Larval Specialization

Among the four suborders of Coleoptera, life histories of the predaceous Adephaga most closely resemble those of the beetles' phylogenetic sister group, the neuropteran orders. Adephagan larvae are generally campodeiform, that is, elongate and slightly dorsoventrally flattened, with long thoracic legs and a posteriorly tapered, dorsally sclerotized abdomen (Fig. 13). They typically have anteriorly directed mouthparts that often include elongate, sickle-shaped mandibles with a reduced mola (Fig. 14). The legs are six-segmented (coxa, trochanter, femur, tibia, tarsus, claws) as in the Megaloptera, Raphidioptera, and Neuroptera. The ninth abdominal tergite usually bears a pair of dorsolateral appendages (urogomphi) that may be short and unsegmented, or longer and variously segmented. These are secondarily evolved structures of the Coleoptera, and not homologous with the cerci of, for example, the orthopteroid orders. Adephagan larvae usually develop through three instars before pupation.

The larvae of Archostemata deviate from this generalized configuration by representing the syndrome that has evolved repeatedly in taxa characterized by the larval wood-boring habit. In these groups, the larvae are lightly sclerotized, more or less tubular, with shortened or reduced legs, and various ampullae on the thoracic and abdominal segments (Fig. 15). The archostematan family Micromalthidae exhibits probably the most bizarre set of larval forms and associated life cycle seen in Insecta. The campodeiform first instar is an active triungulin. It molts to become a legless, feeding cerambycoid larva, which in turn may undergo four types of molt. It may pupate directly to become an adult diploid female. Alternatively, it may develop into one of three kinds of larviform reproductive: a thelytokous pedogenetic female that parthenogenetically produces viviparously a number of diploid triungulins; an arrhenotokous pedogenetic female that lays a single egg, from which hatches a stump-legged curculionoid larva that in turn devours the mother, pupates, and then emerges as an adult haploid male; and an amphitokous pedogenetic female, which may produce either form. The hormonal controls of this system are not known, although production of the various larval types seems to be affected by environmental conditions.

The demographic consequences of this life cycle include the ability to quickly multiply and to use available rotting wood in the production of numerous dispersive adults. The triungulin larvae (Fig. 16) can expand the infestation to adjacent portions of the rotten log or timber. The cerambycoid larvae (Fig. 17), more typical of other archostematan larvae, can efficiently feed in confined galleries in rotting wood. The pedogenetic form (Fig. 18) can itself produce many more triungulins, enhancing the rate of increase of the population. The adults (Fig. 19) are produced in massive numbers, with these winged colonists establishing new colonies. Natural

Micromalthus Debilis

FIGURES 15-19 (15) Tenomerga concolor mature larva (Cupedidae), dorsal view. [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.67a. Kendall/Hunt, Dubuque, IA. Figures 16-19, Micromalthus debilis (Micromalthidae), dorsal view. (16) Triungulin first instar larva. (17) Cerambycoid larva. (18) Pedogenetic larva. (19) Adult female. (Drawings, Figs. 16-19, courtesy of the copyright holder, the Royal Entomological Society, London.)

FIGURES 15-19 (15) Tenomerga concolor mature larva (Cupedidae), dorsal view. [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.67a. Kendall/Hunt, Dubuque, IA. Figures 16-19, Micromalthus debilis (Micromalthidae), dorsal view. (16) Triungulin first instar larva. (17) Cerambycoid larva. (18) Pedogenetic larva. (19) Adult female. (Drawings, Figs. 16-19, courtesy of the copyright holder, the Royal Entomological Society, London.)

infestations have been reported in large Quercus (oak) or Castanea (chestnut) logs across the beetles' native range in northeastern North America. Other human-associated infestations have been reported from timbers deep in a South African diamond mine, and in thick oak paneling used to line the vaults of the Federal Reserve Bank in New York City.

The small suborder Myxophaga is characterized by adults and larvae of extremely small size, with both larvae and adults living interstitially in riparian areas, where they feed on algae. As opposed to the Archostemata and Adephaga, the larval legs are five-segmented, with the tarsus and claws fused into a single segment, the tarsungulus. The abdomen may or may not bear urogomphi on the ninth tergite. Like many other beetle species that feed on small particulate matter (pollen, spores, conidia, etc.), the larval mandibles bear a basal mola. Because they are aquatic in all stages, the adults bear a plastron, and the larvae may breathe by means of a plastron that covers the spiracles or via vesicular gills (i.e., a balloonlike expansion of the spiracular peritreme with an apical opening).

It is in the order Polyphaga that divergence of larval and adult lifestyles becomes evolutionarily significant. Among basal polyphagans in the superfamilies Staphylinoidea and Hydrophiloidea, larval anatomy remains generally of the campodeiform type, although mouthparts may be specialized for feeding on fungal food through development of broadly papillate molar regions on the mandible (Figs. 20-21). As in the Myxophaga, the larval leg has five segments. Aquatic forms may bear lateral gills on the thorax or abdomen (Fig. 22). Urogomphi of various configurations also may be present.

The larvae of the superfamilies Dascilloidea (Fig. 23), Byrrhoidea, and Bostrichoidea exhibit a dorsally convex body

Dascilloidea

FIGURES 20-21 Anisotoma errans larva (Leiodidae). (20) Head capsule, anterior view. (21) Right mandible, ventral view. Note large asperate mola at base. [From Newton, A. F., Jr. (1991). Leiodidae, pp. 327-329. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Figs. 34.152a and 34.154. Kendall/Hunt, Dubuque, IA.

FIGURES 20-21 Anisotoma errans larva (Leiodidae). (20) Head capsule, anterior view. (21) Right mandible, ventral view. Note large asperate mola at base. [From Newton, A. F., Jr. (1991). Leiodidae, pp. 327-329. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Figs. 34.152a and 34.154. Kendall/Hunt, Dubuque, IA.

configuration that has evolved into the much more exaggerated C-shaped grub characteristic of the Scarabaeoidea (Fig. 24). Scarab grubs can develop in a variety of microhabitats. Primitive scarabaeoids such as stag beetles (Lucanidae) and bess beetles (Passalidae) develop as saprophagous larvae in rotting wood. Larvae of the Geotrupidae and scarab subfamilies Scarabaeinae and Onthophaginae develop in mammalian herbivore dung where they also feed on fungi. Flowering plant roots are fed on by larvae of species in the more highly derived scarab subfamilies Melolonthinae, Rutelinae, and Dynastinae. Many species in these subfamilies are of economic concern, because they feed on commodities such as corn, small grains, vegetable crops, grasses, turf, fruits, and nursery stock. The C-shaped larval configuration results in an increased abdominal capacity relative to the head and thoracic forebody. This increased capacity is directly connected to the scarab larva's penchant for feeding on large amounts of food in order to pupate at a large size. Scarabs are well represented among the largest beetles, with the impressive Goliathus beetles of Africa and Asia attaining the greatest body mass of any beetle known.

Dascilloidea

FIGURES 22-24 Beetle larvae. (22) Berosus metalliceps (Hydrophilidae), dorsal view. [From Spangler, J. P. (1991). Hydrophilidae, pp. 355-358. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.296. Kendall/Hunt, Dubuque, IA.] (23) Dascillus davidsoni (Dascillidae), lateral view. [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.323a. Kendall/Hunt, Dubuque, IA.] (24) Popillia japonica (Scarabaeidae), lateral view. (© New York Entomological Society.)

FIGURES 22-24 Beetle larvae. (22) Berosus metalliceps (Hydrophilidae), dorsal view. [From Spangler, J. P. (1991). Hydrophilidae, pp. 355-358. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.296. Kendall/Hunt, Dubuque, IA.] (23) Dascillus davidsoni (Dascillidae), lateral view. [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.323a. Kendall/Hunt, Dubuque, IA.] (24) Popillia japonica (Scarabaeidae), lateral view. (© New York Entomological Society.)

Spiracles Insert

FIGURES 25-28 Larvae of phytophagous Chrysomeloidea and Curculionoidea. (25) Donacia sp. (Chrysomelidae), lateral view, sharp respiratory horns (rh) insert into underwater stems of water lily, providing air to the spiracles located at base of horns. (© New York Entomological Society.) (26) Neocimberis pilosus (Nemonychidae), lateral view (sp, spiracle). [From Anderson, W. H. (1947). Ann. Entomol. Soc. Am. 40, 489-517. © Entomological Society of America.] (27) Apion griseum (Apionidae), lateral view. (28) Hypera nigrirostris (Curculionidae), lateral view.

FIGURES 25-28 Larvae of phytophagous Chrysomeloidea and Curculionoidea. (25) Donacia sp. (Chrysomelidae), lateral view, sharp respiratory horns (rh) insert into underwater stems of water lily, providing air to the spiracles located at base of horns. (© New York Entomological Society.) (26) Neocimberis pilosus (Nemonychidae), lateral view (sp, spiracle). [From Anderson, W. H. (1947). Ann. Entomol. Soc. Am. 40, 489-517. © Entomological Society of America.] (27) Apion griseum (Apionidae), lateral view. (28) Hypera nigrirostris (Curculionidae), lateral view.

A C-shaped larva has evolved independently in another phytophagous group with concealed larval stages; the Curculionidae. The curculionid sister group, the Chrysomeloidea, is primitively characterized by larval stages superficially similar to those of Eucinetoidea and Dascilloidea, that is, larvae of moderately convex dorsal habitus (Fig. 25). Primitive weevils retain evidence of thoracic legs (Fig. 26); however, all evidence of thoracic appendages has been evolutionarily erased in higher weevils (Fig. 27). As phytophagous weevils have specialized, taxa have moved from being internal feeders to foraging on the external surfaces of their hosts. External feeders such as the lesser clover-leaf weevil gain a foothold on their host plant through ventral abdominal ampullae (Fig. 28), analogous to the prolegs of Hymenoptera and Lepidoptera. A parallel transition from hidden feeders to exposed foliage feeders has also evolved in the weevil sister group, the Chrysomeloidea.

The Cerambycidae comprise one basal division of the chrysomeloids, with all their larvae internal feeders. The Palophaginae represent the earliest divergent lineage of Chrysomelidae, based both on late Jurassic fossils (> 145 mya), and phylogenetic analysis of living species. Larvae of this subfamily attack the male strobili of Araucaria (Coniferales: Araucariaceae).

The subfamily Aulacoscelinae represents another early-diverging chrysomelid lineage. Larvae of this group are internal feeders on cycads. From this syndrome of hidden feeding, leaf beetle larvae have evolved to live on open plant tissues of many of the world's angiosperms. Where plants have evolved the ability to incorporate secondary chemical compounds in their tissues, herbivorous chrysomelids have evolved to use these chemicals to recognize food and stimulate oviposition. They have also evolved the ability to sequester these broadly toxic chemicals into their tissues to gain protection from predators. Today it is commonplace to observe brightly colored larvae and adults of protected leaf beetles congregated on exposed plant surfaces, serving as a communal warning to predators regarding their unpalatability.

The wood-boring larval body plan of the Archostemata is well represented in the Polyphaga, having independently evolved in the Buprestidae (Fig. 29), Eucnemidae, and Cerambycidae (Figs. 30-31). Larvae in all these families can bore through freshly dead or dying wood by using their well-developed, anteriorly directed mandibles. Laterally expanded thoracic segments or abdominal ampullae serve to anchor these larvae in their tunnels, facilitating purchase by the

Lidsk Kostra
2.0 mm 2.0 mm

29 30 31 32

FIGURES 29-32 Larvae of wood-boring beetles. (29) Agilus anxius (Buprestidae), dorsal view. (30) Unidentified lepturine larva (Cerambycidae), ventral view, scale unknown. [Figs. 29, 30 from Boving, A. G., and Craighead, F. C. (1930). An illustrated synopsis of the principal forms of the order Coleoptera. Entomol. Am. 11, 1-125. © New York Entomological Society.] (31) Platyzorilespe variegata (Cerambycidae), lateral view. [From Gardner, J. C. M. (1944). On some coleopterous larvae from India. Ind. J. Entomol. 6, 111-116. © Entomological Society of India.] (32) Hemicrepidius memnonius (Elateridae), dorsal view. [From Dietrich, H. (1945). Cornell University Agricultural Experiment Station Memoir 269, plate IV.2. © Cornell University.]

mandibles on the wood surface. Leg reduction has proceeded during diversification of cerambycid borers, with larvae of more basally divergent subfamilies such as the Prioninae and Lepturinae having shortened thoracic legs (Fig. 30), whereas larvae of the highly derived subfamily Lamiinae (Fig. 31) are legless.

Where wood-boring beetles have gone, similarly shaped predatory beetles have followed. These tubular larvae in the Elateroidea and Cleroidea may be highly sclerotized, and they bear well-sclerotized head capsules and/or urogomphal plates (Fig. 32) that armor them appropriately for their habitats (e.g., under bark, within wood-boring beetle galleries). Elaterid larvae have diverse feeding habits, with many groups being phytophagous or saprophagous. However, all forms, regardless of food habit, imbibe their food as an extraorally predigested liquid.

Other elateroid larvae, such as fireflies (Lampyridae) and soldier beetles (Cantharidae), lack the heavy armor of the concealed gallery feeders, and prey on other arthropods among leaf and ground litter. These larvae use grooved mandibles to suck up the liquefied contents of their prey. In Elateridae, and independently in Phengodidae and Lampyridae, larval and adult stages have evolved the ability to produce light using organs composed of modified cuticular cells. The significance of larval luminescence has been variously explained. For example, night active Pyrearinus larvae in the elaterid subfamily Pyrophorinae use light organs to attract flying insect prey to Brazilian termite mounds where they make their home. Phengodid larvae of the genus Phrixothrix possess medial photic organs on the head that use red light to illuminate potential prey. But they also possess lateral abdominal light organs that emit green light. These abdominal light organs are homologous with those of Lampyridae and most likely serve to advertise that the larvae are chemically protected. Increasingly complicated light communication systems have evolved in the adult stages of various phengodid and lampyrid taxa.

Cucujoidea and Tenebrionoidea are diverse superfamilies whose larval forms blend imperceptibly into each other morphologically and biologically. It is in these groups that saprophagous and mycophagous feeding habits are associated with extensive larval diversification. Primitive larvae of both superfamilies are similar and typical of Polyphaga in many ways (e.g., five-segmented legs, urogomphi, moderate degree of sclerotization, etc.) Evolutionary trends in one often are mirrored in the other. Cucujoid and tenebrionoid larvae are usually small to moderate in size, and somewhat dorsoventrally compressed. Many are cryptozoic, occurring in leaf litter, under bark, in fungus, or in rotting wood, where they feed on fungi or on fungus-altered plant matter. Groups specialized for feeding on spores, conidia, loose hyphae, or other small particles exhibit various specializations correlated with microphagy. Most notably, these include a well-formed mandibular mola and prostheca (Fig. 33).

Extreme dorsoventral compression of the body has occurred repeatedly in response to the selective pressures of

FIGURE 33 Left larval mandible, ventral view, of Anchorius lineatus (Biphyllidae), showing basal mola (lower left) and prostheca with comb hairs; mandible width, 0.16 mm. [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.]

FIGURE 33 Left larval mandible, ventral view, of Anchorius lineatus (Biphyllidae), showing basal mola (lower left) and prostheca with comb hairs; mandible width, 0.16 mm. [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.]

occupying subcortical and interstitial leaf litter habitats. Sometimes (Fig. 34), the body form is simply flattened, with a reorientation of the head to a protracted, prognathous condition and a migration of the leg articulations to more lateral positions. Flattening, however, may be accompanied by an additional transition to an onisciform (or pie-plate-shaped) body through extensive development of tergal flanges, resulting in a broadly oval body outline in some Cerylonidae, Corylophidae (Fig. 35), Discolomidae, and Nilioninae (Tenebrionidae). Larvae specialized for life under bark, in fungi, or in rotting wood typically have short, stout,

Corylophidae Larvae

FIGURES 34-35 Flattened beetle larvae, dorsal view. (34) Dendrophagus americanus (Cucujidae). [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.527. Kendall/Hunt, Dubuque, IA.] (35) Corylophidae, genus unknown [From Lawrence, J. F. (1991). In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.628a. Kendall/Hunt, Dubuque, IA.] Figures 36—37 Larvae of Coccinellidae, dorsal view. (36) Pre dace ous Stethoris histrio. (37) Phytophagous Epilachna varivestis: sc, scolus; ve, verruca. [From Le Sage, L. (1991). Coccinellidae, pp. 485-494. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.570. Kendall/Hunt, Dubuque, IA.]

34 35 36 37

FIGURES 34-35 Flattened beetle larvae, dorsal view. (34) Dendrophagus americanus (Cucujidae). [From Lawrence, J. F. (1991). Order Coleoptera. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.527. Kendall/Hunt, Dubuque, IA.] (35) Corylophidae, genus unknown [From Lawrence, J. F. (1991). In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.628a. Kendall/Hunt, Dubuque, IA.] Figures 36—37 Larvae of Coccinellidae, dorsal view. (36) Pre dace ous Stethoris histrio. (37) Phytophagous Epilachna varivestis: sc, scolus; ve, verruca. [From Le Sage, L. (1991). Coccinellidae, pp. 485-494. In "Immature Insects," Vol. 2 (F. W. Stehr, ed.), Fig. 34.570. Kendall/Hunt, Dubuque, IA.]

unarticulated, and unsegmented urogomphi. The apex is typically recurved to point dorsally. This form of urogomphi is thought to help larvae move about in cramped habitats.

Several tenebrionoid and cucujoid groups experienced parallel transitions to a parasitic lifestyle, including Meloidae, Rhipiphoridae, and some Cucujidae and Bothrideridae. The Rhipiphoridae provide a glimpse at parasitism involving both externally and internally feeding stages. In Rhipiphorinae, the triungulin first instar locates and attaches itself to an adult of a suitable hymenopteran host. After being carried back to the host's nest, the triungulin detaches itself and searches for a host larva. Once the host has been located, the larva burrows inside, where feeding continues (endophagy) until the parasitoid becomes greatly swollen. As the host larva reaches maturity, the parasitoid exits from its thorax, switching to feed externally (ectophagy), eventually killing it.

In Rhipidiinae, the reverse sequence of internal and external feeding occurs: the triungulin locates a cockroach as a potential host, inserts its head and thorax into a membranous region on its venter, and begins to feed while most of its body remains outside the host. Later, the larva transforms into a less mobile, legless form and moves entirely inside the host, where it begins to grow rapidly. Near the end of its development, the larva molts to a form with legs and emerges from the host to pupate.

Cucujoid and tenebrionoid taxa that are adapted for external feeding typically have a more eruciform (caterpillarlike) shape resulting from elongation of the legs, reorientation of the head to a more hypognathous position, and dorsoven-tral inflation to a more cylindrical shape. These external feeders also tend to exhibit defensive modifications. Aposematic coloration is common in these groups. Tergal and pleural armature, which is absent or modest in most cucujoids and tenebrionoids, becomes exaggerated in some predators (e.g., stethoris, Coccinellidae, Fig. 36), surface feeding herbivores (e.g., the coccinellid genus Epilachna, Fig. 37), and fungus feeders (e.g., the erotylid genus Aegithus), to form various structures such as setose, tuberculate verrucae, and complexly branched scoli.

Within Cucujoidea and Tenebrionoidea there is a recurring evolutionary transition from mycophagy/saprophagy to a lifestyle of true phytophagy as a borer in healthy herbaceous stems or wood. This entire sequence can be observed within individual families (e.g., Melandryidae), where there is a range of larval feeding that extends from boring in fungus sporophores to boring in fungus-infested wood and finally to boring in sound wood. Cucujoid and tenebrionoid wood borers tend to have fleshy bodies with conspicuous sclerotized plates usually restricted to the anterior end of the body. The head capsule tends to be prognathous and often bears a median endocarina, an internal keel on the dorsum associated with the development of especially powerful mandibular muscles.

Predatory larval forms have arisen repeatedly within Cucujoidea and Tenebrionoidea, most notably in the

Coccinellidae. Accompanying this trophic transition is a suite of morphological changes to produce a campodeiform body (Fig. 36). The head typically has a more prognathous orientation. The mandibles are more prominent and lack a mola.

The beetle pupa is adecticous and usually exarate (i.e., the mandibles are fixed in position, and the head and thoracic appendages are free). Several groups have independently evolved the obtect condition; among them staphylinine Staphylinidae, Ptiliidae, and Coccinellidae, and the hispine Chrysomelidae. If the pupa rests concealed in a pupal chamber, it lies on its dorsum elevated from the substrate by numerous thoracic and abdominal setae. Pupae may be enclosed in a cocoon made of silk (aleocharine Staphylinidae, Tenebrionidae, Curculionidae), fecal material (Passalidae and some Scarabaeidae), or the larval fecal case (cryptocephaline Chrysomelidae).

Exposed pupae, as in Coccinellidae, Chrysomelidae, and Erotylinae (Erotylidae) may remain attached to their host plant or fungus via the sloughed-off last larval cuticle, which encircles the anal portion of the pupa. Such exposed pupae may be protected by defensive secretions remaining in the shed larval skin. Beetle pupae retain the ability to move the abdomen by using the flexible abdominal intersegmental membranes. Sclerotized processes on opposing margins of the abdominal segments, called gin traps, have been suggested as defensive devices used to pinch and drive off mites and other predators.

Bee Keeping

Bee Keeping

Make money with honey How to be a Beekeeper. Beekeeping can be a fascinating hobby or you can turn it into a lucrative business. The choice is yours. You need to know some basics to help you get started. The equipment needed to be a beekeeper. Where can you find the equipment you need? The best location for the hives. You can't just put bees in any spot. What needs to be considered when picking the location for your bees?

Get My Free Ebook


Post a comment