Hymenoptera

Hymenopterans belong to the Phylum Arthropoda, Subphylum Labiata, Superclass Hexapoda, Class Insecta, and Order Hymenoptera. It is a large order of insects with over 150,000 described species comprising the sawflies, woodwasps, ichneumonids, chalcids, wasps, bees, and ants. In addition, there are over 2,000 extinct taxa. The order ranks third in the top five orders of insects in terms of total number of species. Many examples of this order are found in Arkansas, including the invasive fire ant.

The Hymenoptera are divided into two suborders: the Symphyta (which have no waist) and the Apocrita (which have a narrow waist). Symphytans include the horntails, sawflies, and parasitic wood wasps. They have an unconstricted junction between the thorax and abdomen and a serrated or saw-shaped ovipositor in many taxa that is used by some to bore into wood. The larvae are herbivorous, free-living, and eruciform, with three pairs of true legs, prolegs on every segment, and ocelli. Apocritans include the wasps, bees, and ants characterized by a constriction between the first and second abdominal segments called a wasp-waist (petiole), also involving the fusion of the first abdominal segment to the thorax. Also, larval apocritans lack legs, prolegs, or ocelli. One additional morphological feature is that the hindgut of the larvae also remains closed during development, with feces being stored inside the body (with the exception of some bee larvae where the larval anus has reappeared through developmental reversion). In general, the anus opens only at the completion of larval growth.

In the geologic record, hymenopterans originated in the Triassic Period to the beginning of the Jurassic Period. The oldest fossils belong to the sawfly family Xyelidae. During the Cretaceous, social hymenopterans first appeared. The best-preserved are those found in amber (fossilized sap from ancient trees).

Molecular analysis reveals the order as a clade, most closely related to endopterygote (complete metamorphosis) orders including the orders Diptera (true flies) and Lepidoptera (butterflies and moths). The Aculeata, Ichneumonomorpha, and Proctotrupomorpha are considered monophyletic. The superfamilies Megalyroidea and Trigonalyoidea are sister clades, as are the Chalcidoidea+Diaprioidea. The Superfamily Cynipoidea was generally recovered as the sister group to the superfamilies Chalcidoidea and Diaprioidea, which are each other’s closest relations. The Apocrita may be paraphyletic, as it has been suggested that the parasitic wood wasps (Family Orussidae) may be the group from which the Apocrita arose.

In terms of size, hymenopterans range from the smallest fairyflies (Family Mymaridae), which are about 0.21 mm (about 0.008 in.) long, to the largest pelicid wasps (Family Pelecinidae), which can exceed 5 cm (about 2 in.) in length. Hymenopteran mouthparts (ectognathous) are adapted for chewing, with well-developed mandibles. Many species have further modified mouthparts into an extended proboscis, with which they can drink liquids, such as nectar. They also have large compound eyes, and typically three simple eyes, ocelli. The forward margin of the hind wing possesses a number of hooked bristles (hamuli) that lock onto the forewing, holding them together. The smaller hymenopterans may have only two or three hamuli on each side, but the largest wasps may have a significant number, keeping the wings adhered together especially tightly. Their wings have relatively few veins compared with many other insects, especially in the smaller species.

In the more ancestral hymenopterans, the ovipositor is blade-like and has evolved for slicing plant tissues. However, in the majority, it is modified for piercing and, in some cases, is several times the body length. In some species, the ovipositor has become adapted as a stinger, and the eggs are laid from the base of this structure, rather than from the tip, which is used only to inject venom. The sting is often used to immobilize prey items, but in some wasps and bees it may be used in defense.

Larval hymenopterans generally have a distinct head region, three thoracic segments, and usually nine or ten abdominal segments. In symphytan larvae, they resemble caterpillars in appearance and, similarly, typically feed on leaves. They possess large chewing mandibles, three pairs of thoracic limbs, and, in the majority of cases, six or eight abdominal prolegs. Unlike in caterpillars, however, the prolegs have no grasping spines and the antennae are reduced to meager stubs. Wood borers or stem borer symphytan larvae have no abdominal legs, and the thoracic legs are smaller than those of non-borers.

Larval apocritans, with rare exceptions, possess no legs, are maggot-like in morphology, and are adapted to living in a protected environment. This could be the body of a host, or a cell in a nest, where the adults will take care of the larvae. In those that are parasitic, the head is often greatly reduced and partially withdrawn into the anterior part of the thorax (prothorax). Sense organs are poorly developed, with no ocelli, very small or absent antennae, and mandibles that are toothlike, sicklelike, or spinelike. Because they have an incomplete digestive tract (a blind sac), they are unable to defecate until they reach adulthood, presumably to avoid contaminating their environment. Larval stinging forms (Aculeata) generally have ten pairs of spiracles (or breathing pores), whereas parasitic forms usually possess nine pairs.

Genetically, sex determination is accomplished by the number of chromosomes an individual possesses. Those eggs that are fertilized receive two sets of chromosomes (one from each parent’s respective gametes) and develop into diploid females, whereas unfertilized eggs contain only one set (from the mother) and develop into haploid males. The act of fertilization is under the voluntary control of the gravid egg-laying female, giving her complete control of the sex of offspring (haplodiploidy). However, the true genetic mechanism of haplodiploid sex determination may be more complicated than simple chromosome number.

Females typically have a special ovipositor for inserting eggs into hosts or places that are otherwise inaccessible. The ovipositor is often modified into a stinger. The young develop through holometabolism (complete metamorphosis)—that is, they have a worm-like larval stage and an inactive pupal stage before they mature. Some hymenopterans take advantage of parthenogenesis, the creation of embryos without fertilization. A particular form of parthenogenesis is thelytoky, in which female embryos are created (without fertilization). The form of thelytoky in hymenopterans is a kind of automixis in which two haploid products (proto-eggs) from the same meiosis fuse to form a diploid zygote. This process tends to maintain heterozygosity in the passage of the genome from mother to daughter. It is found in several ant species including the desert ant (Cataglyphis cursor), the clonal raider ant (Cerapachys biroi), the predaceous ant (Platythyrea punctata), and the electric ant or little fire ant (Wasmannia auropunctata). It also occurs in the Cape honeybee (Apis mellifera capensis).

In terms of food habits, various species of hymenopterans show a wide range of dietary habits. The vast majority of symphytans are phytophagous, feeding both externally on leaves and internally as leaf miners and wood and stem borers on both monocots and dicots. The notable exception is the Orussidae, which are parasitic on wood-boring hymenopterans and coleopterans. Stinging wasps are predators and will provision their larvae with immobilized prey, while bees mainly feed on nectar and pollen. Collectively, members of the order are most important to humans as pollinators of wild and cultivated flowering plants, as parasites of destructive insects, and as producers of honey. In turn, hymenopterans are sought as food by other animals, including skunks, badgers, field mice, and shrews. The larvae of the wax moth (Galleria mellonella) live in bee nests, where they eat beeswax, thus damaging the nest. The cuckoo bee (Anthophorinae), a close relative of the bumblebee, lays its eggs in bumblebee nests, where the larvae are cared for and nourished by bumblebee workers.

A large number of species are considered parasitoids as larvae. Parasitic wasps lay their eggs inside caterpillars, and the larvae slowly devour their living hosts before crawling out and weaving cocoons. For example, the eggs of the endangered Homerus swallowtail (Papilio homerus) are parasitized at a rate of seventy-seven percent, mainly by Hymenoptera species. Some species are even hyperparasitoid, with the host itself being another parasitoid insect. Habits intermediate between those of the herbivorous and parasitoid forms are shown in some hymenopterans, which inhabit the galls or nests of other insects, stealing their food, and eventually killing and eating the occupant. A diurnal species of Strepsiptera, Xenos peckii, uses non-parasitic paper wasps as its host, and adult female X. peckii never leave their host.

Bee and wasp stings can cause medical problems, usually minor. Stings from bees, wasps, and fire ants can cause significant reactions, ranging from localized pain and swelling to serious and even potentially fatal conditions. In fact, at least 90 to 100 deaths occur each year in the United States as a result of serious sting reactions. The majority of people will have only a localized reaction to a sting such as reddening of the skin and dull pain. Swelling and/or itching may also occur, but the pain usually disappears within a few hours. About three percent of individuals stung will have an allergic reaction to the sting, and up to 0.8 percent of sting victims experience the severe and life-threatening allergic reaction known as anaphylaxis. The treatment of choice for severe allergic reactions is epinephrine.

The red imported fire ant (Solenopsis invicta) is a nuisance in Arkansas and other surrounding states. They have been in Arkansas since at least the 1950s and can be found all over the southern half of the state. The species first entered the United States around the 1930s via the port of Mobile, Alabama. Apparently, ships from South America entered the port and used fire ant–infested soil as ballast. Another species, the black imported fire ant (S. richteri), occurs in Arkansas but is mostly found along the Mississippi River. By March 2017, there was quarantine on fire ants in parts of fourteen states and Puerto Rico, and thirty-nine counties fell under the federal S. invicta quarantine in Arkansas. Under this quarantine, certain materials like potted plants, grass sod, or baled hay that have come into contact with soil cannot be shipped to non-quarantine areas.

A great deal of research has been done on various aspects of the biology of hymenopterans in Arkansas, with the majority of studies being conducted on ants (Formicidae). At least 131 ant species from seven subfamilies and thirty-four genera are known to occur in the state. Interestingly, fifty-six species have been reported from Petit Jean State Park (Conway County). In addition, at least nine exotic ant species are known to occur in the state, including Brachymyrmex patagonicus, Cardiocondyla venustula, Hypoponera opaciceps, Linepithema humile, Monomorium pharaonis, Pyramica metazytes, S. invicta, S. richteri, and Tetramorium bicarinatum.

There is no comprehensive list of hymenopterans in Arkansas. An older report documented six species of bumblebees (Bombus spp.) from the state. An endemic parasitoid wasp, Idris leedsi, is known only from Baker Spring in Johnson County. It is a parasitoid of eggs of various spiders. Another study reported forty-seven species of symphytans from Steel Creek (Newton County) in the Buffalo National River. There is no doubt that there remains a void and a specific need to provide a summary of the species of hymenopterans in Arkansas as well as their geographic distribution.

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Chris T. McAllister
Eastern Oklahoma State College