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Hymenoptera

(Sawflies, ants, bees, and wasps)

Class Insecta

Order Hymenoptera

Number of families About 84

Photo: A female sawfly (family Tenthredinidae) guards her eggs in the Amazon of Peru. (Photo by George D. Dodge. Bruce Coleman, Inc. Reproduced by permission.)

Evolution and systematics

Hymenoptera is a worldwide order of at least 100,000 described species, more biologically diverse than any other insect order. The order is divided into two suborders— Symphyta (wood wasps and sawflies) and Apocrita (wasps, bees, and ants)—with 22 superfamilies and about 84 families. The fossil record dates from the Triassic (245–210 million years ago). The Hymenoptera may be the sister group of the Antliophora, made up of Diptera (flies), Siphonaptera (fleas), and Mecoptera (scorpionflies and hangingflies); and the Amphiesmenoptera, comprising the Trichoptera (caddisflies) and Lepidoptera (butterflies and moths).

Physical characteristics

Adult hymenopterans range in size from minute to large, at 0.006–4.72 in (0.15–120 mm) and from slender (e.g., many wasps) to robust (e.g., the bumble bees). The head usually is very mobile. The compound eyes often are large and sometimes strongly convergent dorsally. Fine setae occasionally emerge from between facets, and ocelli may be present, reduced, or absent, especially in forms with reduced wings. The antennae are long and multisegmented, and their surfaces are covered with various sense organs. The mouthparts vary from the generalized biting type to the combined sucking and chewing type (e.g., bees). Mandibles typically are present and are used by the adult to cut its way out of the pupal cell, for defense, for killing and handling prey, and in nest construction.

The first abdominal segment of the Apocrita is attached firmly to the metathorax and usually is separated from the remaining abdominal segments (metasoma) by a narrow waist (petiole). In Apocrita thoracic segments plus the first abdominal segment are called the mesosoma, and the incorporated first abdominal segment is the propodeum, followed by the remainder of abdomen. There are generally two pairs of wings. Venation is most complete in Symphyta and mostly reduced in small Apocrita. The hind wings have rows of hooks (hamuli) along the leading edge that couple with the hind margin of the forewing in flight. The legs frequently are cursorial (adapted for running), sometimes with fossorial (adapted for digging) forelegs; the hind legs are modified to carry pollen.

Sensory structures (sensilla) on the ovipositor enable the female to recognize suitable egg-laying sites. In some ants, bees, and wasps the ovipositor has lost its egg-laying function and is used as a defensive, venomous stinger. Although the smooth stingers of ants and wasps allow for repeated use, the barbed stings of the honeybee can be used only once. As the honeybee struggles to leave the stinger and venom behind, it is disemboweled and soon dies.

Bees have several morphological adaptations associated with pollen collection, including plumose (branched) hairs. Moreover, the hind tibia and basitarsus are enlarged, with long hairs on their outer surfaces. These hairs either form a brush (scopa) or are reduced to a fringe surrounding a bare area or concavity (corbicula, or pollen basket). Leaf cutter bees have a well-developed scopa on the ventral surface of the abdomen.

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Order: Hymenoptera

A tree wasp (Vespula sylvestris) worker leaves its nest hole in a dead tree stump. (Photo by Kim Taylor. Bruce Coleman, Inc. Reproduced by permission.)

Distribution

Hymenopterans are found worldwide.

Habitat

Hymenoptera occur in soil and litter or on vegetation. Most are active on bright, sunny days, hunting insects, gathering pollen and nectar, or assembling nest-building materials. Some parasitic species are active at night, when their nocturnal hosts are active.

Behavior

Symphyta lay their eggs on or in leaves, stems, wood, and leaf litter, and females sometimes stand guard over their egg masses. The larvae are almost exclusively phytophagous. Pupation takes place within the plant tissue or in the ground. Most have a single generation a year and overwinter as fullgrown larvae. The larvae of many species of Apocrita are parasitoids in the immature stages of other insects (or other invertebrates), while the adults are free living.

Idiobiont parasitoids prevent any further development of the host after initial parasitization. Koinobiont parasitoids allow the host to continue its development and often do not kill and consume the host until the host has reached its maximum size. The development of a secondary parasite, or hyperparasite, at the expense of a primary parasite is more

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frequent in Hymenoptera. The Apocrita species that have the ovipositor modified into a sting are grouped together into the Aculeata. Female Scoliidae locate and attack large subterranean beetle larvae in their burrows or earthen pupal cells. The sting does not kill the host but only immobilizes it. Then the female lays her eggs and departs, leaving her offspring to develop without further assistance. Aculeate hymenopterans may or may not use their stings to immobilize the host; mutillids bite through the host cell and lay an egg on the mature larvae or pupae of the host.

Somewhat more complex behavior is exhibited by many Pompilidae, in which the female captures and paralyzes her prey and then drags it to a cavity or crevice on the ground. She lays an egg on the host and usually seals the cavity before leaving. Cleptoparasites, such as cuckoo bees of the genus Chrysis (actually a metallic blue or green, thick-bodied wasp), construct no nests of their own and instead rely primarily on the food stores of hosts (Chrysis). In other aculeates (for example, other Pompilidae), the female wasp prepares a nest before locating prey and can relocate her nest when she returns with prey.

Female Eumenes (Vespidae) lay eggs in the empty cell before prey are introduced. The female then provides the cell with prey and seals it before the larva begins to develop. This is called mass provisioning, because the initial food that is amassed must be sufficient to feed the larva during the entire course of its development. Progressive provisioners, such as many Sphecidae and Vespidae, provide additional food at intervals. Large, multicelled nests, in which each cell is stocked with many small prey, are characteristic of many behaviorally advanced species. Bees (Megachile and Xylocopa) are similar to wasps in this respect, except that they provision larval cells with pollen and nectar rather than arthropod prey.

Truly social or eusocial hymenopterans have a division of labor, with a caste system involving sterile individuals that assist the reproducers, cooperation among colony members in tending the young, and overlap of generations capable of contributing to colony functioning. Among hymenopterans exhibiting primitive eusociality are paper wasps. The highly eusocial hymenopterans comprise the ants, some wasps, and many bees. Common construction materials for nest building are mud, leaves, and masticated wood chips that are formed into a paperor carton-like nest.

Feeding ecology and diet

Adult parasitoids require carbohydrates in the form of honeydew, nectar, or other plant secretions. Many female parasitoids also feed extensively on the body fluids of hosts, to sustain egg production. Most symphytan larvae are phytophagous (plant eaters); larvae of Siricidae are wood borers and utilize cellulases produced by fungi to feed on wood; the enzymes necessary for processing the wood fragments are acquired from the fungus ingested. Apocritan larvae have diverse feeding habits; they may be parasitic or gall forming, or they may be fed with prey or nectar and pollen by their parents or other colony members. Adult hymenopterans mostly feed on nectar, pollen, or honeydew produced by Homoptera; only a

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few consume other insects. Many wasps feed their young macerated or paralyzed insects and spiders. Most bees feed young pollen and nectar. Many members of the order visit flowers for nectar or pollen. Leaf cutter ants feed on fungi, which they cultivate in the nest. The ants live in an obligate mutualistic association with a fungus. The fungus has lost its capacity for sexual reproduction; dispersal occurs by means of the queen ant carrying fungal hyphae to a new nest site.

Reproductive biology

Males of parasitic species commonly search for females at their emergence sites and occasionally fight for possession of such sites. Some males form female-attracting swarms. Among most hymenopterans the females produce pheromones, chemicals that attract and sexually stimulate males of the same species. Courtship in the Apocrita is common and complex, involving sequences of antennal contact, leg and wing vibrations, and mandibular movements. Sex in most Hymenoptera species is determined by the fertilization of the egg; fertilized eggs develop into females, and unfertilized eggs usually develop into males. Thus, females determine the sex of their offspring and can manipulate the rate of increase of their populations.

The adult female searches for a host by responding to a series of cues in the environment. When she locates a potential host, she examines it often with her antennae or the tip of the ovipositor, to decide whether it is acceptable as a site for egg deposition. Females of some species lay their eggs on a broad range of similar hosts within a particular habitat, whereas others are highly specific to a single host or a few closely related species. Most parasitoid species lay their eggs on or in the body of the host, and many have a long ovipositor to reach hosts in cocoons, burrows, or other protected situations. In some cases only a single egg is laid on a host (solitary parasitism); in others, several to many eggs may be laid on the same host (gregarious parasitism). The larva may be ectoparasitic, that is, developing externally, or endoparasitic, developing within the host. Sometimes early instars are endoparasitic, and later instars are ectoparasitic. Pupation typically occurs within or beside the host remains.

Hymenoptera are holometabolus insects, that is, they have a life cycle progressing from egg to larva to pupa to adult. The eggs are ovoid or sausage-shaped, with a respiratory stalk in some parasitic species and with hooklike or sucker-like attachment devices in some ectoparasites. The outer covering typically is thin and smooth, sometimes with little yolk. Polyembryony, that is, development of several individuals from one egg, occurs in some parasitic species.

In Symphyta the larva is eruciform. The head is well sclerotized, and there are three pairs of thoracic legs and abdominal prolegs (commonly on the second through eighth and the tenth abdominal segments). There are as many as eight larval instars, with females often having one more than males. In Apocrita the larva is vermiform, apodous, grublike, or maggot-like, and the head capsule is weakly sclerotized. Primitively, they probably have five larval instars, but the number is reduced in many endoparasitoids.

Order: Hymenoptera

A leaf cutter ant (Atta) carries a leaf section to its nest in Corcounado National Park, Costa Rica. (Photo by Carol Hughes. Bruce Coleman, Inc. Reproduced by permission.)

Larval heteromorphosis occurs in many parasitic species. In these species, whereas the final instar larva is vermiform, the first or intermediate instars are of diverse forms. Pupae of the exarate type have free appendages that are not glued to the body; they may form in a cocoon in the host or in special cells. The cocoon is spun with silk from the labial glands. Some hymenopterans may spend a very long period in diapause within the cocoon.

Conservation status

The 2002 IUCN Red List includes 152 hymenopteran species. Of these, 3 are listed as Critically Endangered; 142 as Vulnerable; 6 as Lower Risk/Near Threatened; and 1 as Data Deficient.

Hymenopterans are susceptible to the indiscriminate use of insecticides and to habitat destruction. The effects of insecticides include the eradication of nontarget organisms, including such pollinators as bees and wasps. Without the pollinating services of bees and other insects, we would have few vegetables, fruits, and flowers and little or no clover. To achieve preservation of these key aspects of our lives, hymenopteran biotopes must be conserved and utilized in a sustainable fashion.

Significance to humans

Certain hymenopterans were considered deities in such civilizations as Egypt. To the Greeks the bee Melitta was

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Order: Hymenoptera

A female leaf cutter bee (Megachile centuncularis) carries a leaf section to her nest. (Photo by Kim Taylor. Bruce Coleman, Inc. Reproduced by permission.)

known as the Goddess Honey Mother. The ant-hunting species of Dinoponera was a symbol of virility for several Amazon tribes and was used in initiation rituals. The Mixe people of Oaxaca (Mexico) believe that they become more powerful through the ingestion of ants; ants thus have come to stand for virility, fortitude, and courage. The bee Melipona beechei in Mexico represented the spiritual world, merriment, and rain. Wasps and bees are represented frequently in literature, music, theater, cinema, and television. The Greek comic playwright Aristophanes, for example, wrote a play called The Wasps, satirizing the Athenians’ love of litigation and characterizing jurors as wasps in their harshness. The Russian composer Nikolai Rimski-Korsakov, inspired by the sound that bees produce, composed the “Flight of the Bumblebee,” a piece of music for strings set within the opera The

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Tale of Tsar Saltan, the story of a prince who is turned into a bee. In modern times, several protagonists of the Pokemon cartoons represent hymenopterans.

From the standpoint of human beings, Hymenoptera probably is the most beneficial order of insects; it contains many insects that are of value as parasitoids or predators of other arthropods, including insect pests. They have been employed successfully as pest control agents in several countries. Bees generally are regarded as the most important group of insect pollinators. Apis mellifera is of great commercial value as a producer of honey that is used extensively as food and in the manufacture of many products. Beeswax is used in making candles, sealing wax, polishes, certain types of ink, models of various kinds, and in other products such as face and hand creams, lipsticks, and lip salves.

Nest-building Hymenoptera can be domestic nuisances. Bees and wasps inject venom when they sting. Many people are highly sensitive to bee or wasp stings and may suffer anaphylactic shock leading to death or disability as the result. The sting of the female hymenopteran can seem unprovoked, but it is, in fact, an aggressive defense of the nest.

Hymenopterans are food for humans in some parts of the world. They are edible at all stages of growth. Boiling tends to break down the poison, which is basically protein and, at boiling temperatures, the stinger softens. Pounding them before boiling makes them more edible. Ants (except the fire ant) and ant larvae are edible and tasty. Australian aborigines living in arid regions derive sugar from species of Melophorus and Camponotus, popularly known as honey pot ants. Specialized worker ants (repletes) are fed with nectar by other workers and store it in their huge distended crops. Honey pot ants in the western United States and Mexico belong to the genus

Myrmecocystus.

Few species of Hymenoptera are harmful. Sawfly larvae cause damage to forests, orchards, and ornamental trees. Wood-boring larvae, in association with fungi, can cause extensive damage to plantations of conifers. A few ants are pests, for example, the leaf cutter ants or the seed harvesters; others protect sap-sucking insects, which are pests themselves.

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1. Yellow jacket (Vespula germanica); 2. Potter wasp (Eumenes fraternus); 3. Tarantula hawk (Pepsis grossa); 4. Ibalia leucospoides; 5. Golden paper wasp (Polistes fuscatus); 6. Apoanagyrus lopezi; 7. Trissolcus basalis; 8. Digger wasp (Scolia dubia); 9. Pelecinus polyturator; 10. Velvet ant (Mutilla europaea). (Illustration by Barbara Duperron)

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1. Mud dauber wasp (Sceliphron caementarium); 2. Cuckoo wasp (Chrysis coerulans); 3. Megarhyssa nortoni; 4. Large red-tailed bumble bee (Bombus lapidarius); 5. Leaf cutter ant (Atta sexdens); 6. Large carpenter bee (Xylocopa virginica); 7. Alfalfa leaf cutter bee (Megachile rotundata); 8. European wood wasp (Sirex noctilio); 9. Pear and cherry slug (Caliroa cerasi); 10. Honey bee (Apis mellifera). (Illustration by Barbara Duperron)

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