Sunday, July 08, 2018

Courtship of Poecilobothrus nobilitatus (Dolichopodidae)

The Dolichopodid fly Poecilobothrus nobilitatus is a common species which can be abundant in the right conditions. It likes the shallow, muddy edges of pools and streams and even muddy puddles in dappled shade. Dolichopodids (long legged flies) have extravagant external male genitalia, they are typically colourful with metallic green and blue colours and iridescent eyes and the legs of males are often ornamented. In this particular species, the males, which are about 5-6 mm long, have the wings blackened over the outer half and milky white tips.

Poecilobothrus nobilitatus male
 Females are usually a bit duller and lack ornamentation. In this species, the wings are not coloured.

Poecilobothrus nobilitatus female
During courtship, Dolichopodid males semaphore with there ornamented legs, or in this case, their coloured wings. In this species, courtship takes place on the surface of wet mud or water. At the start of a courtship bout, the male approaches a female and and waves its wings in short bursts when a few centimeters away, it then jumps over the female and repeats from the other side. If the female shows any interest, the second phase involves the male hovering in a short, wavering flight, in front of and to either side of the female.

There is a small pond in my garden in the shade of a Scot's Pine tree. This obviously provides the right conditions for P. nobilitatus and large numbers are present at the moment - and courting furiously.

Poecilobothrus on the pond - these are mostly females
 I wanted to try and photograph this courtship behaviour, but I knew it would be difficult. The flies are small and, to get a good image, would need a reproduction ratio close to 1:1, which means getting quite close. At these short distances, the rapid movements involved would make it very difficult to capture. In these circumstances, electronic flash provides sufficient illumination to achieve the small apertures necessary for the depth of field and also the short duration of the flash has a chance of freezing the movements.

Photos were taken using a 100mm macro lens at around f16. The main illumination was from Canon MT-24 dual flash with a Yongnuo 560 III providing background illumination. A Yongnuo RF605C radio unit was used to trigger the background flash. The Canon 80D camera was set in manual mode and the flash power set as low as possible - around 1/8 or 1/16 power. This reduces the flash duration allowing for high speed imaging. The maximum flash sync speed for this camera is 1/250th sec, so the shutter speed was set to 1/250 or 1/200th. At 100 ISO, f16 and 1/250 sec, the available light level would lead to underexposure of about 4-5 stops, so the exposure is totally due to the flash, therefore the camera's shutter speed is irrelevant (providing the shutter is fully open when the flash fires).

Male (right) approaching a female (left) wing waving

Male wing waving

Male leaping over a female