A drone’s life.
Most people could tell you that a drone is either a small unmanned military aircraft or a male honey bee, but few realise that the latter are in fact quite fascinating creatures. Drones are larger than worker bees (although smaller than queens) and more robust, with much bigger eyes. They do not possess a sting and take little part in the day-to-day life of the hive; their main function being to mate with a receptive queen.
Unlike workers and new queens, which are produced from fertilised, diploid eggs laid by the queen, drones hatch from unfertilised, haploid eggs containing only one set of chromosomes (as opposed to the normal two). These eggs are produced by workers via a process called arrhenotokous parthenogenesis. Sex is determined by an allele which is polymorphic, with approximately 18 known forms, and females result from the presence of different alleles, so haploid bees are obligatory males. Often queens will also produce some diploid males (i.e. with two identical sex alleles) but these are identified and destroyed by nurse bees. A ‘spotty brood pattern’ is produced by empty cells due to the removal of diploid drones from the brood comb. This form of reproduction, which is also utilised by other social insects, is called haplodiploid sex determination. All of the sperm produced by a drone is identical, and results in workers that are more closely related to each other than full sisters of other sexually-reproducing organisms.
Haploid drones therefore have only one parent, a mother, whereas there are both male and female ‘grandparents’. Three generations removed there are two female and one male ancestors, and so on. This results in a numerical progression also known as the Fibonacci sequence: 1, 1, 2, 3, 5, 8, 13, 21, etc. Each number can be calculated as the sum of the two preceding numbers. The Fibonacci sequence was first described in Indian Sanskrit texts and is named after early an Italian mathematician who introduced the concept to western scholars. Fibonacci numbers are repeatedly of importance in mathematics, and in the natural world can be observed in the arrangement of tree branches, leaves on stems, and in the geometry of ferns, pineapples and artichokes.
Although their primary focus is reproduction, drones may nevertheless perform a useful role within the hive. Along with workers, they contribute to the maintenance of temperature by either shivering (to generate heat) or rapid movement of their wings to displace hot air. Drones may also surround and attempt to disorientate intruders.
Where cold winters are prevalent, drones may be driven out of the hive in autumn, and will not reappear until the following spring. Drone numbers peak around the swarm season in late spring and early summer, at which time each colony may contain about 200 drones. They are particularly active in the early afternoon, when they assemble at drone ‘congregation areas’ to await the arrival of virgin queens. Congregation areas are located at some distance from the hive, and are typically 30-200 metres in diameter and 10-40 metres above ground. They are usually over open country and afford some protection from the wind.
Congregation areas are reused year after year, which suggests that they are delineated by environmental factors. There is also some evidence that drones detect magnetic orientation. Up to 25,000 drones, from 200 different colonies, have been observed at a single congregation site; ensuring the presence of considerable genetic diversity. Each drone will visit a number of sites, commonly making several flights every day and returning to the hive in between to feed.
Sight and smell are used to detect the arrival of a virgin queen (hence the drones’ large eyes), which is then actively pursued in an attempt to mate. Drones far outnumber queens, so although queens mate with up to 20 drones on their ‘nuptial flights’, only a small percentage of drones (estimated at less than 0.1%) are ever successfully in mating.
Mating occurs in mid-air and lasts less than five seconds. The drone straddles the queen and then everts his penis (or endophallus) with great force into her open sting chamber. The violent ejaculation causes the endophallus and other abdominal tissues to be ripped from the drone’s body, resulting in immediate paralysis and, shortly thereafter, death. Drones that don’t manage to mate have a life expectancy of about 90 days. Semen is stored in the queen’s spermatheca (part of her reproductive tract) to be used in the establishment of the new colony, which will have a number of ‘fathers’. Apiarists may choose to artificially control genetics by limiting mating to drones from a single, known source.
Also of interest is that the potentially devastating parasitic mite, Varroa destructor, prefers to inhabit the brood cells of drone larvae, as they are relatively large and have a longer period of development.