Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK.
J R Soc Interface. 2012 Jun 7;9(71):1194-207. doi: 10.1098/rsif.2011.0617. Epub 2011 Nov 9.
The alula is a hinged flap found at the base of the wings of most brachyceran Diptera. The alula accounts for up to 10 per cent of the total wing area in hoverflies (Syrphidae), and its hinged arrangement allows the wings to be swept back over the thorax and abdomen at rest. The alula is actuated via the third axillary sclerite, which is a component of the wing hinge that is involved in wing retraction and control. The third axillary sclerite has also been implicated in the gear change mechanism of flies. This mechanism allows rapid switching between different modes of wing kinematics, by imposing or removing contact with a mechanical stop limiting movement of the wing during the lower half of the downstroke. The alula operates in two distinct states during flight-flipped or flat-and we hypothesize that its state indicates switching between different flight modes. We used high-speed digital video of free-flying hoverflies (Eristalis tenax and Eristalis pertinax) to investigate whether flipping of the alula was associated with changes in wing and body kinematics. We found that alula state was associated with different distributions of multiple wing kinematic parameters, including stroke amplitude, stroke deviation angle, downstroke angle of incidence and timing of supination. Changes in all of these parameters have previously been linked to gear change in flies. Symmetric flipping of the alulae was associated with changes in the symmetric linear acceleration of the body, while asymmetric flipping of the alulae was associated with asymmetric angular acceleration of the body. We conclude that the wings produce less aerodynamic force when the alula is flipped, largely as a result of the accompanying changes in wing kinematics. The alula changes state at mid-downstroke, which is the point at which the gear change mechanism is known to come into effect. This transition is accompanied by changes in the other wing kinematic parameters. We therefore find that the state of the alula is linked to the same parameters as are affected by the gear change mechanism. We conclude that the state of the alula does indeed indicate the operation of different flight modes in Eristalis, and infer that a likely mechanism for these changes in flight mode is the gear change mechanism.
小翅是大多数短角亚目双翅目昆虫翅膀基部的铰接瓣片。小翅在食蚜蝇科（Syrphidae）中占翅膀总面积的 10%，其铰接排列方式允许翅膀在休息时向后扫过胸部和腹部。小翅通过第三腋片来驱动，第三腋片是参与翅膀缩回和控制的翅膀铰链的一部分。第三腋片也与苍蝇的齿轮变速机制有关。该机制通过施加或去除与限制翅膀在下半冲程运动的机械止动件的接触，允许在不同的翅膀运动模式之间快速切换。小翅在飞行中处于两种明显的状态——翻转或扁平——我们假设它的状态表明在不同的飞行模式之间切换。我们使用自由飞行的食蚜蝇（Eristalis tenax 和 Eristalis pertinax）的高速数字视频来研究小翅的翻转是否与翅膀和身体运动学的变化有关。我们发现小翅的状态与多个翅膀运动学参数的不同分布有关，包括冲程幅度、冲程偏转角、下冲程迎角和旋前时间。以前，所有这些参数的变化都与苍蝇的齿轮变换有关。小翅对称翻转与身体对称线性加速度的变化有关，而小翅不对称翻转与身体角加速度的变化有关。我们得出的结论是，当小翅翻转时，翅膀产生的空气动力会减小，这主要是由于翅膀运动学的伴随变化。小翅在中下降冲程时改变状态，这是齿轮变换机制已知开始生效的点。这种转变伴随着其他翅膀运动学参数的变化。因此，我们发现小翅的状态与受齿轮变换机制影响的参数有关。我们得出的结论是，小翅的状态确实表明在 Eristalis 中存在不同的飞行模式，并且推断这些飞行模式变化的可能机制是齿轮变换机制。
