The Order Hymenoptera contains well over 100,000 species, and includes sawflies, wasps, gall wasps, bees and ants as well as a bewildering assortment of ichneumons and other parasites. In Britain alone there are an estimated 7,100 species.
The range in size varies tremendously from huge spider hunting wasps through to minute members of the Mymaridae family known as fairy flies which spend their larval stage in the eggs of other insect species.
They develop through their life using a complete four stage metamorphosis (egg, larvae, pupa and adult) and have a Haplo-diploid genetic make up, which means that only the females have a full set of chromosones, while the males have only half.
One very distinct advantage of this, is that the ratio of make to females can be controlled by the parent; sex of the offspring is determined at fertilisation - fertilised eggs become female, and non-fertilised eggs become male. This factor allows population control by the previous generation.
A typical member of this Order has two pairs of membranous wings (although some are wingless), which are coupled together by a row of minute hooks on the leading edge of the hind wing, which generally are very much smaller than forewing and not always easy to distinguish, especially when at rest.
The venation of the wing is reduced to the point where it produces a networks of large, often square cells, not seen in any other insect order.
There is frequently a pigmented Pterostigma towards the tip of the forewing along with an elongated cell beyond it. This elongated cell is often known as the marginal cell. Just behind the Pterositgma and marginal cell are two or three sub marginal cells, which can be of great use in identifying the species or family.
|Image by Noel Cornwall.|
The head is usually rather hard and extremely mobile, being attached to the thorax by slender neck. As well as the compound eyes there are usually three ocelli present. The antennae are extremely variable in form, and are often longer in the males than the females.
The Pronotum is often no more than a narrow collar at the front of the thorax, but sometimes extends back to the tegulae, which are swellings scales or swellings over the base of the forewings.
The Order is then split into 2 Sub-orders
Symphyta - sawflies
Apocrita - which is by far the largest sub order and contains a wide selection of species including both parasitic and social species. The most obvious difference between this sub order and Symphyta is the traditional wasp waist, which sawflies do not have and their abdomen is joined to the thorax across its full width.
Although The waist appears to divide the thorax from the abdomen it is actually made from the first two sections of the abdomen. Known as the propodeum, the first section is fused to the rear of the thorax and the waist comes in behind it. The narrow front part of the abdomen is known as petiole and may consist of one or two segments, or just part of one segment. In a few families it is attached to the top of the propodeum, but is generally attched at the bottom. The whole of the abdomen behind the propodeum is known as the gaster.
Apocrita is then further split into 2 sections;
Aculeata in which the ovipositor is modified into a sting and used for paralysing prey or as a defence.
Parasitica, which are almost all parasites or parasitoids and have an ovipositor which is used to place their eggs on or inside the hosts body. The length of the ovipositors varies tremenously and in some is long enough to pierce deep inside plants or other animals.
Nearly all parasitica will inject into the host a small amount of venom, either prior to, or with the egg. In some species it will paralyse the victim but in many other where the paralysis is only temporay it contains a varied and complex mixture of chemicals and agents which help to modify the hosts tissues ( especially obvious in gall wasps ).
Tissue modification is a factor in nearly all venoms and enables the host to become more nutritious to the larvae as well as helping to overcome the hosts immune system. The latter being especially important for endoparasites, where the hosts immune system will attempt to encapsulate the egg to prevent invasion of the body. Many ways of circum navigating this are present, such as laying the egg directly into the brain (ganglion) where the immune system can not encapsulate it, but possibly the most devious is the use of Pol-DNA-Viruses which are injected with the venom of many endoparaites and have been shown to target and disable the hosts immune system, and so protecting the developing parasitiod larvae.
Which ever the method used, The larvae must also protect itself from infection as well as the host, who may die from bacterial or fungal infection before the larvae is ready to pupate. In order to do this the many larvae excrete chemicals with antibiotic and antiseptic properties as they move around the hosts body cavity. They also tend to avoid damaging the hosts gut (a massive source of bacteria) until later, by eating non essential areas first such as body fat and reproductive organs. Another way used by many species is the use of Teratocytes, which are bundles of cells that emerge from the egg with the larvae, that absorb food from the hosts body cavity and are fed on by the larvae, thus removing the need to feed directly on the host until absolutely neccesary.
Members of Parasitica are sometimes known as Protelean parasites, indicating that they are parasitic only in their larval form.
They can be seperated into 3 groups, one of which is the gall forming wasps and the remaining two groups are known as Koinobionts and Idiobionts, where the wasps fall into the relevant groups according to the way that they paralyse thier prey. The prey of koinabionts is only partially paralysed by the wasps venom and soon recovers and continues with its development until killed by the larvae reaching maturity. Conversely the prey of idiobionts are totally paralysed by the venom and their development is terminated at that point. It is generally thought that koinabionts evolved from idiobionts.
Other differences between the two are shown in the table below, but are generalisations and there are several species that will differ from the rule in one or more respects but will generally fall into one of the two groups.
Parasites that lay eggs into in organism that is already living parasitically inside a host are known as hyperparasites.
|Host specialists with very precise physiological adaptions to specific hosts. Alot of hosts modification is present along with many ways of preventing encapsulation.||Large host range with a greater choice of species available. The location of the host is more important and often have highly developed means of finding and reaching concealed hosts.|
|Uses a specific venom that only works on a small number of species.||General type of venom that effects a wider variety of host species.|
|Generally Endoparasitoids which develop inside the body cavity of the host, with afew exceptions such as Polysphincta tuberosa.||Ectoparasitoids that develop outside the body of their hosts and sucking the nutrients through the skin.|
|The hosts growth and development continues, so although each species specialises in a single stage, they are able to attack all stages.||Oviposition must take place when the host is large enough to fully feed the parasitoid so later stages are preferred.|
|Oviposition on exposed hosts because the host is still active and able to defend itself.||Oviposition on concealed hosts which is paralysed and unable to protect itself from predators.|
|Pro-ovigenic - They produce many small eggs that are fully developed the moment that the wasp hatches.||Synovigenic - They produce a few large eggs that develop sequentially over the life of the adult wasp.|
|Slow larval development, which is delayed in the early stages to allow the host to grow.||Quick larval development because the host is paralysed and the larvae must eat it as fast as possible before it dies or is eaten by something else.|
|Very little host feeding - a phenomenon where the adult female drinks some of the hosts blood (haemolymph).||Host feeding is common in order to gain protein to produce more eggs.|
|Simpler sex ratio which is usually percentage driven||Sex ratio is influenced by host suitability. Males are usually smaller and so can survive to maturity on smaller hosts.|
|Short adult life||Longer adult life|
Polyembryony is another complex strategy employed by parasites with an aim to ensure that the maximum number of offspring will emerge from the fewest eggs and thus saving energy and resources from wastage. This is basically the process of one egg being layed which then splits and divides many times, cloning itself into many independant larvae. In exteme cases this can result in thousands of larvae from one original egg. Some species have even been shown to produce different types of larvae from the same egg, some of these will develop normally into adult wasps, and others that never mature but act as guards to protect the larvae from other parasitic larvae.
The larva of both Parasitica and Aculeata are always surrounded by food and are poorly developed when compared with other insect larvae. They are always legless and the head is commonly reduced, especially in parasitic species.
The young parasites grow inside or firmly attached to the outside of their host species and there may be just one or several hundred of them on one host, depending on the size of the larvae and the host which is gradually eaten alive, but care is taken not to eat any of the vital organs until the larvae are almost ready to pupate, at which time the host is killed and completely devoured. One obvious exception to this rule is Dinocampus coccinellae (Perilitus coccinellae) which cuts the motor nerves of its ladybird hosts legs and pupates underneath the ladybird which then starves while the pupa is protected by the colour of the host above it. Parasites that cause the death of their hosts are known as Parasitoids.
Clearly these parasites have extra-ordinary powers of detection that seems to be based on scent and vibration, which enables them to find their hosts, and in Ichneumon eumerus in particular, sound which enables it to here the blue butterfly larvae in the ant nest.
There is no hard line deviding Parasitica from Aculeata and there are several transitional families, eg ruby tailed wasps, which have parasitic larvae but the adults are anatomically closer to Aculeata than Parasitoca.
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