Author: ISS

Aerodynamics of Bee Flight Insects such as fruit flies and dragon flies usually fly by creating rolling vortices along the edge of their wings that first contacts the air to produce lift. They beat their wings in large strokes, typically flapping of arcs of 145° and 165° at a frequency which is determined by their body size. This generates enough forces sufficient for flight. Bumblebees have very small wings in comparison to their size and must compensate for this in other ways. Bumblebees show an unusual pattern of wing beats: Their wing sweeps back in a 90° arc, and then flips over as it returns – an amazing 230 times a second. The forces that are created as the wing changes direction are different to those which are produced during normal insect flight and are thought to dominate during flight. Additional vortices are created by the rotation of the wing – like a propeller. The wing also flaps back on its own wake (the region of recirculating air flow behind its body) leading to increased forces than if it were to flap in still air.
Altshuler D, Dickson W, Vance J, Roberts S, Dickinson M (2005) Short-amplitude high-frequency wing stokes determine the aerodynamics of honeybee flight. Proceedings of the National Academy of Sciences of the United States of America, 102:18213-18218.

Wing and muscle structure and function
Bees have two pairs of membranous wings with large cells and veins. The veins increase wing mechanical rigidity. Coupling of the costal edge of the hind wings to the forewings by small hooks called hamuli allow them to move in unison. Three hinges (horizontal, vertical and torsional) located at vein intersections allow the wing to move in different rotations during flight. This allows greater manoeuvrability during flight. There are two main groups of muscles involved in wing activation; the dorsoventral muscles, which contract to raise the wings and the dorsolongitudinal muscles, which contract to lower the wings. This form of flight is called indirect flight because the muscles affect the wings indirectly through deformation of the thorax. To sustain flight, the wings need to oscillate at an extremely high frequency (>100Hz). The adaptation of asynchronous flight muscles allows for this because the thoracic muscles contract many times from a single nerve impulse. This allows them to avoid the drawbacks of typical synchronous flight systems, in which there is one contraction per neural activation, such as low power output and efficiency.
Heinrich B (2004) Bee Economics. Harvard University Press, Cambridge Heinrich B, Esch H (1994) Thermoregulation in bees. American Scientist 82:164-170 Meyer JR (2007) External anatomy: wings. http://www.cals.ncsu.edu/course/ent425/tutorial/wings.html. Accessed 5 December 2011 Knospe CR (1998) Insect flight mechanisms: anatomy and kinematics. http://people.virginia.edu/~crk4y/research/flight.PDF. Accessed 5 December 2011 Syme DA, Josephson RK (2002) How to build fast muscles: synchronous and asynchronous designs. Integrative and Comparative Biology 42:762-770

Figure 1. Bee wing structure

It is a common misconception using current aerodynamic models that bumblebees (Bombus sp.) should be unable to fly. This is not the case, it is a highly evolved and complex biomechanical system The wing shape and movement contribute to a unique way of flying which takes advantage of vortexes and produces lift, or at least drag, on both the upward and downward stroke. This poster highlights the most important aspects that are involved in bumblebee flight in the current scientific literature.

Three hinges allow greater dexterity of the wings

Flight costs are approximately 375cal g-1 h-1

Pile - soft setae (hair) covering the body for insulation

Wings that beat 240 times a second

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All locomotion originates from the insect’s thorax

Forces and Power Bumblebees can produce enough vertical force and thrust to support their body weight and overcome body drag .The bumble bee has a ratio of 30% muscle to body weight with a power output of approximately 56W kg-1 body mass. The overall power output depends on the overall efficiency of the flight muscle. Force and power values are difficult to obtain for the bumblebee flight however computational fluid dynamics have been used to investigate bumblebees power and force output. And have shown that at hovering and low flight speed , vertical force is mainly produced by both half strokes and contributed by wing lift and at medium and high speeds vertical force is mainly produced during downstroke and is contributed too by both wing lift and wing drag . Power output is greatly influenced by muscle length and body size: power output increases with bee mass, however flight performance of bumble bees is compromised at greater body masses. Buchwald R, Dudley R (2010) Limits to vertical force and power production in bumblebees (Hymenoptra : Bombus impatiens). The Journal of Experimental Biology 213: 426-432 Dudley R, Ellingtion C (1990) Mechanics of forward flight in bumblebees II. Quasi-steady left and power requirements. The Journal of Experimental Biology 148 : 53-88 Jianghao W, Sun M (2005) Unsteady aerodynamic forces and power requirements of a bumblebee in forward flight. Acta Mech Sinica 21 : 207-217

Thermoregulation To fly, bumblebees must thermoregulate to within a small range. Temperatures vary between different bumblebees but a general range of temperatures where flying can occur is between 30-45°C. Below 30°C bumblebees cannot fly. Ambient temperature is usually below this so to reach this temperature they must increase their metabolism. The main method taken to do this is to disconnect the wings and rapidly contract and relax the muscles that control the wings. The thick hairs that cover the insects help insulate the animal and shifting heat between the thorax and the abdomen (which has a higher SA:VOL) helps control the temperature. How long the bee takes to warm itself to above 30°C will depend, among other factors, on the ambient temperature. Apart from shifting temperature between body parts they can decrease their temperature by evaporative methods: tongue-lashing, regurgitating water onto the head, and convective methods: altering air speed over itself (flying faster, changing the motion of their wings). Heinrich B (1975) Thermoregulation in Bumblebees II. Energetics of Warm-up and Free Flight. Journal of comparative physiology 96: 155-166
Roberts SP and Harrison JF (1998) Mechanisms of Thermoregulation in Flying Bees. American Zoologist. 38:492-502

Time Energy Budgets
A bumblebee has two main types of energetic costs: Thermoregulation and physical activities. A foraging bumblebee spends its time; flying between the nest and foraging area, flying or walking between flowers and extracting nectar or pollen from flowers1. Bumblebees fly between flowers horizontally, and within clusters of flowers vertically, however energetic costs are assumed to be equal for the two different modes of flight. All flights will result in an energetic cost to a bumblebee of 375 cal g-1 h-1. The metabolic rate of a resting bumblebee is related to the thoracic temperature. Increasing the thoracic temperature of the bumblebee is the main component of energy expenditure in grounded bumblebees. The energetic cost of non-flight activity is shown by: c(Tth) cal/(g body wt x hr). (Tth = Thoracic temperature, g body weight is assumed to be 2.95 x thoracic weight in grams, hr = hours spent doing non-flight activity. This equation is derived from oxygen intake, which is assumed to be 21.1J of energy produced per 1ml of Oxygen consumed. Because activities of a bumblebee can be split between non flight activities, and flying activities, deriving total energy expenditure – energy expenditure from non-flying activities can allow energy expenditure of flight to be derived. Bumblebees cannot fly without increasing their thoracic temperature. The lower the ambient temperature (TA) the greater the metabolic cost. However, energy expenditure of this action is negligible compared to other activities.
1. Pyke G H (1980) Optimal foraging in bumblebees: Calculation of net rate of energy intake and optimal patch choice. Theoretical Population Biology 17: 232-246