Schwarziana quadripunctata is a small, stingless bee found in a stretch of the South American Amazon from Goiás, Brazil, through Paraguay, to Misiones, Argentina.Michener, C.D. (2007). The Bees of the World. Baltimore: Johns Hopkins University Press. This highly eusocial insect constructs earthen nests in the subterranean level of the subtropical environment, an unusual feature among other stingless bees. The species ranges in sizes from and feeds on a diverse diet of flowering plants found abundantly on the forest floor, including guacatonga (Casearia sylvestris) and the mistletoe species Struthanthus concinnus.
Taxonomy and phylogeny
Schwarziana quadripunctata was first described by the French entomologist and former president of the French Entomologist Society Amédéé Louis Michel le Peletier in 1836. Although originally placed in the genus Trigona, more recent taxonomic evaluations have since placed it under its current genus, following the 1943 work of Padre J.S. Moure, occasionally referred to as "the Father of Brazilian Bee Taxonomy". However, some still consider the term Schwarziana as a subgenus rather than genus and instead incorrectly classify it under the closely related genus Plebeia.
The closest relative of S. quadripunctata is the species Schwarziana mourei, determined through morphological and genetic evidence collected in the early twenty-first century. Until recently (2015) it was believed that these were the only two existing species under the genus Schwarziana in the world. However, two more species were discovered in the high altitude environment of South America – S. bocainensis in southeastern Brazil and S. chapadensis in central Brazil.
Description
Bees The stingless bee S. quadripunctata varies in size from . Worker bees and dwarf queens tend to be on the lower end of this spectrum while queens tend to lie on the higher end. Dwarf queens and workers tend to have an average weight of about 30 mg, but have been known to weigh as much as 40 mg or as little as 22 mg. Mated queens (those in charge of the nest) are, in comparison, much larger. The average queen bee weighs in at about 130 mg. However, some have been measured at over 160 mg, about a quarter of the weight of an average paperclip. Colonies contain a larger queen with greater fecundity than dwarf queens, causing the size discrepancy between the two. Unusual for most other eusocial insects, worker bees and dwarf queen bees tend to be similar in weight and size. The bees have a pale coloration of brown or reddish-brown with occasional yellow markings on the head. They maintain a punctate thorax and abdomen and a dorsal thoracic area sporting a few hairs. Glands are present on the head and thorax. The glands are larger in the heads of general worker bees and larger in the thorax for nursing worker bees.
Nests Nests are entirely underground on the subtropical floor and consist of vast and expansive cavities. Each nest is occupied by a single colony consisting of an individual queen and several thousand workers. Brood cells are arranged in spiral combs, each cell housing only one individual. Cells housing queens are typically much larger than the surrounding cells housing males and workers.
Distribution and habitat
The distribution of S. quadripunctata ranges from the central highlands of Brazil down towards the northern edge of Argentina on the eastern side of South America. Altitudes can reach up to fifteen hundred meters () in some areas. Lying so near the equator, temperatures are fairly warm year-round, averaging twenty-five degrees Celsius ().
Nests are built into the fertile topsoil of the subtropical rainforests. The niche breadth of S. quadripunctata includes a wide variety of flowering plants that overlaps with many other eusocial bees native to the area, including members of the tribes Meliponini and Trigoni, as well as Africanized honeybees. Each nest features a single, simple rounded entrance, as opposed to a triangular entrance or one with a tube network, several layers, or multiple openings. S. quadripunctata nest entrances in São Paulo, Brazil consisted of an average entrance area of , relatively small compared to the body of the bee. This may account for the lower traffic levels of the stingless bee through the entryway, an average of 17.4 bees per minute per nest.
Members of this species are fairly common in their large South American region, with nests containing over several thousand members each. However, their population is believed to have declined by nearly sixty-five percent (65%) over the past few decades due to competition with the introduced Africanized honeybees along with human deforestation. This decline is projected to continue into the future.
Colony cycle
Colonies are founded by the swarming of individuals consisting of a single queen followed by several thousand worker bees. This can only be achieved if both the queen and her workers “align” themselves towards the same goal in production of fertile females. Once a colony has been established, housing combs are produced by the workers, with larger ones allocated to potential queen larvae and the more numerous, normal-sized cells produced for the worker bee eggs. Dwarf queens are also reared in worker cells. Queen cells are typically located near the edge or periphery of the comb while worker and dwarf queen cells are randomly aggregated towards the center. Although less than one percent (1%) of dwarf queens were seen to emerge from worker cells, they account for nearly eighty-six percent (86%) of the total queen population.
A colony will continue to grow in size until the queen bee dies or swarms to a new location to begin a new colony. In rare cases, two queens may swarm to the same location, due to balanced attractiveness, to begin a new colony. In these special cases, the younger queen typically produces more eggs than the older queen and begins to take over the colony, phasing out the older member.Engels, Wolf (ed.) (1990). Social Insects: An Evolutionary Approach to Castes and Reproduction. New York: Springer-Verlag.
Behavior
Dominance hierarchy
In the stingless bee S. quadripunctata, as in many social insects, queens reign at the top of the dominance hierarchy, followed by dwarf queens and finally workers. However, dwarf queens have been observed to lead nearly one in five colonies. Although, this accounts for only seventeen percent (17%) of dwarf queens and is significantly lower than the rate at which they were reared (constituting 86% of all females). This does show, however, that becoming a dwarf queen can be beneficial.
Dwarf queens have a lower fecundity and reproduction rate than normal queens. This appears to be the consequence of a reduced egg-laying rate and a lower average ovary weight. A queen’s ovary is nearly four times larger than a dwarf queen’s.
In some aspects, however, workers occasionally have a slight advantage over queens and dwarf queens. Worker bees are in charge of delivering food supplies to developing larvae. Thus, workers can force underdeveloped dwarf queen larvae and queen larvae to be converted into a worker bee, or to be eradicated completely, through extreme limitations of their food rations.
Division of labor
The division of labor often, in some way or another, reflects the evolutionary sex ratio of a species. This is especially true in S. quadripunctata, where worker bees can outnumber queens by about one thousand to one (1000:1).
The main purpose of the colonial queen, or gyne, is to lay eggs within the brood cells housed in the combs. They also play a vital role in the establishment of new colonies. Using their attractive senses, queen bees are able to lead a swarm of workers and the occasional dwarf queen to new locations. Until fairly recently, it was hypothesized that these dwarf, or miniature, queens were the result of developmental mistakes. However, through Tom Wenseleers' "Caste Conflict Hypothesis," it is believed that dwarf queens are evolutionarily advantageous in that they develop in order to avoid becoming a worker bee. Their role was found to become the head of a colony in the absence of viable, full-sized queens.
The roles of the worker bee in S. quadripunctata include the maintenance and protection of larvae (queen, dwarf, or worker) placed within the brood cells. They are responsible for delivering nutrients to the developing larvae until they hatch. Worker bees are also responsible for accompanying the establishments of new colonies by a queen.
Reproduction and reproductive suppression
Unlike typical honeybees, which practice multiple mating, stingless bees have been suspected of single mating between drones and queen bees. This, along with the consequential pairing of the high relatedness rates within colonies of S. quadripunctata, serves to explain the high degree of kin selection among stingless bees.
After a female queen mates and lays her brood, the worker bees package them into specifically designed cells – queens in larger “royal” cells and workers and dwarf queens in smaller ones. Each cell is then capped with a layer of wax-like substance. With the queen’s role in production complete, the combs are left to the tending of worker bees, which distribute nutrients and vital resources to the growing larvae. With so much control, worker bees occasionally discriminate or display reproductive suppression tendencies against dwarf queens by either uncapping the cell before the dwarf hatches, halting the supply of nutrients, or killing the newly developed individual upon emergence of the cell.
Communication
A major strength of any eusocial insect is the ability to communicate amongst one another. Often, this method of communication comes in the form of chemical signals between members of the same colony. In particular, cuticular hydrocarbon levels are thought to be of great importance to S. quadripunctata communication and can vary greatly according to age, sex and caste.
Hydrocarbons in the stingless bee can vary in length from nonadecane with nineteen carbons to tritriacontane with thirty-three carbons, as discovered in a study conducted by Nunes et al. (2009). It was found that cuticles of older worker bees contained higher concentrations of heptacosene (C27) and hentriacontene (C31) than younger workers and virgin queens, indicating that these hydrocarbons play an important role in nest mate recognition. It was also found that younger stingless bees lacked some of the major hydrocarbons found in older individuals, most likely due a result of less exposure to the wax present in the nest. The relative concentrations of hydrocarbons in members of a colony differed between unrelated sites, alluding to the hypothesis that the wax of each nest contains a unique chemical blend from which the developing larvae absorb.
Navigation
Along with, and often closely related to, communication, effective navigation within an environment is vital to the successful reproduction of eusocial insects. Although some organisms’ methods of navigation seem quite apparent (i.e. through modes of sight and scent) the navigational system of Schwarziana quadripunctata had remained elusive until quite recently. It has been determined that S. quadripunctata navigates, like many other bees, through the use of magnetoreception – utilizing magnetic fields in the atmosphere to recognize differences in location, altitude and directionality.