Hymenoptera (Hi-men-op-tura) is one of the largest orders of insects, comprising the ants, bees, wasps, and sawflies, among others. There are at least 100,000 described species of hymenopterans, placed into the two suborders of Apocrita (bees, wasps, and ants) and Symphyta (saw flies and wood wasps) (Gzimek et al. 2004). They are found worldwide.
While advancing their own survival, reproduction, and maintenance, hymenopterans also provide important values to the ecosystem and to human beings. Bees, in particular, are noted for their importance as pollinators of flowering plants, which is significant both ecologically and for agriculture. Many hymenopterans are important parasites or predators of agricultural pests, and some even serve as food. Honey and beeswax are important commercial products of hymenopterans. Although some can be pests, and even dangerous to those sensitive to the venomous sting, in general, hymenopterans also add to the diversity in nature and, thus, to human enjoyment.
Hymenopterans are a biologically diverse group of insects, ranging in size from the minute (0.15 millimeters or 0.006 inches) to large (120mm or 4.72 inches) and from slender to robust (Gzimek et al. 2004).
As arthropods, hymenopterans have jointed appendages, an exoskeleton (hard, external covering), segmented body, ventral nervous system, digestive system, open circulatory system, and specialized sensory receptors. The term "jointed appendages" refers to both legs and antennae.
As insects, hymenopterans have three pairs of jointed legs; an abdomen that is divided into 11 segments and lacks any legs or wings; and a body separated into three parts (head, thorax, and abdomen), with one pair of antennae on the head. As true insects, they also are distinguished from all other arthropods in part by having ectognathous, or exposed, mouth parts.
In hymenopterans, the mouthparts may be of a type used for biting, chewing, or sucking . Adult hymenopterans typically have two pairs of wings with reduced venation, with venation most complete in species belonging to the suborder Symphyta and most reduced in small species of the suborder Aprocrita (Grzimek et al. 2004). The hind pair characteristically is smaller than the front pair. Some hymenopterans, such as ants, have a wingless worker caste. The name hymenoptera refers to the membranous wings of the insects, and is derived from the Ancient Greek ὑμήν (humẽn), meaning "membrane," and πτερόν (pteron), meaning "wing." The hindwings are connected to the forewings by a series of hooks called hamuli.
Hymenopterans have compound eyes, which tend to be large and convergent dorsally, and the antennae are long, multisegmented, and covered with sense organs (Grzimek et al. 2004). Females have an ovipositor—an organ used for laying eggs—that in some species has been modified for stinging, sawing, or piercing. In some ants, bees, and wasps the ovipositor does not have an egg-laying function, but a defense function. In sawflies, females use the ovipositor to cut into plants where they lay their eggs. A few species of sawflies have long, thin ovipositors used to drill holes deep into wood.
In species belonging to Aprocrita (wasps, bees, and ants), the first abdominal segment is firmly attached to the metathorax and usually separated by a narrow waist (petiole) (Grzimek et al. 2004). Sawflies (suborder Symphyta) are distinguishable from most other Hymenoptera by the broad connection between the abdomen and the thorax.
Hymenopterans are holometabolus insects, meaning they undergo complete metamorphosis in which the larvae differ markedly from the adults. Insects that undergo holometabolism pass through a larval stage, then enter an inactive state called pupa, and finally emerge as adults (imago).
The fossil record for Hymenoptera dates to the Triassic (245 to 210 million years ago) (Grzimek et al. 2004), with the oldest fossils belonging to the family Xyelidae. Social hymenopterans appeared during the Cretaceous.
Hymenopterans are found worldwide, but are more abundant and diverse in tropical and temperate regions.
Hymenopterans include both free-living and parasitic species. The larva of many insects of Apocrita are parasitoids that are parasites in the immature stages of other animals, but the adults are free living. The adults of these species lay their eggs in, or on, the host animal to develop. Many other species of Apocrita are stinging forms that lay their eggs in nests provided with food. In Symphyta, the larva are almost exclusively phytophagous (eating plant matter).
Hymenopterans may be solitary or may live in various types of communities. In some species, groups of cohabiting females may be sisters, and if there is a division of labor within the group, then they are considered semisocial. The most advanced of the social communities are eusocial colonies. In these, in addition to a division of labor, the group consists of a mother and her daughters. Such eusocial colonies can be primitively social or highly social. If the castes are purely behavioral alternatives, the system is considered "primitively eusocial," and if the castes are morphologically discrete, then the system is "highly eusocial." Paper wasps are hymenopterans that exhibit primative eusociality. Highly eusocial hymenopterans include the ants, many bees (such as honeybees), and some wasps.
Among the hymenopterans, sex is determined by the number of chromosomes the individual receives. Fertilized eggs get two sets of chromosomes, and so develop into diploid females; unfertilized eggs only receive one set, and so develop into haploid males. This phenomenon is called haplodiploidy.
Note, however, that the actual genetic mechanisms of haplodiploid sex determination are more complex than simple chromosome number. In many Hymenoptera, sex is actually determined by a single gene locus with many alleles. In these species, haploids are male and diploids heterozygous at the sex locus are female, but occasionally a diploid will be homozygous at the sex locus and develop as a male instead. This is especially likely to occur in an individual whose parents were siblings or other close relatives. Diploid males are known to be produced by inbreeding in many ant, bee, and wasp species.
The consequence of haplodiploidy is that females on average actually have more genes in common with their sisters than they do with their own daughters. Because of this, cooperation among kindred may be unusually advantageous, and has been hypothesized to account for the high incidence of eusociality in this order.
The suborder Symphyta includes the sawflies, horntails, and parasitic wood wasps. The group appears to be paraphyletic, as it is often believed that the family Orussidae may be the group from which the Apocrita arose. They have an unconstricted junction between the thorax and abdomen, and the larvae of free-living forms are herbivorous, have legs, prolegs (on every segment, unlike Lepidoptera), and ocelli.
The wasps, bees, and ants together make up the suborder Apocrita, 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, the larvae of all Apocrita do not have legs, prolegs, or ocelli.
Commonly, Parasitica is the term used for four superfamilies of Aprocrita—Ichneumonoidea, Chalcidoidea, Cynipoidea, and Proctotrupoidea—while the remaining superfamilies are called Aculeata (Grzimek et al. 2004). The Parasitica tend to be parasites of other insects while the Aculeata are stinging forms, although some Parasitca are phytophagous and some Aculeata are parasites (Grzimek et al. 2004). With the exception of the phytophagous species of Parasitica, these hymenopterans lay their eggs on, or in, animal hosts (insect, spider), while species of Aculeata used nests provided with food (Gzimek et al. 2004).
Hymenopterans are very significant to humans. Grzimek et al. (2004) note that "Hymenoptera probably is the most beneficial order of insects." Almost all extant species of bees subsist on nectar and pollen, and as such are very important pollinators. Indeed, bees are the major type of pollinators in ecosystems that contain flowering plants. As such, bees are extremely important as pollinators in agriculture. It is estimated that one third of the human food supply depends on insect pollination, most of this accomplished by bees. In addition, honeybees are important for production of honey, and beeswax is used commercially to produce candles, polishes, sealing wax, and in various other products, such as hand creams and lipsticks (Grzimek et al. 2004). Many hymenopterans also are important because they are parasitic or predatory on other arthropods and are thus useful for biological control of insect pests (Grzimek et al. 2004). Some hymenopterans (such as ants) serve as food, being edible in all stages of growth, with boiling tending to break down the poison and soften the stinger (Grzimek et al. 2004).
On the other hand, a number of hymenopterans can be pests and even dangerous. Bees and wasps can inject venom into people, and those sensitive to the stings can go into anaphylactic shock, perhaps leading to death or disability (Grzimek et al. 2004). Some hymenopterans, such as larva of particular species of sawflies, can damage trees. Ants, likewise, can be pests.
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