Many insect hearts have a property unheard of in most other animals: they can beat backwards as well as forwards. But in the tiny fruit fly Drosophila melanogaster, not enough was known about heartbeat reversals in adults or the structure of the heart, which would give more clues as to why reversals might happen. According to Lutz Wasserthal from the University of Erlangen Nuernberg, Germany, heartbeat reversals could help with gas exchange. As non-oxygen carrying insect blood, called haemolymph, is shifted between the front and the back of the body, it could act as a hydraulic fluid, expanding the fly's `lungs', the tracheal air sacs. To shed some light on backwards beating in adult flies, Wasserthal measured the heartbeat in adults, and scrutinised the heart's anatomy(p. 3707).
The fly's heart is a 1 mm long muscular tube that runs along the dorsal side of the abdomen, and contains a number of intake valves. At the anterior end of the abdomen, nearest the fly's waist, the heart narrows and becomes the aorta, which travels through the fly's thorax and opens up in the head. Haemolymph is pumped out of this opening into the body cavity, where it travels backwards through the fly's body and is taken up into the heart again via the intake valves.
To record the heartbeat, Wasserthal delicately attached flies to his apparatus by their wings, ensuring that they were completely undamaged. He projected an infrared light through the abdomen of the flies that was picked up by a modified sensor chip with 5 mini-sensors similar to those used in bar code readers. As the heart relaxes, it fills with haemolymph, meaning that more light gets through to the sensor. The changes in light levels recorded by the chip's sensors corresponded to the contracting and relaxing of the heart,and the relative timing of the waves of contraction told Wasserthal whether the heart was beating forwards or backwards.
He found that when beating forwards, the heart beats at slower rate, around 4 Hz, and for longer periods of time, around 14 s. When the direction reverses the heart beats faster, around 5 Hz, and for a shorter time, around 5 s. The heartbeat switched from forwards to backwards beating and then back again once every 20 to 25 s.
What was currently known about the anatomy of the fly's heart, though,couldn't explain where the haemolymph would flow during reversed heartbeat. By carefully preparing his samples for scanning electron microscopy and analysing the tissues, Wasserthal found another pair of input valves at the end of the heart near the waist, which would allow haemolymph to get into the heart from the thorax nearby. These input valves are fed by a pair of newly discovered channels that allow haemolymph to flow from the thoracic cavity to the heart. He also found an opening at the posterior end of the heart, allowing haemolymph to flow out in the opposite direction. These results made it clearer how and why the heart beats both ways: as heartbeat reversals shift haemolymph out of the front half or the back half of the fly's body, the tracheal sacs compensate for the change in haemolymph volume, emptying or filling with air.