species of Ellipticus (Erotylidae) have robust mandibles capable of gouging off chunks of hymenium and its supporting tissues (Fig. 39). Larvae of Holopsis (Corylophidae) have found another method of tapping this resource. They use a slender, snoutlike elongation of the head to graze on the inner surface of individual spore tubes (Fig. 40). The Nannosellinae (Ptiliidae) exhibit another evolutionary solution, namely,

FIGURES 39-42 Feeding structures of fungus-feeding beetle larvae. (39) Larval mandible, ventral view, of Ellipticus sp. (Erotylidae); multidentate apex and setose lobe near base (lower left) are used to bite large chunks off fungal substrate, which are then swallowed whole (scale, 100 ^.m). [From Lawrence, J. F. (1989). Mycophagy in the Coleoptera: Feeding strategies and morphological adaptations. In "Insect—Fungus Interactions" (N. Wilding, N. M. Collins, P. M. Hammond, and J. F. Webber, eds.), p. 16, Fig. 21. Academic Press, London.] (40) Holopsis sp. (Corylophidae), lateral view, with long feeding rostrum bearing apical mandibles, allowing feeding inside pore tubes of sporocarp fungi (scale, 100 ^.m). [From Lawrence, J. F. (1989). Mycophagy in the Coleoptera: Feeding strategies and morphological adaptations. In "Insect—Fungus Interactions" (N. Wilding, N. M. Collins, P. M. Hammond, and J. F. Webber, eds.), p. 16, Fig. 17. Academic Press, London.] (41) Larval head, anterior view, of Dasycerus sp. (Staphylinidae), showing brushy mandibular apices used to remove spores or hyphae from the substrate to the mouth (scale, 50 [From Lawrence, J. F. (1989). Mycophagy in the Coleoptera: Feeding strategies and morphological adaptations. In "Insect—Fungus Interactions" (N. Wilding, N. M. Collins, P. M. Hammond, and J. F. Webber, eds.), p. 11, Fig. 12. Academic Press, London.] (42) Larval right mandible, ventral view, of Nosodendron unicolor (Nosodendridae), showing food press near base (lower right) that concentrates particulate food while ejecting liquid (scale, 100 ^.m). [From Lawrence, J. F. (1989). Mycophagy in the Coleoptera: Feeding strategies and morphological adaptations. In "Insect—Fungus Interactions" (N. Wilding, N. M. Collins, P. M. Hammond, and J. F. Webber, eds.), p. 6, Fig. 6. Academic Press, London.]

miniaturization: fully grown adults, only 0.4 mm in length, crawl inside individual spore tubes to feed directly on the soft spore-bearing tissue.

Specialists on fleshy mushrooms [e.g., Oxyporus (Staphylinidae),] face different challenges. Unable to fly around to look for new mushrooms, larvae must complete their feeding on their ephemeral host before it decays. Many of the fungus beetles that specialize on soft mushrooms exhibit greatly accelerated development. Their mandibles are more bladelike and are capable of slicing through large chunks of soft fungal tissue.

Beetles preferring small, scattered fungal spores or conidia as food often have a suite of features related to their microphagous habits. The mouthparts tend to be brushy and capable of sweeping tiny particles from the substrate into their mouth. These modifications often involve the maxillae, but in larval Dasycerus (Staphylinidae) the mandibular apices are modified for this function as well (Fig. 41). The mandibular mola is also commonly modified in spore feeders. Spores are ground between opposing molar grinding surfaces on each mandible, with an action much like that of a millstone grinding wheat into flour. Nosodendridae, which feed partially on yeasts that occur in sap fluxes, use their brushy mouthparts to filter the fungal cells from the fluid

Less exploitative symbiotic relationships with fungi also are widespread and diverse within Coleoptera. The best-studied examples of mutualism with fungi are the relationships occurring in the bark and ambrosia beetles (Platypodinae and Scolytinae of the Curculionidae). Perhaps the most familiar case is that of Dutch elm disease. At the corners of this "ecological triangle" are the bark beetles (Scolytus spp.), the fungus (Ceratostomella ulmi), and the host elm trees (Ulmus spp.). Adult beetles nibble on tree twigs and thereby inoculate them with fungal spores. Following germination of the spores, the fungus attacks the tree and ultimately kills it. The beetles prefer to oviposit on recently killed Ulmus trees, many of which were recent victims of C. ulmi. Upon hatching, their larvae bore about, feeding on fungus-infested wood. The final link in the cycle is completed when newly emerging adults pick up fungal spores as they move around the gallery before flying off to dine on some living elm twigs.

A broad range of variants stems from the basic pattern observed in Dutch elm disease. In some cases the link between the fungus and the beetles weakens to the point of being merely incidental. In Lymexylidae and at least some Platypodinae, the relationship is a tighter, obligatory one in which the beetles farm a fungus to feed their brood. The wood of the host tree is important to the beetle only as a substrate for the fungal garden. In these evolutionarily linked relationships, the beetles often have specialized pockets called mycangia on their body to aid in the transportation of spores or conidia to new substrates (Figs. 43—44). Mycangia sometimes have associated glands that help to keep the fungal tissue viable until it is needed to start a new garden. There also is a tendency for these ambrosia fungi to be less invasive and destructive to the tree, instead staying near the galleries in which they are cultivated. Neither the fungi nor the beetles in these closer relationships can exist independently.

Another solution to digestion of plant matter is seen in some Cerambycidae and Anobiidae. Instead of using fungi to externally convert plant matter to digestible food, they rely on endosymbiotic yeasts and bacteria to accomplish the feat internally. Although yeasts and bacteria are common inhabitants of the gut in many insects, the relationship between some yeasts and beetles is one of obligatory symbiosis. Endosymbiotic yeasts may be harbored in the lumen of the gut, in diverticula (Fig. 45), or in specialized cells in the cytoplasm called mycetocytes. Clusters of

FIGURES 43-45 Beetle mycangia and mycetome. (43) Scolytoplatypus sp. (Curculionidae), transverse section of front part of adult pronotum, showing mycangial cavity filled with spores. [From Crowson, R. A. (1981). "The Biology of Coleoptera," p. 562, Fig. 286. Academic Press, London.] (44) Eurysphindus hirtus (Sphindidae), left adult mandible, dorsal view, showing spores of myxomycete inside dorsal cavity that is presumed to serve as a mycangium. [From McHugh, J. V. (1993). A revision of Eurysphindus LeConte and a review of sphindid classification and phylogeny. Syst. Entomol. 18, 57-92. © Blackwell Science Ltd.] (45) Foregut (F) and anterior portion of midgut of Lixus sp. larva (Curculionidae), showing mycetomes (M). [From Crowson, R. A. (1981). "The Biology of Coleoptera," p. 562, Fig. 286. Academic Press, London.]

FIGURES 43-45 Beetle mycangia and mycetome. (43) Scolytoplatypus sp. (Curculionidae), transverse section of front part of adult pronotum, showing mycangial cavity filled with spores. [From Crowson, R. A. (1981). "The Biology of Coleoptera," p. 562, Fig. 286. Academic Press, London.] (44) Eurysphindus hirtus (Sphindidae), left adult mandible, dorsal view, showing spores of myxomycete inside dorsal cavity that is presumed to serve as a mycangium. [From McHugh, J. V. (1993). A revision of Eurysphindus LeConte and a review of sphindid classification and phylogeny. Syst. Entomol. 18, 57-92. © Blackwell Science Ltd.] (45) Foregut (F) and anterior portion of midgut of Lixus sp. larva (Curculionidae), showing mycetomes (M). [From Crowson, R. A. (1981). "The Biology of Coleoptera," p. 562, Fig. 286. Academic Press, London.]

mycetocytes can form small organs called mycetomes. Yeasts may permit the breakdown of cellulose and provide various nutrients to their host. In the drugstore beetle, Stegobium paniceum, endosymbiotic yeasts are credited with providing riboflavin, niacin, pyridoxine, pantothenic acid, folic acid, and biotin.

To perpetuate endosymbiotic relationships, the gut of offspring must be charged with endosymbionts early in development. Yeasts are passed from adult beetles to larvae in various ways. The egg chorion may be inoculated with yeast so that the young are charged upon chewing out of egg and ingesting the chorion. In some Cucujidae, Silvanidae, Lyctidae, and Curculionidae, yeasts migrate into the egg within the female before the chorion is secreted. A third method of yeast transmission results following migration into the testes of the father. The yeast and sperm then enter the egg through the micropyle.

Formerly classified as fungi and studied by mycologists, the Myxomycetes are now recognized as protozoan animals. Despite their phylogenetic position, Myxomycetes are similar to fungi in some respects, and as a result beetle-myxomycete interactions share parallels with beetle—fungus interactions. In the plasmodial stage, Myxomycetes flow around their

FIGURE 46 Anisotoma basalis (Leiodidae) feeding on a Stemonitis myxomycete fruiting body. [From McHugh, J. V., and Wheeler, Q. D. (1989). Cornell Plantations Q. 44(3), cover figure. © Cornell Plantations.]

environment, consuming bacteria. Rhysodine Carabidae and Cerylonidae feed, at least facultatively, on the plasmodial stage. When these colonial protozoans well up as plasmodia to form a sporocarp, they take on many funguslike features. This stage has attracted specialist beetles from no fewer than seven families: Leiodidae (Fig. 46), Staphylinidae, Clambidae, Eucinetidae, Cerylonidae, Sphindidae, and Lathridiidae. Pits in the mandibles of Sphindidae, an entirely myxomy-cophagous family (Fig. 44), and the venter of slime-mold-feeding latridiid species have been found to house myxomycetan spores.

Whereas other holometabolous insect orders such as the Hymenoptera and Diptera include parasitoid lineages of great diversity, the Coleoptera have not diversified to any great extent via parasitism on animal hosts. In addition to meloid and rhipiphorid hymenopteran parasites, parasitism of single host individuals has been infrequently observed. Aleocharine Staphylinidae parasitize the pupae of higher flies (order Diptera, suborder Cyclorrhapha). Within the Carabidae, the bombardier beetles, or Brachinini, parasitize the pupae of Gyrinidae, and species of the genus Lebia parasitize chrysomelid leaf beetles. Lebia beetles imitate various alticine flea beetle species with which they co-occur. The quick-jumping alticines are protected from predatory birds by their ability to disappear via a jump, suggesting that the Lebia have evolved a similar appearance through mimetic evolution. Coccinellid predatory larvae approach the specialization seen in some parasitoids, as some of the smaller species require only one to several homopterous prey individuals to complete larval development. Nonetheless, these species can switch prey species depending on the density of various hosts.

Platypsyllus castoris beetles of the family Leiodidae are specialists on beavers, with both the flattened, highly modified adults (Fig. 47) and the larval stages living in the animals' fur. Related leiodids in the subfamily Leptininae live

FIGURE 47 Platypsyllus castoris (Leiodidae), parasitic on beaver (Castor spp.). [From Crowson, R. A. (1981). "The Biology of Coleoptera," p. 549, Fig. 280. Academic Press, London.]

on the bodies of rodents, though they exhibit much less extreme body forms, and a lower level of host specificity, than the beaver beetles. The highly specific host relationship of Platypsyllus probably evolved from a more general predaceous habit. Such nest inquilines are found in a variety of lineages within the Staphylinidae, with adults and larvae variously preying on flea larvae or other nest-associated scavengers.

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