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General Overview of the Hymenoptera

wasp photo, frontal aspect

Hymenoptera is one of the most diverse orders of insects, including over 115,000 described species. Only the Coleoptera (beetles) outnumber the Hymenoptera in species described, and the Diptera (flies) and Lepidoptera (moths and butterflies) rival them in species diversity. Common names highlight the diversity of Hymenoptera. While most insect orders can be associated with a single common name (i.e. Coleoptera are beetles, Diptera are flies), Hymenoptera includes such varied insects as ants, bees, wasps, sawflies, and many others without any common names at all. However, only by comparing the Hymenoptera with a well known group such as the vertebrates is their diversity truly demonstrated: there are only half as many species of vertebrates (which includes all mammals, birds, fish, reptiles, and amphibians) as there are Hymenoptera. In addition, most vertebrate species have been catalogued and described, while a large proportion of the Hymenoptera have not. Hymenoptera are not only diverse in terms of structure, size, and numbers of species, but also in their habits and life histories. Some are phytophagous (plant-feeding), while others are herbivorous, predatory, or even parasitic. Many Hymenoptera lead a solitary lifestyle, while some of the bees, ants, and wasps show some of the highest degrees of social organization of any animals.

Hymenoptera are also important to the balancing and functioning of most ecosystems on the planet. Bees are important pollinators of many flowering plants, and tiny fig wasps are solely responsible for pollinating the the many species of figs. Ants have a large impact in many communities because they are often the most numerous creatures there. Parasitoid wasps play important roles in regulating populations of herbivorous and predatory insects. Hymenoptera are also one of the most beneficial orders for the human economy. Not only do bees pollinate many of our crops, but they also produce goods such as wax and honey. Parasitic wasps are often the most successful way to control pest insects without excessive use of insecticides.

Although the Hymenoptera is a diverse group of insects, there are many characteristics that unite it as a natural grouping. First, all Hymenoptera have membranous wings, much like many primitive insects. However, their venation is much simpler. The base of the forewing is covered by a small roundish sclerite (plate) called the tegula, while the fore and hind wings are connected by small hooks called hamuli. Although all of these characters help distinguish winged Hymenoptera from other insects, they do little good for the numerous species with rudimentary or absent wings. A more technical character shown only in the Hymenoptera is that the first segment of the abdomen is fused with the last segment of the thorax. In more advanced Hymenoptera, which possess a strong constriction between the first and second abdomenal segments, the first segment is referred to as the propodeum. Therefore, the strong constriction separates not the thorax from the abdomen, but the thorax plus the first abdominal segment (the mesosoma) from the rest of the abdomen (the metasoma). This combination of characters should allow anyone to differentiate between Hymenoptera and any other of the insect orders. It is important to note that Hymenoptera also have holometabolous develeopment (complete metamorphosis); that is they go through larval stages, and a pupal stage before becoming adult. While insects such as grasshoppers have life stages that look similar througout their life cycle, Hymenoptera have larvae which are as different from the adults as butterfly caterpillars are from butterfly adults.

Sawflies ("Symphyta")

The sawflies, members of the suborder Symphyta, are the most primitive members of the Hymenoptera. They contain 14 families, and comprise less than 5% of all hymenopteran species. With the exception of the tiny family Orussidae, all sawflies are phytophagous, feeding on many kinds of plants. Some of the few hymenopterans to be considered economic pests, sawflies can cause considerable damage in forests around the world. Immature sawflies look much like caterpillars of moths and butterflies (see picture to the right), and it is this stage that causes the most of the damage to plants. The adults look somewhat like small wasps, except they lack the constricted waist common to the rest of the Hymenoptera and have a more complete wing venation. Sawflies do not possess stingers as do bees and wasps. Instead, they have an egg laying structure (ovipositor); in many species, the ovipositor is sawlike and is used to cut open plant tissue and lay eggs inside.
photo of a sawfly larva

photo of an ophionine ichneumonid

Parasitic Wasps ("Parasitica")

The parasitic wasps are perhaps the most diverse members of the Hymenoptera, and also some of the most poorly known. Most parasitica attack other insects and are termed parasitoids. They lay their eggs in or on the intended victims (called hosts) and the larvae feed on them until they transform into pupae. The pupae usually develop inside the empty shell of the host, but a few spin cocoons. While the parasitoid life style is preponderant, there are others including phytophagy (including gall-formation) and predation. They are often referred to as the suborder Parasitica, although this is not a natural grouping. However, the parasitic wasps plus the Aculeata, which will be discussed later, form a difinitively natural group called the Apocrita. Parasitic wasps (and the rest of the Apocrita)possess the constricted waist common to more typical bees and ants but have an ovipositor more like that of the sawflies. Instead of the saw-like ovipositor of the sawflies, however, parasitic wasps have a needle-like one, used to pierce the outer shell ("cuticle") of host insects or other surfaces such as seed casings. In many cases, the ovipositor injects venom, that stuns the host, and/or other substances which defeat the host's immune reaction to the inserted egg or eggs. Many species of parasitic Hymenoptera will attempt to use the ovipositor in a defensively, but, with the exception of some larger species of Ichneumonidae, most are incapable of penetrating human skin and inflicting a sting. The larvae of Parasitica are also very different from those of sawflies. They completely lack legs and eyes, and spend their entire existence inside or attached to the surface of the host.

Ants, Bees, and Stinging Wasps ("Aculeata")

The most advanced group within the Hymenoptera is the Aculeata, which includes ants, bees, and stinging wasps, the typical Hymenoptera recognized by most people. Unlike most major divisions within the Hymenoptera, this is certainly a natural group. Most Aculeates are predators, although this is not a hard and fast rule. Some are parasitoids while others, such as the common honey bee, feed primarily on pollen and nectar from flowers. They all have a constricted waist as do the Parasitica, but they have a significantly different ovipositor. Instead of using the ovipositor to inject eggs into a host, the Aculeates can sting their prey and inject venom into them. As the ovipositor is only found in female insects, it follows that male Aculeates cannot sting. The Aculeates also contain the largest of the Hymenoptera: tarantula hawks, wasps in the genus Pepsis, can reach a wingspan of 10 cm and provide paralyzed tarantulas for development of their offspring. The Aculeates also demonstrate a diverse array of social organization, from solitary wasps to communal nesting bees to the highly organized societies of ants. Aculeate larvae are morphologically similar to those of the Parastica. However, because aculeate eggs are laid in a protected, enclosed space, and developing larvae are provisioned sufficient food for their entire development, they need only to be able to eat the food provided to them. Thus, they have different structural adaptations than Parasitica that usually live within or upon developing hosts.

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