Why the Silk Moth Cannot Fly

Wings That Do Nothing

An adult silk moth has a full set of wings and cannot fly. It is not injured and it is not weak from captivity — the capacity is simply gone, bred out over thousands of years of selection that never once favoured a moth that could leave.

What Else Went With It

Flightlessness is one item on a list. The adult Bombyx mori also:

• Does not feed. The adult stage exists only to mate and lay eggs, and then it is over.
• Does not disperse. A moth placed somewhere stays roughly there.
• Cannot survive outside human care at any life stage.

Why Selection Went That Way

Because none of it was selected against, and some of it was selected for. A rearing operation wants animals that stay in the tray. A moth that flies is a moth that is lost, taking its eggs with it. Over enough generations, choosing the animals still present when you come to collect them is itself a selection pressure.

The feeding loss follows different logic. Adult life in this species is short and singular — the animal emerges, mates, lays, dies — and there is no advantage in maintaining a feeding apparatus for a stage that does not need one. Wild relatives kept it; the farmed animal did not need to.

The Comparison That Proves It

Tussar, eri and muga moths are wild or semi-domesticated and are often reared outdoors on standing trees. They fly. They find their own host plants. They survive unattended.

Same order of insect, same industry, different degree of domestication — and the difference is visible in whether the adult can leave the tray.

The genomic version of this is in the 40-genome study: 354 genes bearing the marks of selection, concentrated in the silk gland, the gut and the reproductive tract. Nobody was selecting for flight.

Related: what silk is made of · inside the silk gland