The case of the missing unborn flies may have a solution: prenatal cannibalism.
Females of the small fly species Emblemasoma auditrix deposit their larvae on cicadas, insects on which the youngsters feed for about 5 days. Those larvae hatch from eggs while still inside their E. auditrix mother. While Mom carries a brood of some 38 wriggling larvae equipped with sharp mouthparts, she deposits only one larva on each cicada that she finds.
Since the mother fly’s hunt for cicadas lasts several weeks, some of the larvae face a long wait for food, says Reinhard Lakes-Harlan of the Justus-Liebig-University Giessen in Germany. To study what happens during that wait, he and his colleague from Germany, Thomas de Vries caught pregnant female flies in Michigan.
In the lab, the researchers kept some of the females away from cicadas and confirmed that the females didn’t deposit larvae anywhere else. Yet females dissected 12 days after capture averaged 9 to 14 fewer larvae than did females checked on arrival. Most of the females that had been in the lab 12 days contained some partial remains of larvae—often just hard mouthparts.
Some of the larvae are eating their siblings, Lakes-Harlan and de Vries suggest in a paper now online for Naturwissenschaften. Prenatal cannibalism has been recorded in other animals, such as a shark species in which up to 25 embryos fight it out until the lone survivor is born. Lakes-Harlan proposes the cicada-hunting fly as the first prenatal cannibal recorded among insects.