HYMENOPTERA is a large order of insects , comprising the sawflies , wasps , bees , and ants . Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones.
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.
* 4 Reproduction
* 4.1 Sex determination * 4.2 Thelytoky
* 5 Diet
* 6 Classification
* 7 See also * 8 References * 9 Bibliography * 10 External links
The cladogram of external relationships, based on a 2008
Boreidae (snow scorpionflies)
HYMENOPTERA (sawflies, wasps, ants, bees)
This clade has been studied by examining the mitochondrial DNA.
Although this study was unable to resolve all the ambiguities in this
clade, some relationships could be established. The
The cladogram is based on Schulmeister 2003.
Cephoidea (stem sawflies)
Siricoidea (horntails or wood wasps)
Xiphydrioidea (wood wasps)
Orussoidea (parasitic wood wasps)
"wasp waist" 200mya
APOCRITA (ants , bees , wasps )
Bombus muscorum drinking nectar with its long proboscis
Hymenopterans range in size from very small to large insects, and usually have two pairs of wings. Their mouthparts are adapted for chewing, with well-developed mandibles (ectognathous mouthparts). Many species have further developed the mouthparts into a lengthy proboscis , with which they can drink liquids, such as nectar . They have large compound eyes , and typically three simple eyes, ocelli .
The forward margin of the hind wing bears a number of hooked bristles, or "hamuli ", which lock onto the fore wing, keeping them held together. The smaller species may have only two or three hamuli on each side, but the largest wasps may have a considerable number, keeping the wings gripped together especially tightly. Hymenopteran 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. In the majority, however, it is modified for piercing, and, in some cases, is several times the length of the body. In some species, the ovipositor has become modified as a stinger , and the eggs are laid from the base of the structure, rather than from the tip, which is used only to inject venom . The sting is typically used to immobilise prey, but in some wasps and bees may be used in defense.
The larvae of the more ancestral hymenopterans resemble caterpillars in appearance, and like them, typically feed on leaves. They have large chewing mandibles, three pairs of thoracic limbs, and, in most cases, a number of abdominal prolegs . Unlike caterpillars, however, the prolegs have no grasping spines, and the antennae are reduced to mere stubs.
The larvae of other hymenopterans, however, more closely resemble maggots , and are adapted to life in a protected environment. This may be the body of a host organism, or a cell in a nest, where the adults will care for the larva. Such larvae have soft bodies with no limbs. They are also unable to defecate until they reach adulthood due to having an incomplete digestive tract, presumably to avoid contaminating their environment.
Main article: Haplodiploid sex-determination system
Among most or all hymenopterans, sex is determined by the number of chromosomes an individual possesses. Fertilized eggs get two sets of chromosomes (one from each parent's respective gametes ) and develop into diploid females, while unfertilized eggs only contain one set (from the mother) and develop into haploid males. The act of fertilization is under the voluntary control of the egg-laying female, giving her control of the sex of her offspring. This phenomenon is called haplodiploidy .
However, the actual genetic mechanisms of haplodiploid sex determination may be 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. Diploid biparental males are usually sterile but a few species that have fertile diploid males are known.
One 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 females may be unusually advantageous, and has been hypothesized to contribute to the multiple origins of eusociality within this order. In many colonies of bees, ants, and wasps, worker females will remove eggs laid by other workers due to increased relatedness to direct siblings, a phenomenon known as worker policing .
Another consequence is that hymenopterans may be more resistant to the deleterious effects of inbreeding . As males are haploid, any recessive genes will automatically be expressed, exposing them to natural selection. Thus, the genetic load of deleterious genes is purged relatively quickly.
Main article: Thelytoky
Some hymenopterans take advantage of parthenogenesis , the creation of embryos without fertilization . Thelytoky is a particular form of parthenogenesis in which female embryos are created (without fertilisation). 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 (little fire ant) Wasmannia auropunctata . It also occurs in the Cape honey bee Apis mellifera capensis .
Oocytes that undergo automixis with central fusion often have a reduced rate of crossover recombination , which helps to maintain heterozygosity and avoid inbreeding depression . Species that display central fusion with reduced recombination include the ants Platythyrea punctata and Wasmannia auropunctata and the honey bee Apis mellifera capensis. In A. m. capensis, the recombination rate during meiosis is reduced more than 10-fold. In W. auropunctata the reduction is 45-fold.
Single queen colonies of the narrow headed ant Formica exsecta illustrate the possible deleterious effects of increased homozygosity. In this ant, colonies with more homozygous queens age more rapidly. The result is reduced colony survival.
Different species of
A huge number of species are parasitoids as larvae. The adults inject the eggs into a host, which they begin to consume after hatching. 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.
Symphyta includes the sawflies , horntails , and
parasitic wood wasps . The group may be paraphyletic , as it has been
suggested that the family
Orussidae may be the group from which the
Apocrita arose. They have an unconstricted junction between the thorax
and abdomen. The larvae are herbivorous, free-living eruciform s, with
three pairs of true legs, prolegs (on every segment, unlike
The wasps, bees, and ants together make up the suborder (and clade) 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 lack legs, prolegs, or ocelli. 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 only opens at the completion of larval growth.
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