Osmia rufa

''Osmia bicornis'' (synonym ''Osmia rufa'') is a species of mason bee, and is known as the red mason bee due to its covering of dense gingery hair.Wild Life Trusts
/ref>
- Swarm-like behaviour of red mason solitary bees - retrieved 2013-08-14INSECTS - Collins gem guide
/ref> It is a solitary bee that nests in holes or stems and is polylectic, meaning it forages pollen from various different flowering plants. These bees can be seen aggregating together and nests in preexisting hollows, choosing not to excavate their own. These bees are not aggressive; they will only sting if handled very roughly and are safe to be closely observed by children. Females only mate once, usually with closely related males. Further, females can determine the sex ratio of their offspring based on their body size, where larger females will invest more in diploid females eggs than small bees. These bees also have trichromatic colour vision and are important pollinators in agriculture.

Taxonomy and phylogeny

This species is part of the order Hymenoptera, which consists of bees, wasps, ants, and sawflies, and a member of the family Megachilidae, which mostly consists of solitary bees, and is among 11 species of ''Osmia'' identified in Britain. ''O. bicornis'' is the current scientific name for this bee, although it was formerly known as ''O. rufa.'' In 1758, Linnaeus described the male of this species under the name ''Apis rufa'' and described the female as a separate species ''Apis bicornis''. In 1802 Kirby recognised that ''A. bicornis'' and ''A. rufa'' were the same species and he named this species ''Apis bicornis''. Subsequently, the opinion was put forth that ''A. rufa'' was the correct name, because it appeared before ''bicornis'' in the Systema Naturae. The use of the name ''rufa'' does not comply with the International Code of Zoological Nomenclature's rules, however; the concept of "line priority" does not exist in the rules, and the action of the first revising author, Kirby, must be followed. Thus the correct scientific name for the species is ''Osmia bicornis'', although ''O. rufa'' is still widely used. Three subspecies of ''O. bicornis'' are often recognized, namely ''O. b. bicornis'', ''O. b. cornigera'', and ''O. b. fracticornis''.

Description and identification

''O. bicornis'' is about the same body size as the honeybee. Sexual dimorphism is observed in this species; females are larger than males, because the female larvae are provisioned with (and eat) more pollen. Body size in ''O. bicornis'' decreases as temperature in brood cells increases. Beyond 25 °C, body growth can be severely truncated, leading to small adult body size or mortality. The male and females are also distinguishable by antenna length, with males possessing an additional antenna segment, (characteristic of almost all Hymenoptera).

Males

Males are 8–10 mm long. They have a grey-white tuft of hair on their faces, including on the clypeus.

Females

The females have two horns and darker hairs on their heads, and are 10–12 mm long. Clypeal hairs are absent in females.

Distribution and habitat

''O. bicornis'' is found in England, southern Scotland (possibly northern Scotland, as well), Wales, Ireland, mainland Europe, Sweden, Norway, North Africa, Georgia, Turkey, and Iran. Of the 11 species identified of ''Osmia'' in England, ''O. bicornis'' is both the largest and most common species present.

''O. bicornis'' occupies a variety of nesting sites within nature and in sites of human construction. These bees have been known to nest in key holes, empty snail shells, plant stems, and empty beetle hollows. ''O. bicornis'' occupies the old shells of these three species: '' Helix nemoralis'', '' Helix hortensis'', and ''Helix pomatia'' and the nests of '' Anthophora'' species. Additionally, these bees make their nests in such sites as sandy banks, decaying trees planted in clay soil like the willow tree, old-mortared walls, flint stone holes, garden shed fifes, and window frame holes and cracks.

The maximum foraging distance for ''O. bicornis'' is about 600 m, though generally high plant density around the nests allow bees to forage closer to the nest and for a shorter duration.

Nest structure

The nest of ''O. bicornis'' consists of an array of partitioned cylindrical cells in holes in wood or reed tubes. These bees accept a diverse range of pre-existing cavities as nest sites. The cells are arranged linearly within a narrow tube. If the internal diameter of the tube exceeds 12 mm, then this linear arrangement may be forced into two rows instead of one. The length of each cell can vary from 10 to 21 mm. The inner sides of the partitions are rough and convex, while the outer sides are smooth and concave. Between the cells and the terminal plug is a space known as the vestibular cell. The vestibule acts as a form of protection against volatile environmental conditions. The bees whose nests are exposed to the sun and heat build vestibules more frequently. The material used to build the nests is mud mixed with their mandibles, but the sides of the tunnel in which the nests are located are usually not lined with mud, with the exception of some irregularly arranged nests. Females construct around six cells per nest on average; however, larger females construct more cells than smaller ones. When it is time for females to lay their eggs, they add pollen to each brood cell and lay one egg in each cell next to the pollen. The sequence of nesting behavior is: cell construction, provisioning, egg-laying, and sealing the cell.

Cells containing females are typically larger than those containing males, due to the sexual dimorphism of the species. Additionally, cells containing females are situated towards the back of the nest, while those with males are closer to the nest entrance. Because of this, male offspring leave the nest sooner than females. Due to the linear arrangement of cells in the nest, the youngest bee leaves earlier than older ones.

Developmental cycle

Although these bees may be seen into late June, they are most active during the spring and early summer. Each year, one generation of bees is formed, making an appearance during the spring. About one week after eggs are laid in the brood cells, the eggs hatch and larvae develop through the summer. The larvae then enter the pupal stage upon spinning cocoons, in which the anterior collar, nipple area, and outer meshwork of the cocoon are spun simultaneously. The adults then hibernate through the winter in the cocoons and finally emerge as mature bees in the spring. The eggs hatch after about one week; the larvae start spinning a cocoon about one month after hatching. The bees become adults in fall, but stay quiescent until next spring.

During cocoon formation, the larva uses saliva to encompass the faecal material and cell. The anterior part of the cocoon is composed of a domed collar and a central, domed nipple region, and the larva weaves salivary "silk" threads in a circular pattern in this region. The larva also uses its digestive contents to form smears on the cocoon, leading to hardening of the cocoon and a colour change to a dark, red-brown. In this stage, in which the organism becomes classified as an imago, the metabolic rate of the imago declines because it must have enough food to survive the winter. Both the imago's body weight and fat body weight decrease.

Male larvae are placed in front of the females within the nest, allowing the males to emerge first in the spring. Specifically, female eggs are laid in inner brood cells, while male eggs are laid in the outer brood cells. Upon emergence, females fly around for about eight weeks. These bees store mostly pollen moistened with a small amount of nectar, which is eaten by the larvae during the summer before they rest through the winter in a cocoon.

Behaviour

Colour vision

''O. bicornis'' bees possess a trichromatic colour system, which they use in foraging for pollen from flowers; the three colours are ultraviolet, blue, and green. A similar colour system is found in these bee species: '' Apis mellifera'', ''Bombus terrestris'', '' B. lapidarius'', '' B. monticola'', '' B. jonellus'', ''Vespula germanica'', and '' V. vulgaris''. Studies comparing the colour systems of ''O. bicornis'' and ''A. mellifera'' show both species share the same spectral sensitivity functions in ultraviolet and blue receptors, while the green receptor in ''O. bicornis'' is sensitive to longer wavelengths than in ''A. mellifera''

Mating behaviour

During mating season, male behaviour with respect to pursuing females is varied, with some males establishing territories close to nesting sites where females emerge and other males observing flowering sites nearby. Males do not normally engage in intrasexual aggression, though they do inspect each other. When a specific mate of interest is present, however, signs of aggression are evident among males. When several males become aware of a receptive female, all males try to mount her; the males do not assault each other directly. In some cases, females may escape and not mate with any of the males.

Females are monogamous, mating with one male within a few days after emergence in the spring. However, males encounter difficulties in completing successful copulation with females, including male inability to determine from where and when females will emerge. Nests are dispersed widely, increasing the number of sites from which new females can emerge. Additionally, females fly away from the nests as soon as they emerge, increasing the mating challenge for males. To counteract these difficulties, males can increase their mating chances by positioning themselves close to foraging sites. Factors including value, patrolling time, and the number of competing males are taken into account when males roam foraging sites for females.

In male-female interactions, males sense potential mates by observing the body shape of females, and by evaluating the female's sense, determine whether a specific female will be receptive to copulation. Females use such cues as the vibrational bursts of the male thorax, which has been suggested to be a sign of male health and overall fitness, color, and odor to select mates. Successful mating of females does not depend on male body size, but on the speed with which males discover female mates. Further, females do not always choose the male with the largest body size, a choice that possibly indicates a preference exists for an optimum male body size; often, females choose males with intermediate body sizes. Yet, the sperm supply of each male limits males to only performing seven copulations in their lifetimes.

Mating technique

''O. bicornis'' females attract males through sex pheromones, which are localized on their cuticle surfaces. Extracts of the cuticle elicit copulatory behavior in males of ''O. bicornis''.

During courtship, the male ''O. bicornis'' stands on the back of the female to try to persuade her to mate. Several indicators of persuasion by the male include vibrating his thorax, rubbing himself against the female, rubbing his antennae over hers, and rubbing his legs over her compound eyes. The female, however, can choose to reject the male and may push him off her back. The three phases to mating in ''O. bicornis'' are precopulatory courtship, copulation, and postcopulatory embrace. In the spring, when a female first emerges, males in close proximity approach her. When a male establishes his position on the female's dorsum, other males retreat. During what is called the precopulatory phase, the male rubs the female's mesothorax with his first two pairs of legs. The male then strokes her antennae with his own to persuade her to copulate. Simultaneously, he rubs her eyes with his front legs. Every stroking motion is recognizable to humans as a high-pitched humming sound, which soon turns into buzzing as the male attempts to copulate. In an attempt to copulate, the male moves backward (on the female) and tries to insert his genitalia into the female's genital chamber, during which he drums on the female's face to produce a tremolo. If the female chooses the male, copulation begins. If she rejects the male, she can bend her abdomen downward to try to shake him off. The male either stops or repeats his attempts at copulation. If the male successfully attains the female, copulation occurs for several minutes. This is followed by a postcopulatory phase which lasts up to 13 minutes. At this time, the male applies an antiaphrodisiac on the female by stroking his abdomen over her in the posterior to anterior direction.

Females of ''O. bicornis'' have a mating plug in their genital chambers after mating. While the mating plug is thought to prevent females from mating with other males, its function is not clear in ''O. bicornis'' as of yet.

Sex allocation

Female body size is indicative of the sex allocation of offspring. Larger females are able to collect more pollen than smaller females, making larger females less prone to open-cell parasitism while away from the nest. To "make the best of a bad job", or counteract the disadvantage they have, smaller females deliberately produce more male offspring and reduce female offspring body size. These changes occur because the smaller females are obtaining less pollen; investing in offspring that require fewer food provisions – males – therefore allows smaller females to combat their handicap. Larger females, in contrast, had more female offspring. In addition to increased foraging efficiency, females hold other advantages over small females, including increased egg production and longevity. Because it does not benefit males to be larger in size, due to the independence of body size on female mating selection, females normally invest more in female offspring.

Female age also predicts sex allocation in offspring. Older females are less efficient at foraging for pollen in nest construction than younger females. Thus, they produce more male offspring and reduce the size of offspring.

Diapause

Diapause allows ''O. bicornis'' to survive harsh winter conditions. Typically in adult insects, reproductive diapause is characterized by a late development of gonads and a buildup of energy reserves. However, diapause in ''O. bicornis'' is somewhat different. The ovaries of females are not completely inactive during overwintering, as the development of oocytes continues in the vitellarium

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