Amegilla dawsoni

This dark-winged bee species is among Australia's largest bees, similar in size and coloring to Xylocopa, or carpenter bee, species.[2] The bee can get up to 23 mm in body length and 45 mm in wing span. Both sexes are densely furry, with the exception of their lower facial regions, which jut outwards and tend to be bare, and colored anywhere from light yellow to dark brown.

The females of this bee species tend to be consistently sized, with head widths around 6.4–7.4 mm.[3] The males, however, are dimorphic. They come in two sizes predominantly, the larger majors and the smaller minors.[4] The majors and minors differ greatly in size, with head widths ranging from 4.9 to 7.3 mm.[3] Minor males have been defined as those with head sizes less than 6.3 mm, while major males have head sizes above 6.3 mm.[5] Males are also allometrically sized, meaning that those with larger heads also have larger mandibles and broader abdomens. The largest of the males are comparable in size to the females.[3] Sexual dimorphism can also be seen in the bee coloring. The males of this species are covered in brown hair. Meanwhile, female A. dawsoni heads and thoraxes are covered in white hair.[6]

This bee species practices solitary nesting, though often the nests are clustered close together.[8] An active nesting colony may contain up to 10,000 burrows.[3] The female bee builds her nest by digging straight down into clay, or other densely packed soil and dirt. She will dig to depths between 15 and 35 centimeters.[8] The female bee will then turn to dig horizontally. In the horizontal shaft, she will dig downwards to create brood cells. The horizontal shaft is extended with each subsequent brood cell that she creates. Occasionally, females will layer two brood cells on top of one another in a doublet formation.[9] The female bee will prepare the inside of each cell by laying down a layer of wax. She fills the layered cell with nectar and pollen from four different plant genera. With this wet mixture in place, she will lay the egg on top of the cell, and then cap the cell with mud. She repeats this until she is done laying her eggs.[8]

The flight season for this bee ranges from the later months of winter to early spring. The bee is univoltine – it produces only one brood per breeding season.[5] After remaining dormant until the following winter, the brood which had been laid the previous year emerges and begins the flight season and mating process.[8] The males of the species will emerge from the nests before their female counterparts. Additionally, the minor males, with head widths less than 6.3 mm, tended to emerge before the major males, with head widths greater than 6.3 mm, over the course of the entire breeding season. Even within the span of single days in the emergence period, minor males tended to surface earlier in the day than major males.[5] This may be due to an evolutionary pressure which forces smaller males to emerge earlier than their larger counterparts to avoid competition for mating with larger individuals. Male emergence is also prolonged, and can last several weeks into the flight season. This results in a large amount of emergence overlap between males and females of the species. This might be due to the increased fitness of late-emerging males to mate with females later in the season, given that the early-emerging competition would already have died.[5]

During the beginnings of their nesting cycles, female A. dawsoni often enter occupied nests with absent hosts. In the event of a clash between intruder and resident, the conflict almost always ends with the resident bee evicting the intruder in a short brawl, lasting only a few seconds. There does not appear to be any advantage of size for bees that are attempting an intrusion, in that larger bees do not seem to be more successful in removing the resident bee from her burrow. One explanation for why this species engages in this behavior, despite its low level of success, is that intruding bees are attempting to find nests where the owners have died or disappeared. This is a cost-saving mechanism by which the intruding bee can save energy which she would otherwise spend on burrowing and brood cell creation. When an owner does happen to return, the intruder allows herself to be removed in a short and non-lethal confrontation so she might continue her search for an abandoned nest.[10]

Two distinct mating patterns arise in this bee species, due to the two size classes of males. The larger males (majors) will patrol the emergence areas of the female bees. Meanwhile, the smaller class of male bee, the minors, patrol the outer perimeter of the emergence area, and wait to mate with females who escape the vicinity of their emergence holes without having mated with a major male. 80% of the male population in A. dawsoni is made up of minors, despite the fact that 90% of females mate with the major bees.[11]

When a male locates a receptive female bee, he will mount her back, and ride her over to the nearest available vegetation, after which he will begin copulation, which occurs in 3 phases. In the first phase, the male will mount the female and will flicker his wings. In the second phase, upon insertion of the male genitalia, the male will rapidly thrust for a few seconds. After this, he will enter the third phase, where he will disengage his genitalia and will probe the external female genitalia. After this, copulation is complete. Female bees will rarely mate more than once – this causes fierce competition between males for mating opportunities.[11]

Female bees of this species indicate sexual receptiveness with a mixture of semiochemicals, signalling chemicals. Cuticular hydrocarbons, long chained fatty acids implicated in the prevention of insect desiccation, have also been implicated in the process of sexual signaling. Emergent virgin A. dawsoni females release a particular mix of CHCs to indicate their receptivity to patrolling males. This blend includes significant amounts of tricosane, pentacosane and heptacosane.[12]

This receptivity blend improves fitness for females, who can signal receptivity to mating, and induce competition, such that the strongest competitors with the best phenotypes and genotypes, will father their broods. Male ability to recognize these signals also allows for males to reduce energy wasted by approaching emerging male bees, or to engage in competition with other males over already mated females.[12]

Upon mating, the female A. dawsoni will begin a chemical shift to indicate that she is no longer receptive to mating attempts. Then she will begin the nesting phase of her life cycle. This shift takes some time. Therefore, recently mated females are still attractive to patrolling bees, and must actively resist copulation. The shift involves actively repressing the production of the receptivity chemicals, and creating a chemical called tetramethylhexadecatrienol, which is unique to nesting females. Additionally, other volatile chemical agents potentially produced in the Dufour's gland have been implicated in male repulsion signaling.[12]

Amegilla dawsoni has been shown to be a panmictic species, where individuals are equally likely to mate with any other individual in the population. This results in completely unmitigated genetic flow, and populations with no genetic differentiation. This is a relatively rare phenomenon, especially in bees, where intraspecies genetic differentiation is common. One explanation that has been offered for this phenomenon in A. dawsoni is the irregular distribution of flowering resources due to the arid conditions of the Australian desert. These conditions could force populations in their flight seasons to aggregate in areas with abundant resources, allowing for populations that are several hundreds of kilometers apart to mix via mating.[13]

Amegilla dawsoni is parasitized by the cleptoparasitic Miltogramma rectangularis, or Miltogrammine fly. These flies enter the brood cells of the bee, and feed off of the provisions meant for the developing brood. They enter the brood cells by means of adult female Miltogrammine flies larvipositing on adult female bees. The adult female bees then enter the cells and deposit the parasitic larvae in the vicinity of their own brood. It has been hypothesized that these parasites, and their proclivity for food theft, have acted as a selective pressure for A. dawsoni to produce smaller males (minors), which require less food, thus explaining the preponderance of minor males despite their reduced sexual fitness. This is unlikely, however, given that the reproductive benefit of producing large males seems to outweigh the losses created by brood parasitization.[15]