Form and function

External structure

Concomitant with diversity of habit in the adult Hymenoptera is a diversity of form. This variety prevails to such a degree that only the briefest, general description applies to the order as a whole.

As in all adult insects, the segmented body consists of three primary body regions: head, thorax, and abdomen. In most forms a narrow constriction at the anterior (front) end of the abdomen distinctly separates it from the thorax. Two pairs of membranous wings are usually present. The vein pattern in the wings is usually reduced, and, in some forms, veins are entirely absent. The hindwings, noticeably smaller than the forewings, are interlocked with the latter by tiny hooklets on the anterior margin. In certain solitary wasps, particularly the wasp family called velvet ants (Mutillidae), females are wingless. All worker ants (Formicidae) lack wings. Winglessness also occurs in some genera of ichneumons and chalcids, usually only in females, sometimes in both sexes, but rarely in the males alone.

The mouthparts are usually modified for biting or for biting and sucking. The compound eyes (i.e., consisting of many mosaic-like facets) are large. There are usually three ocelli, or simple eyes, arranged in a triangle on the top of the head. The antennae vary greatly in form. Rarely are they shorter than the head is wide. Usually they are moderately long, sometimes longer than the body, and composed of many segments. Often the basal segment, or scape, is greatly elongated. In some, segments near the tip are modified into a club, whereas others may have branched segments. The leg is nearly always characterized by five segments, of which the fifth is the tarsus, or “foot.” The abdomen of the female has an ovipositor at the tip. In the sawflies, the ovipositor is modified into a sawlike tool used for making slits in the leaves or stems of plants in which the eggs are deposited, but in all other Hymenoptera it is modified for stinging or piercing.

Internal structure

The form of the digestive system in Hymenoptera is relatively uniform throughout the order. In ants, an infrabuccal chamber located under the mouth has as its apparent purpose the trapping of indigestible particles that have been ingested along with food. This solid residue is regurgitated as a pellet.

In stinging forms the esophagus enlarges near the stomach into a crop, or honey stomach, which serves as a reservoir for liquids to be later regurgitated. In honey ant repletes, the crop may be greatly distended. In honeybees, it may contain as much as 75 milligrams (0.003 ounce) of nectar, which can be about one-third the insect’s total weight. In bees and wasps, the stomach, or ventriculus, is the largest part of the digestive system; in most ants, solitary wasps, and other forms, it is quite small.

Two pairs of salivary glands are well developed, particularly in bees. One pair is found in the head and the other in the thorax. The ducts leading from them unite to form a single canal that passes into the pharynx. Drones and queen bees also have a mass of salivary gland cells in the head near the ocelli. Worker bees have one pair of pharyngeal glands that produce food, especially royal jelly, for the young larvae. The pharyngeal glands are rudimentary in drones and absent in queens.

The function of the mandibular glands, which open near the inner angle of the mandible, is not fully understood. However, in gall wasps their secretion is known to cause an abnormal growth of cells in leaves. Among social forms, this gland in the queen produces both the substance that inhibits ovary development in workers and one that attracts males during her mating flight.

The so-called Nassonow gland, opening on the dorsal side of the abdomen, produces a substance that is used to mark the entrance to the bee hive as well as food sources away from the hive. Honeybees, bumblebees, stingless bees, and many solitary bees have wax glands on the sternites (ventral body plates). The wax is used in the construction of brood cells and cells for the storage of pollen and honey.