The goldenrod gall-fly lays its eggs on the terminal buds of goldenrod plants. Larvae chew through the buds and into the stems,
where their saliva induces the plant to generate a gall, or outgrowth of tissue, that then provides food and shelter for the developing larva. The larvae are prey to both parasitoid wasps and to birds; wasps selectively prey on larvae inside the smallest galls while birds selectively prey on larvae inside the largest galls. Goldenrod gall-flies are therefore subject to what selection?
This occurs when selective pressures occur against the two extremes of a trait (wasps prey on larvae inside the smallest galls; birds prey on larvae inside the biggest galls). As a result, this would select for larvae in medium sized galls, while the number of larvae in small and large galls decreases.
The phenomenon is called an equinox. More specifically when it is September it is called the autumnal equinox ,and when it is March it is called the vernal equinox. Hope this helped!
