Hymenoptera
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| Order: | Hymenoptera Linnaeus, 1758
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Hymenoptera is one of the larger orders of insects, comprising the sawflies, wasps, bees, and ants. The name comes from the membranous wings (Greek hymen, a membrane), of which most forms have two pairs when present, the front wings larger than the back.
Females typically have a special ovipositor for inserting eggs into hosts or otherwise inaccessible places, often modified into a stinger. The young develop through complete metamorphosis - that is, they have a worm-like larval stage and an inactive pupal stage before they mature. (See holometabolism.)
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. (See Haplo-diploid sex-determination system.)
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.
Suborder Symphyta
Hymenopterans classified in the suborder Symphyta include 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.
Suborder Apocrita
The wasps, bees, and ants together make up a suborder of the Hymenoptera called the 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.
Evolution
Hymenoptera originated in the Triassic, the oldest fossils belonging to the family Xyelidae. Social hymenopterans appeared during the Cretaceous. The evolution of this group has been intensively studied by A. Rasnitsyn, M. Engel, G. Dlussky, and others.
References
- Rasnitsyn, A.P. and Quicke, D.L.J. (2002). History of Insects. Kluwer Academic Publishers. ISBN 1-4020-0026-X.
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