Classification

The Hymenoptera (membranous wings), with more than 130,000 described species, constitutes an enormous order within the insect class (Insecta or Hexapoda). They can be divided into 2 suborders, the Symphyta (sawflies) and the Apocrita (wasps, bees and ants). Members of the first suborder have no venom apparatus, but rather a saw-like ovipositor at the posterior end which they use to place their eggs in plant substrate.

The Apocrita also possess such an ovipositor, which in this suborder is modified to varying extents. This suborder is further divided into the Terebrantes and the Aculeata. The offspring of the Terebrantes grow parasitically; the adult animals use the ovipositor to deposit their eggs into or on host animals (e.g. butterfly caterpillars, larvae of sawflies or spiders). Once the larvae hatch, they feed from the body matter of the host. In particular in those species that inject their eggs into the host, the ovipositor is associated with venomous gland secretions that serve to paralyse and “conserve” the host.

In a certain sense, the Aculeata represent a further development of the Terebrantes, since in the former the ovipositor is fully developed as a venom apparatus and is usually no longer associated with oviposition. Of the roughly 50,000 species in this infraorder, most are solitary and are of no significance for humans as venomous animals. Dangerous aculeates are the social species that form communities, which often come into contact with humans. These are the bees (Apidae: bees and bumble bees), wasps (Vespidae; Vespinae: wasps and hornets) and ants (Vespidae; Formicinae).

A current taxonomic tree for the hymenopterans can be seen at: Tree of life web project.

Morphological characteristics and venom apparatus (Aculeata)

The division of the body into 3 parts, i.e. head, thorax (anterior part of the body) and abdomen (posterior part of the body), which is typical for insects, is clearly visible in the Aculeata (Fig. 4.46, 4.47, 4.48). The mouth parts for licking and biting are located on the head, as well as a pair of antennae and 2 large ommatea. Three pairs of legs are located on the thorax, as well as a pair of transparent, membranous wings. The rear pair of wings is considerably smaller than the front pair and often not easy to find. Among the ants, only the animals that are able to reproduce have wings.

The venom apparatus is located on the posterior section of the abdomen and consists of glands and a complex, chitinous sting apparatus (Fig. 4.45a). There are two glands, the so-called acid and alkaline glands, which open outwards at the base of the sting. The sting is divided into 3 parts in the following manner: 2 movable and often barbed lancets within the sting stylus which together form the cavity of the venom duct (Fig. 4.45b). At their base, the two lancets lead into a multi-part lever system composed of various chitinous plates. When the sting is ready to be used, the lever system is employed to slide the lancets alternately up and down the stylus so that the whole sting enters the victim by means of these sawing motions.

In the tribe Meliponini (stingless bees, Apidae) and several subfamilies of ants the sting has regressed and disappeared completely over the course of their phylogenetic development.

Fig. 4.45 Schematic representation of the sting apparatus in the Aculeata.
a Overview (adapted from Müller 1988).
b Cross-section of the sting (adapted from Edery et al. 1978).

Range of venom effects

See Clinical flowchart: Hymenopterans.

Social Organisation

Apart from bees, wasps and ants, most aculeates are solitary and use their venom apparatus primarily to catch prey. They are thus rarely the cause of hymenoptera stings. In the social hymenopterans the venom apparatus is associated to an increasing extent with defence, and bees and ants no longer use it to catch prey.

The spectrum of social structures ranges from simple groups with a small number of individuals and a basic division of labour to highly complex social communities in which thousands of individuals may live together in a colony. Such distinct social groupings are characterised to a high degree by division of labour. Morphologically and physiologically distinct individual animal types form separate castes with varying areas of responsibility. A queen – sometimes more than one – is solely responsible for oviposition. Workers (females) feed the queen and take over the offspring, which are then raised in special chambers. Most of the growing larvae in turn become infertile workers. They represent the overwhelming majority of the animals in a community and are responsible for building and maintaining the nest, gathering food, looking after the brood and defending the colony. Males (drones) are only raised for the purpose of fertilising new queens in times when a colony becomes too large and a number of animals swarm out to establish a new colony. The extremely complex interactions within a colony are regulated by ritualised behaviours and the use of pheromones, among other factors.

Epidemiology

As a basic principle, with regard to hymenoptera stings it is necessary to distinguish between toxic and allergic effects. A single sting in a non-sensitised person is barely more than unpleasant but does become problematic if it occurs in the region of the oral cavity (danger of asphyxia). If one considers that wasps in particular are frequently attracted to foodstuffs and drinks, it is distinctly possible that such a sting may occur if caution is not taken when eating or drinking. Otherwise, life-threatening envenoming can only be expected if several hundred or even thousand stings occur. In fewer than 5% of fatalities caused by hymenoptera stings was actual envenoming the cause of death (Müller 1988). The vast majority of fatal stings (mainly bees and wasps) are attributable to allergic reactions, in particular anaphylactic shock. In this regard, the honey bees and several social wasps are particularly important, as humans are generally stung several times by these animals in the course of their lives and thus the possibility of sensitisation arises.

In many regions outside of the tropical and subtropical zones, hymenopterans are the most important venomous animals, even more so than venomous snakes, in that they cause the most fatalities in those regions. During the 1950s in the USA an average of 46 people per year died due to accidents with venomous animals. Around 50% of these fatalities were caused by hymenoptera stings (predominantly bees and wasps), 30% by snakebites and around 14% by spider envenoming (Parrish 1963). In more recent times the annual number of fatalities caused by bee and wasp stings in the USA is estimated at 30–120 (de Moor C, Vankawala HH, Park R: Hymenoptera stings). In England and Wales around 4 fatalities due to hymenopterans are recorded each year, whereby half of the victims were not known to have had an existing allergy to bee or wasp venom (Youlten 1987). In Switzerland in the period 1961–1983, 61 fatalities (2.7 per year) caused by hymenoptera stings were recorded (Müller 1988). For Europe it is estimated, that at least 200 people die each year due to anaphylaxis from hymenopteran sting (Müller 2010).

Investigations of the prevalence of bee and wasp venom allergies in the general population in North America yielded results of 3.3% in adults and 0.8% in children (Golden et al. 1989, Settipane et al. 1972).

Prevention

For persons suffering from an allergy there is the possibility of desensitisation. These individuals are strongly advised to carry an emergency kit with them (see Diagnosis & Treatment: Hymenopterans).

General advice:

• Do not go too close to beehives or wasp nests.
• Stay calm in the presence of bees or wasps; quick movements provoke defensive stings.
• Do not walk barefoot on lawns or meadows during the warmer seasons. Bees are attracted to field flowers, and there are often numerous wasps close to fruit trees with ripe fruit.
• Take care when eating and drinking if there are wasps around. Wasps will often alight on glasses or food and this can result in dangerous stings in the oral cavity. 
• Conspicuously bright and patterned clothes as well as certain scents (perfumes, hair sprays etc.) can attract wasps and bees.
• When riding a motorbike wear a helmet (with visor) and gloves to protect against bees and wasps that might fly into you.
• Nests in and around the home should ideally be removed by experienced persons (apiarists, fire brigade). If this is not possible, the nest should be removed at night, as hymenopterans have a limited sense of direction in the dark. The entire body should be protected by thick, loose clothes, gloves and a face net.

Persons suffering from an allergy should take particular note of the above points and avoid places where hymenopterans occur in great numbers. These individuals should always carry an emergency kit with them.

Review articles

Hymenopterans in general: Akre and Reed 1984, Meier 1985, 1995, Müller 1988
Bees: Gould and Gould 1988, O'Toole and Raw 1991, Michener 2000
Social wasps: Edwards 1980, Kemper and Döhring 1967, Spradbery 1973
Ants: DeRéaumur 1977, Hölldobler and Wilson 1990, 2009