Hymenoptera

The order Hymenoptera comprises insects, comprising sawflies, wasps, bees, and ants. There are about 150,000 living species of Hymenoptera excluding the over 2,000 extinct ones. Most of the species of Hymenoptera are considered to be parasitic. Hymenoptera is considered to be the largest order of insects and the term is derived from the ancient Greek words for hymen and pteron. Hymen means membrane and the term pteron is translated to the wing. One of the key characteristics of the order Hymenoptera is that their forewings and the hindwings are connected to each other with the help of a series of hooks.

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Fig: Order Hymenoptera

This particular order is most commonly known as stinging insects. The sting apparatus present in all Hymenoptera species is a modification of the egg-laying appendages. Since the female Hymenoptera are naturally associated with the egg-laying structures, therefore, only the female wasps can sting. They typically comprise a special ovipositor that is used for inserting eggs into hosts and this ovipositor most often gets modified into a stinger. The young Hymenopteras are developed through complete metamorphosis or holometabolism. As a result, these creatures have a worm-like larval stage and has an inactive pupal stage before attaining maturity. It has been found that some of the male Hymenoptera species are extraordinarily adept at mimicking the structure as well as the behaviour of the females and can eventually fool some experienced workers too.

Hymenoptera - Scientific Classification and Origin

The order Hymenoptera is included under the class Insecta and is further divided into two suborders: Apocrita and Symphyta. Given below is the scientific classification of the Hymenoptera.

The origin of Hymenoptera traces back to the Triassic period. The oldest Hymenoptera fossils that were discovered belonged to the family Xyelidae.  Although several studies have been done on this clade through examination of mitochondrial DNA, it all failed to solve the ambiguities in this clade. But it also helped in establishing some relationships. 

Classification of Hymenoptera

The order Hymenoptera is further categorised into two sub-orders: 

• Symphyta (which have no waist), and 

• Apocrita (which have a narrow waist)

Symphyta

This is a sub-order that includes the sawflies, horntails, and parasitic wood wasps. Symphyta can be paraphyletic, as the family Orussidae is found to be the group that led to the occurrence of the Apocrita. An unconstricted junction between the thorax and abdomen can also be seen in the group Symphata. The Symphyta larvae are herbivorous, eruciform and free-living. They also consist of three pairs of true legs and ocelli. Unlike Lepidoptera, the larvae of Symphyta consist of prolegs on every segment and do not have crochet hooks at the ends.

Apocrita

The Apocrita is a suborder that includes the wasps, bees, and ants. This clade can be characterized by the presence of a wasp-waist, called the petiole. It is a constriction between the first and second abdominal segments involving the fusion of the first abdominal segment to the thorax. Unlike Symphyta, the larvae of all Apocrita species lack legs, prolegs, or ocelli. During development, the hindgut of the larvae remains closed and the faeces are stored inside the body. Therefore, the anus only opens during the completion of larval growth. 

Anatomy of Hymenopterans

The size of Hymenopterans ranges from very small to large and consist of two pairs of wings. Their mouthparts are ectognathous, i.e., they have well-developed mandibles which are adapted for chewing. Also in many Hymenoptera species, it has been found that the mouthparts are further developed into a lengthy proboscis, mainly used for drinking liquids such as nectar. They have large compound eyes, and typically three simple eyes, ocelli. 

The hind wing of the Hymenopterans has several hooked bristles known as "hamuli" on their forward margin. These hooks are used to lock the hind wing onto the fore wing to keep them 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. The smaller Hymenoptera species can consist of only two or three hamuli on each side, but the larger species such as the wasps are found to have a considerable number of hamuli. This, as a result, will keep the wings gripped together tightly. As compared to other insects, the Hymenopteran wings have relatively few veins, especially in the smaller species. 

The blade-like ovipositor, which can be seen in many ancestral hymenopterans have evolved eventually for the purpose of slicing plant tissues. In the majority of the species, these ovipositors have also been modified for piercing and can also be as large as the length of the body. In certain Hymenoptera species, the ovipositor is modified as a stinger which is generally used for immobilising prey but is also used in defence in the case of some wasps and bees. Some species lay their eggs from the base of the ovipositor instead of the tip, using the tip only to inject venom.

Generally, the hymenopteran larvae consist of a distinct head region along with three thoracic segments and 9-10 abdominal segments. The larvae of the suborder Symphyta is known to resemble a caterpillar in appearance and mostly feed on leaves. They are also found to have three pairs of thoracic limbs, 6-8 abdominal prolegs and large chewing mandibles. Unlike caterpillars, the Symphytan larvae, however, have no grasping spines on their prolegs and the antennae are reduced to mere stubs. Other Symphytan larvae such as the wood borers or stem borers also lack abdominal legs and their thoracic legs are smaller as compared to other non-borers.

While there have also been some rare exceptions with regard to this. For example, the larvae belonging to the suborder Apocrita lack legs and have a maggot-like form generally adapted to live in a protected environment. In several parasitic forms of Hymenopterans, the head is often found to be reduced and is partially withdrawn into the anterior part of the thorax, known as prothorax. They also have poorly developed sense organs with no ocelli, very small or absent antennae and have mandibles that are toothlike, sicklelike or spinelike. These forms of Hymenoptera species are unable to defecate till they reach adulthood. This is because of their incomplete digestive tract (a blind sac) and also presumably to avoid contaminating their environment.

Reproduction in Hymenopterans

In all Hymenoptera species or Hymenopterans, sex is determined by the number of chromosomes that is possessed by the particular species. The fertilized eggs (consisting of both the parents’ gametes) get two sets of chromosomes, which later develop into diploid females. While, on the other hand, the unfertilized eggs from the mother only consist of one set of chromosomes which develops further into haploid males. The egg-laying female voluntarily controls the act of fertilization. This process is known as haplodiploidy. 

But the genetic mechanisms of haplodiploid sex determination can be more complex than the determination of simple chromosome numbers. In most of the Hymenoptera species, sex is generally determined with the help of a single gene locus consisting of many alleles. Also, the haploids found in these species are male, while, the diploids heterozygous present at the sex locus are female. But it has also been found that a diploid can occasionally be homozygous at the sex locus and eventually can develop as a male, instead. This mainly occurs in an individual having parents who were either siblings or other close relatives. Diploid males are produced in many ant, bee, and wasp species by inbreeding. Diploid biparental males are generally sterile but can also be found as fertile diploid males in some species. 

One of the outcomes of haplodiploidy is that on average the females have more genes in common with their sisters as compared to that with their own daughters. It has been found that the worker females tend to remove the eggs that are laid by the other works. This can be seen often in the colonies of bees, ants, and wasps due to increased relatedness to their direct siblings. This phenomenon is known as worker policing.

Another outcome of haplodiploidy is that the hymenopterans can be more resistant to the deleterious effects of inbreeding. As a result of haploid males, any of the recessive genes are automatically expressed and are thus exposed to natural selection. As a result, the genetic load of deleterious genes is purged relatively quickly.

Parthenogenesis in Hymenopterans

Parthenogenesis is the formation of a zygote without the involvement of fertilization. Some of the hymenopterans thus, take advantage of the process of parthenogenesis. Here, the female embryos are created without fertilization by thelytoky, which is a particular form of parthenogenesis.  In hymenopterans, the thelytoky is a kind of automixis, where the two haploids (from the same meiosis) fuses to form a diploid zygote. This process thus helps to maintain heterozygosity while passing the genome from mother to daughter. This process can be found in several Hymenoptera ants. For example, desert ant (Cataglyphis cursor), clonal raider ant (Cerapachys biroi), predaceous ant (Platythyrea punctata) and little fire ant (Wasmannia auropunctata). Apart from the Hymenoptera Ants, it can also be found in the Cape honey bee (Apis mellifera capensis).

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Reproduction in Hymenoptera Ants 

While undergoing automixis with central fusion, the oocytes often have a reduced crossover recombination rate, which helps maintain heterozygosity and avoid inbreeding depression. Few species that undergo central fusion along with a reduced recombination rate are Platythyrea punctata, Wasmannia auropunctata and the Cape honey bee (Apis mellifera capensis). In the Cape Honey Bee, the rate of recombination during meiosis is reduced more than tenfold. 

The single queen colonies of narrow-headed ant (Formica exsecta) shows possible deleterious effects of increased homozygosity. The colonies consisting of more homozygous queens will undergo more rapid ageing and will result in reduced colony survival.

What do the Hymenoptera Feed Upon?

A wide range of feeding habits has been found in different species of Hymenoptera. Most of the primitive forms of Hymenoptera are phytophagous and are found to feed on flowers, pollen, foliage or stems. The stinging wasps are known to be predators which provide their larvae with immobilised prey, while the bees are seen feeding on nectar and pollen. The larvae of many Hymenoptera species are found to be parasitoids. The adults, on the other hand, inject the eggs into a host, which they start consuming after hatching. 

For example, the eggs of the endangered Papilio homerus are parasitized mainly by the Hymenoptera species at a rate of 77%. Some of the Hymenoptera species are also hyperparasitoid, where the host itself is another parasitoid insect. Their habits form an intermediate between that of the herbivorous and the parasitoid forms, as seen in some of the hymenopterans. These species are found inhabiting the galls or nests of other insects, along with stealing their food. They also eventually kill and eat the occupant. 

Did You Know?

• The Hymenoptera species such as the bees, ants and wasps have chemoreceptors on their antennae as well as on their mouthparts, which are used for taste and smell.

• All species of the order Hymenoptera including the wasps, bees and ants are considered to be social insects. Apart from the termites, all these social insects are included under the order Hymenoptera.

• The Hymenopterans were first seen about 200 million years ago.

• A 100 million-year-old bee was discovered fossilized in amber recently. As per the preliminary studies of this bee, it has been confirmed that the bees are descended from wasps.

• All the species are Hymenoptera order are haplodiploid, i.e., all the females are derived from fertilized or diploid eggs, while the male Hymenopterans are derived from haploid or unfertilised eggs. As a result, the male Hymenoptera species have no father.

Thus, we can summarise from this article that Hymenoptera is a large and diverse order consisting of ants, bees and wasps along with all the social insects excluding the termites. Around 130,000 recognised species of Hymenoptera are found worldwide, while there are many more species that are yet to be described, thus making it one of the largest order of insects.