Deer fly eggs

A half-dozen deer flies, Chrysops figidus(?), were laying eggs just above the waterline on a long-decommissioned piling. In their preparation for laying eggs, it is these female deer flies that inflict painful bites as they seek blood that is ultimately used to feed their larvae. (The non-biting males don’t bother with blood, but stick to nectar.) 

A deer fly on the same region of an old piling in 2010

A sighting of egg laying seems to require looking at just the right time on just the right day. By the following morning, the flies are gone and the egg casings have all darkened.

Yet, beyond a fortuitous observation, there was something rather odd: eight years ago, deer flies laid eggs on the same small portion of the same piling. Two observations do not establish a pattern — yet, might something explain it? In each case the eggs were laid on the shady side of the piling about 30 cm above the waterline.

Water is the destination of the larvae that emerge from the darkened eggs. Once there, they eat aquatic insects. Presumably, this is why this area of the piling gets chosen: it is the moister shady side just above the water which will be the larvae’s new home.

A deer fly positions its eggs so larvae can drop into the water.