Apoidea

The superfamily Apoidea (Pierre André Latreille, 1802) is a major group within the Hymenoptera, which includes about 20,000 species, the best known of which is the honeybee (Apis mellifera).

The morphology of the Apoidea reflects their specialization in pollinating insects: the body is more or less covered with hair, the mouthparts are adapted to the collection of nectar, the legs have modifications suitable to favor the collection of pollen; the stinger is only for defense.

Scientific classification

DomainEukaryota
KingdomAnimalia
SubkingdomEumetazoa
SuperphylumProtostomia
PhylumArthropoda
SubphylumTracheata
SuperclassHexapoda
ClassInsecta
SubclassPterygota
CohortEndopterygota
OrderHymenoptera
SuborderApocrita
SectionAculeata
SuperfamilyApoidea

Anatomy

The mouthparts of Apoidea are of the chewing-sucking, chewing-lambing and chewing-lambing-sucking type and are mostly suitable for sucking sugary liquids (nectar, honey, honeydew). The mandibles completely lose the ability to chew and the liquids are sucked by means of the maxillo-labial complex: the galea and the labial palpi are developed in length and flattened; at the ligula, the insect forms a suction channel through which the liquid food is sucked.

The species belonging to the most primitive families (Colletidae, Andrenidae, Stenotritidae, Halictidae, Melittidae) have a short ligula that allows them to forage only flowers that have a shallow corolla. Others (Megachilidae, Apidae) have a ligula more suitable for deeper corollas.

They have legs basically ambulatory type, but which have particular formations aimed at scraping the pollen attached to the body and collect it in a real organ of transport located in the posterior tibiae. A particular characteristic of Apoidea is the exceptional development of the first tarsomere, much larger than the following ones.

They have a particular adaptation of the front legs, in correspondence of the first tarsomere and the tibia, which aims to facilitate the cleaning of the antennae. The ventral side of the first tarsomere, near the tibio-tarsal joint, has a semicircular recess covered by a series of short bristles, which simulate a brush. The closure of the tibio-tarsal joint causes the distal spur of the tibia to oppose the opening of the recess by closing a subcircular lumen through which the antenna is passed. The spur, in this way, exerts a thrust that forces the antenna to slide inside the hollow of the tarsomere, while the bristles scrape the pollen left attached to the antennae.

The corbiculae or pollen basket is a concavity present on the outer face of the hind tibiae, on the edges of which are inserted sparse, long bristles. The insect brushes the pollen from the body, helping itself with the anterior and posterior legs and moistens it by compacting it on the pollen basket; the bristles naturally have the function of forming a cage that holds the pollen. This structure is particularly evident when the foraging workers return to the nest or to the hive: the collected pollen appears in the form of two globular masserelles, of a generally yellowish color, more or less orange, (prevailing colors, but not exclusive, in pollens), at the sides of the hind legs.

The brush or broom is a series of dense, strong bristles arranged in several transverse rows on the inner face of the first tarsomere of the hind legs. It is used to scrape pollen from the thorax, wings, and abdomen.

Systematic discrimination of Apoidea uses the morphology of the wing veins.

Social Life

On the basis of social behavior, we distinguish:

  • solitary bees (e.g. Colletes, Anthophora, Xylocopa): after fertilization, each female builds a nest, usually simple cavities dug in the soil or wood, consisting of a series of cells, then fills them with nectar and pollen mixture to form the so-called “bee bread”; finally, they lay an egg in the cell. Their larvae develop exclusively on the basis of these provisions, without receiving any other care from their mother;
  • community bees (e.g. Andrena, Megachile): the females use a common nest in which each builds and supplies its own cells;
  • quasi-social bees (e.g. Nomia): females cooperate in nest construction and cell provisioning, but without any division of labor;
  • semi-social bees (e.g. Halictus): females cooperate in nest construction and cell provisioning, and are divided into two functional castes: alongside fertilized females, there are also sterile worker females;
  • social bees (e.g. Apis, Bombus, some species of Halictus): they are characterized by the overlapping in the nest of several generations and by a division of labor based on the presence of different castes.

Nutrition

The diet of Apoidea is extremely specialized: they are totally dependent on flowers for their food. Adults feed on nectar, while larvae feed on pollen and nectar. On the basis of floral preferences we distinguish:

  • oligoleptic species: these are those that feed on pollen on a limited number of species; they are in turn distinguished into strictly oligoleptic species when they forage on a few species of a single genus (e.g., Colletes cunicularius feeds on pollen and nectar. Colletes cunicularius feeds exclusively on Salix pollen, Andrena florea is specialized in Bryonia), and largely oligoleptic species when they feed on pollen of species belonging to different genera of the same family (e.g. Andrena agilissima, which limits its choices to genera of the family Cruciferae, or A. fuscipes, specialized in Ericaceae);
  • polyletic species: they are those that forage on several species of different families; the domestic bee, Apis mellifera, is the most representative example of polyletic apoid.

It is not uncommon to observe species considered oligoleptic which modify their behavior according to the momentary unavailability of pollens of the preferred species: Colletes cunicularius, for example, can direct its foraging activity to species different from Salix in case of unavailability of the latter.

There are also some apoids that show a specialization for pollen collection on flowers of non-nectariferous species (e.g. Ophrys), and must necessarily resort to other species to satisfy their carbohydrate needs.
Some non-nectariferous species sometimes host colonies of aphids whose honeydew accumulates in the flower spur: this is the case, for example, of the orchid Himantoglossum robertianum, which hosts many colonies of the aphid Dysaphis tulipae, whose honeydew acts as an attraction for the pollinating insect Bombus hortorum.

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