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An Introduction to Medical Entomology

For educational purposes. 

 

Arthropoda: Insecta

HYMENOPTERA

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GENERAL CHARACTERISTICS OF THE HYMENOPTERA

 

 

[Also See: ID Keys:  Great Britain,   Palearctic Region,   European Russia-1]

 

       The order has two suborders, the Symphyta and the Apocrita:  The Symphyta includes species with a very generalized form, both as adults and as larvae. None of them show the specialized habits that typify most of the other Apocrita, and they are primarily phytophagous. The first abdominal segment is not completely fused to the metathorax nor does the constricted waist that is characteristic of the remaining Hymenoptera accompany the fusion.  The ovipositor is used as an apparatus for piercing plant tissues. The trochanter has two joints. Larvae are eruciform and in addition to thoracic legs some the abdominal segments may have prolegs that are without distal crotchets or spines (Please see Glossary for terminology).


       This group includes the wood-wasps, the ovipositors of which are used as drills for perforating timber in which the eggs are laid. The 6-legged, strong larva bores through the wood (in the case of Sirex gigas, this stage lasts for two years).  Pupation occurs near the surface of the affected timber, from which the adult bites its way out. The sawflies with saw-like ovipositors, are most important as agricultural pests, and are different from the wood-wasps by having softer bodies, their smaller size, and by the presence of two apical spurs on the anterior tibiae.


       The Apocrita includes all the other Hymenoptera. The second abdominal segment is constricted to form a narrow waist or petiole, the first segment being firmly joined with the thorax.  Larvae are apodous when full-grown.   Ichneumon flies have slender curved antennae, and there is a stigma on the wing. The ovipositor is generally long and projects forward from the tip of the abdomen. The larvae of Lepidoptera and of sawflies are their usual hosts.


       
Cyanamid "flies" also have slender antennae, but there is no stigma on the wing and there is a reduced venation.  Many of these form galls on plants, while others are parasitic on fly larvae.


       Chalcid wasps also have the wing venation reduced with no closed cells. The antennae are geniculate or elbowed. Though most of these small wasps are parasites, e.g. of lepidopterous and dipterous larvae, and of homopterous nymphs, a few feed on plant tissues.

 

       Ichneumonids, chalcids and cynipids have the ovipositor coming from beneath the abdomen well in front of its tip, and these insects differ in this feature from the Proctotrypidae where the ovipositor is terminal.  Dipterous larvae are often parasitized by these insects, as are also the eggs of Orthoptera and Hemiptera. Many hyperparasites occur in this family.


       The ants (Formicoidea) are social, polymorphic insects in which two segments form the abdominal petiole. Moreover, this petiole always has one or two nodes.  The females have a well-developed stinging apparatus which is a modified ovipositor. Polymorphism reaches its highest degree of complexity in this group, as many as 30 different castes having been found.  Some of these are pathological phases due to infection by parasites, e.g. nematode worms, or other Hymenoptera. In colonies that have winged forms of both sexes, mating takes place during a nuptial flight in which several colonies are involved at the same time. This permits interbreeding between individuals from different colonies. The females then cast off their wings and begin colonies in the ground.. The workers are sterile females, whose ability to lay eggs in certain circumstances may be restored. For example, when a colony loses a queen several workers may, under the stimulus of diet, take her place.

 

       In addition to the environmental complexity, which a social existence involves, association with other organisms complicates the lives of ants. Some have adopted an agricultural habit, living on fungi that they cultivate. Others gather seeds from which they destroy the radicle to prevent germination. Special chambers or granaries in the nest are constructed for their storage. A pastoral habit is found in others, a symbiotic relation being set up with such insects (e.g. aphids) because they exude fluids that are coveted to the ants.  There are numerous other associations of a different nature that range from the symbiotic to the parasitic. Finally there are the slave-makers: Formica sanguinea, e.g.,, captures from the colonies of F. fusca pupae which on emergence serve as slaves in the colony which has adopted them (Borradaile & Potts, 1958).


       The wasps of the superfamily Vespoidea are both social and solitary in habit. In these, the abdominal petiole is smooth and, species with a worker caste are always winged. The prothoracic tergum extends back towards the wing base. Wasps are primarily  carnivorous.  Rarely are they phytophagous as in some solitary masarine wasps that feed their larvae on pollen and honey. Among solitary species may be mentioned Odynerus which deposits caterpillars in its nest when its larvae are developing. Pompilid wasps are entirely predatory on spiders.  Other groups have adopted the 'cuckoo' habit, laying their eggs in the nests prepared and provisioned by other species.  Social wasps, e.g. Vespa, live in nests usually constructed of paper obtained in the form of wood pulp. The larvae, living in cells on horizontal combs, are fed on insect food gathered by the workers. In early summer the social wasps feed on such insects as plant lice, etc., but later in the season they search for sweet fruits, which make them annoying, both in the garden and in structures. In autumn the colony dies, fertilized females being the only survivors. The inability to store animal food on which the larvae rely explains the disappearance of colonies in autumn. Only in tropical regions where food is plentiful throughout the year do wasp colonies persist.


       Closely resembling the Vespoidea are the wasps belonging to the superfamily Sphecoidea, the distinctive character of which is the possession of a prothoracic tergurn that does not extend back as far as the wing bases. These are all solitary predaceous forms, which sting their prey and paralyze them before placing them in the larval cells, which have been previously prepared.


       The superfamily Apoidea includes social and solitary bees. Bees are recognizable by their dilated hind tarsi and the plumose hairs of the head and body that acquire pollen. There are also inner metatarsal spines of the posterior legs that comb the hairs free of pollen.  The pollen is then transferred to the outer upturned spines (pollen basket) of the hind tibia of the opposite side. These legs are further adapted with spines for the manipulation of wax plates when being removed from the abdomen. The median glossa is also typical and in certain solitary forms, e.g. Anthophora and all the social bees, e.g. Apis and Bombus, is greatly elongated along with the parts other than the mandibles for gathering nectar from deep-seated flower nectaries. Larvae are fed exclusively on pollen, nectar and salivary fluids. Megachile, the leaf-cutter, is a solitary bee that makes cells of neatly cut leaf fragments. Each cell containing an egg is stored with honey and pollen. Such cells are commonly made in the walls of houses, the mortar being removed for this purpose. Andrena constructs burrows in the ground and, though solitary, is usually found in groups of individuals occupying a common terrain that may include a 'village' of several hundred nests. Nomada has adopted the 'cuckoo' habit
(Borradaile & Potts, 1958).


       Bombus spp. are  similar to the Vespa spp. in that only impregnated females survive the winter. The colony of the honeybee Apis mellifera is more permanent, only the males dying off in the autumn to leave the rest of the colony to hibernate. The nest is constructed of wax, an exudation from abdominal glands of the worker (sterile female), and a material
of vegetable origin fastens parts of the nest together thereby making the whole weatherproof.


        The workers of Apis are graded according to age into nurses, who see to the welfare of the larvae by incorporating salivary juices with their food, ventilators who, by wing-fanning, set up currents in the nest or hive to reduce the temperature and to evaporate the honey, scavengers or cleaners, and foragers who collect pollen and nectar. The changes from nursery work to housework and to fieldwork are necessitated by changes in glandular capacity as age increases. Though the density of the population of the colony determines to some extent when a queen with a number of workers will depart from the hive as a swarm, it appears that this event is also dependent on other factors not as yet clear, one of which is the relative proportions of the above age-groups among the worker caste. The sexes are determined by a cytological mechanism. Thus, in bees, wasps and ants, haploid parthenogenesis results in the production of males. A fertilized (diploid) female has control over the fertilization of eggs that she lays. If an egg is fertilized by sperm from the spermatheca a female (diploid) offspring develops; if not, a male offspring (haploid) develops. Whether a young female becomes a worker (sterile) or a queen (capable of fertilization) depends on nutrition
(Borradaile & Potts, 1958).

 

       The mouthparts of the Hymenoptera are adapted primarily for biting and often for sucking.  There are two pairs of membranous wings joined by hooks on the anterior border of the hind wing joined with a groove on the posterior border of the forewing.  The hind wings are smaller.  The first segment of the abdomen is fused to the thorax, and a constriction behind this segment usually is present. There is always an ovipositor that is modified for piercing, sawing, or stinging.  Metamorphosis is holometabolous.  The larvae are usually without legs and rarely erusiform with thoracic and abdominal legs.  The pupae are exarate

And regularly protected in a cocoon.

 

       The Hymenoptera are remarkable for their great specialization of structure, for their varying degrees of social organization and for the highly developed condition that parasitism has reached. 

 

       The mouthparts are complex in some cases but they seem hardly ever to have wholly lost the various parts recognizable in the generalized Orthoptera body plan. The high point of their development is in Apis, the honeybee, and their least modified condition is in sawflies.


       There also have developed certain parts of the head capsule that are common to all but the more generalized Hymenoptera. Thus the head articulates with the thorax by a narrow neck and the occipital foramen that is small and bounded below by a strong
hypostomal foramen.  The union of the post genae forms this.  Great mobility is thereby possible of the head; the hypostomal bridge forms a strong base for the attachment of the maxillo-labial complex. In all Hymenoptera this complex is formed by the union in a common membrane of the maxillae and labium that are thus placed in a close working relationship with each other. The working of maxillae and labium as a functional unit is further ensured by their basal segments, cardo and stipes, submentum and prementum, being so arranged as to bend in a common plane.  Folding of the mouthparts under the head when at rest, as well as their forward extension when in use, is facilitated. In these several features, most of which are present in generalized forms, there are the foundations on which the structural evolution of the higher forms is based and without which these might never have developed (Borradaile & Potts, 1958).

 

       Among the sawflies are to be found the most generalized mouthparts. Wasps, too, are easily referable in these respects to the primitive omnivorous types with the additional feature of adaptation to licking of fluids by an extension of the bifid glossa and the setose maxillary galea. The mandibles here are well suited by their toothed form to feeding on solid food.

 

       Among the sawflies are to be found the most generalized mouthparts. Wasps, too, are easily referable in these respects to the primitive omnivorous types with the additional feature of adaptation to licking of fluids by an extension of the bifid glossa and the setose maxillary galea. The mandibles here are well suited by their toothed form to feeding on solid food.

       At the other end of the scale of specialization there is the elaborate elongated and extensible mouthparts of Apis, the honeybee.  The mandibles are large, smooth, spatulate structures articulated to the gena of the cranium. They are used for manipulation of wax and pollen within the hive and not for the gathering of food.


       The labium has a short triangular postmentum, to the front border of which is articulated a long prementum. From this there projects forwards a long tongue, formed from fused glossae, and which is setose externally and grooved ventrally. At the base of the tongue are the short curved paraglossae, holding it in such a way as to conduct fluid from the ventral glossal groove to the upper surface of the tongue base and so to the mouth that lies above.  Arising also from the distal end of the prementum are the labial palps consisting of several long segments whose inner surfaces, being concave, can partly encircle the bee's tongue ventrally for the whole of its length.


       In line with the postmentum lies the maxillary cardo at each side. Basally each cardo is articulated to a cephalic apodeme that projects inwards to the head cavity. At its distal end it articulates both with the stipes and with a V-shaped sclerite, the lorum. This lies in the membrane that unites the labium with the maxillae and probably develops as a specialization of it. The locum thus connects the two maxillae with each other, and into its apex fits the proximal angular border of the postmentum. The stipes of each maxilla lies at the side of the prementum and is of about the same length. Distally, on its outer side, lies the much-reduced maxillary palp, and on its inner side a similarly reduced lacinea. From between these two there projects the curved, blade-like, long galea. The two galeae have concave inner surfaces, like the labial palps, and with these latter complete the encirclement of the tongue dorsally.


       Food can be drawn up the ventral groove of the tongue by capillary action, but it can also pass in larger quantities into the space surrounding the tongue enclosed by the galeae and the labial palps, passing within the folds of the paraglossae and being thereby directed to the mouth, which opens above this point.  Such a feeding mechanism is the climax in an evolutionary process which has involved in succession the fusion of the glossa lobes, as in the sawflies, the lengthening of the basal joints of the labium and maxilla as in Colletes, and the elongation of the glossa, e.g. Apis and Bombus.


       
The highly complex social organization in the bees, ants and wasps, in which caste development is of prime importance, is foreshadowed in the interesting behavior of solitary wasps and bees. The supply of food to the larva by progressive feeding, instead of mass provisioning, appears to help the parent to become acquainted with its offspring, and this establishment of family life may be regarded as the forerunner of the complex social state of the higher forms. For instance, in the wasp Odynerus the egg is laid in a cell and sufficient caterpillars stored to serve as food for the whole of the larval life Some African species of this genus supply their growing larvae daily with fresh caterpillars (Borradaile & Potts, 1958).

.
       Another important aspect in the development of social life has been that of trophallaxis. Among wasps the worker taking food to a grub receives in turn a drop of saliva from the grub. The workers eagerly look for this, and it is thought that it is the mutual exchange of food between young and adult that creates in the adult an interest in the welfare of the colony. That the exploitation of a particular form of abundant food has contributed to the development of the social system is obvious. As examples there is pollen and honey for bees and dung as a basis for the simpler social life of some beetles. No feature determining cohesion of the bee colony seems to be of such paramount importance as the ability of the queen to satisfy the craving for a secretion produced by her (queen substance) which all members of the colony experience. The absence of a queen is rapidly sensed by the colony and its communal behavior consequently greatly disturbed. Ants and termites appear to be similarly dependent on the queen.


       The complex environment in which a social insect lives has produced a form of behavior simulating intelligence. Bees, for example, can with great effect inform one another of the presence of a food source. They can further inform each other by scent and, dance of the position of the food source with considerable precision provided the sky is not wholly overcast.  The direction of the dance movement refers to the position of the sun in the sky relative to the hive. Because this position can be determined by the bees in a sky when the sun is not visible, though in which some blue sky is present, the ommatidia of the compound eye enable the bee to analyze the degree of polarization of light emerging from a blue patch.  It may be concluded that associated with the social state's development, there has come about a complexity of behavior that ultimately depends in turn on the enhanced sensitivity of the members of the colony.


       A kind of parasitism known as "parasitoidism" is highly developed in the Hymenoptera, with the ichneumons, chalcids and proctotrypids being almost entirely parasitic. Almost all orders of insects are affected by the activities of these groups, the egg, larval, pupal and adult stages all being parasitized.  Insects with parasitic habits are divided into (1) Koinabionts and (2) Idiobionts (Please see <
koiidio.htm> for comparison ).


       The Hymenoptera contains some of the most economically important insects. The sawflies are important as agricultural pests. Flower-visiting bees are of great value in the pollination of flowers. Carnivorous wasps devour other insect pests such as aphids, while to a large extent the parasitic Hymenoptera are useful in regulating the populations of phytophagous insects as has been proven by numerous biological control campaigns.


       Two main types of larvae are found in this order, the legged larva of the sawflies and the legless form of bees, wasps and ants.  The sawfly larva has an outward resemblance to the lepidopterous caterpillar, but is easily distinguished by its single pair of ocelli and the absence of crotchets or spines on the abdominal legs. The prolegs of the abdomen occur on different segments (
Borradaile & Potts, 1958).

 

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Detailed Morphology & Habits

 

       The Hymenoptera ranks second in the number of insect species next to the Coleoptera.  They also contain the greatest number of beneficial insect species.  Two-paired and clear wings characterize them the front wings being largest.  A stigma often occurs on the coastal margin.  Venation is very variable, however.  Some species are wingless, which is especially true in the parasitic groups.  Hamuli or hooks are present on the costal margin of the hind wing, which engage with a ridge on the posterior portion of the forewing to hold the wings together.  This enables the wings to operate together as a single unit. Three ocelli are usually present. 

 

       The mouthparts are usually of the chewing type, but there many species with a lapping-sucking type.  The mandible is used for chewing, cutting, molding wax, forming cells, etc.  The labium and maxillae are considerably modified in the bees.

 

       The meso- and metathorax are well developed, but he prothorax is reduced.  The first segment of the abdomen is fused with the thorax and is called the propodeum.  The second abdominal segment is often a petiole and the remaining portion the gaster.  Most narrow-waisted species are beneficial while broad waisted species are usually harmful (e.g., sawflies).

 

       The ovipositor is used to bore, pierce or to reach into crevices in order to deposit eggs.  It is often associated with poison glands and ducts.  For example, some wasps sting lepidopterous caterpillars with just enough poison to immobilize them.  This then serves as fresh food for the developing wasp larvae.

 

       The pupae are exarate and may be either naked or enclosed in a cocoon, but much variation may occur within one family.

 

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Subdivisions and Classification

 

       There are two suborders:  Symphyta (Chalastogastra) are the sawflies and Apocrita (Clistogastra) are all other groups. 

 

       The Symphyta have eruciform larvae and their prolegs are without crochets may occur on all abdominal segments.  The adults have the abdomen broadly joined with the thorax.  The ovipositor is adapted for piercing so that their eggs may be laid in hard wood.  There are many pestiferous species in this group.

 

       The Apocrita contains the largest number of species of Hymenoptera.  Their larvae are grub like without legs.  Some develop as grubs on other animals and their mother nourishes some.  The adults have a distinct petiole, and in some ant species both the second and third abdominal segments may form the petiole.  A node is usually present.

 

HYMENOPTERA OF PRIMARY MEDICAL IMPORTANCE

 

       The Hymenoptera as a group are considered more important to humanity than for the few groups that inflict injury, and even death, to humans and animals by their poisonous stings.  Their attributes as pollinators of food plants, honey production are well known.  However, probably far outnumbering any other group are the parasitic Hymenoptera that by their constant interaction with other insect populations maintain stability in the ecosystem.  There are probably over one million species of the parasitic group.

 

       Stressing their adverse effects as stinging insects, Matheson (1950) pointed out that they are provided with a sting, which is a modified ovipositor and which is connected with special poison glands.  He noted that all stinging insects belong to the Hymenoptera, which include the families Apidae (honeybees), Bombidae (bumblebees), Vesidae (wasps & hornets), Sphecidae (digger wasps), Mutillidae (velvet ants), Formicidae (stinging ants) and others of lesser importance for troubling animals and humans.

 

       Avoidance is probably the best tactic for control of stinging insects.  It is especially important to avoid contact with their nests.  Wasps are especially provoked by human proximity to their nests and will attack in large numbers, causing death in susceptible individuals.  The unfortunate hybridization in Brazil of Italian and African strains of the honeybee has produced a very aggressive new strain (the "Killer Bee") that spread throughout the Americas, and which has as of 2016 resulted in the death of many humans and animals (Legner 1990).

 

       Available to the general public are a number of pesticides that when applied to adults as direct killing agents or in poisoned baits are temporarily effective.  It is important to restrict adulticides to nighttime applications when wasps and killer bees are inactive.  To control ants, such as the Argentine ant, applying poisoned baits around a dwelling will reduce the numbers entering the house.  However, many of the products available have little or no effect due to insecticide resistance.  And the overuse of any product in one area will reduce its effectiveness as the local population develops resistance.

 

 

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  Key References:     <medvet.ref.htm>    <Hexapoda>

 

Matheson, R. 1950.  Medical Entomology.  Comstock Publ. Co, Inc.  610 p.

Service, M.  2008.  Medical Entomology For Students.  Cambridge Univ. Press.  289 p

Legner, E. F.  1995.  Biological control of Diptera of medical and veterinary importance.  J. Vector Ecology 20(1): 59-120.

Legner, E. F..  2000.  Biological control of aquatic Diptera.  p. 847-870.  Contributions to a Manual of Palaearctic Diptera,

          Vol. 1, Science  Herald, Budapest.  978 p.

 

 

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