The weird and wonderful Vapourer Moth

While having lunch, a bright, russet moth outside flew against the bay window glass again and again. After a while, I decided to capture it to check what it was. The moth was very unsettled, and carried on flying in the bug pot while we finished our meal. Shortly after, another similarly russet moth, a bit larger, flew against the glass and landed on a silky cocoon on the window frame. I went out and realised the reddish moths were in fact male vapoureurs attracted by a female on the cocoon. The second male was now mating with the female (above). The Vapourer, Orgyia antiqua is a very odd moth indeed. They have a striking sex dimorphism. Males have large, feathery antennae and often fly during the day. They are a rich russet colour with a white spot on each forewing. Females, in contrast, don't look much like moths, they cannot fly as their wings are vestigial, they have small antennae and enormous bulging bodies, full of eggs, and are pale grey in colour. Females will barely move in their short lives, attracting the males with pheromones they release shortly after emerging from her cocoon. The large feathery antennae of the males helps them locate the females quickly.

I took this photo from inside the house.

After mating for about 15 minutes, the male left (13:20). We could then had a closer look at the female. Eggs were visible through her thin abdominal skin. She looks velvety and heavy.

The female quickly started to lay eggs on the cocoon surface, the photo below taken at 15:15. Note that the caterpillar hairs, which are irritating, are embedded on the cocoon itself, so they may act as a defensive device for the eggs themselves.

After egg laying, female dies, so the adult stage is very short, about two days and they do not feed. The eggs overwinter and the caterpillars will be born the following spring.
  There are several British species of a few moth families showing this pattern of female flightlessness, amongst them the Winter Moth. The limited mobility of the females is compensated by the highly dispersive larvae, which might be able to balloon when little. In the UK adults are found from July to September.

On this photo you can see the vestigial wing: just a small hairy flap (the head is down and the abdominal tip up, egg laying).

At 16:41 she had pretty much finished laying.

 I searched around for more cocoons nearby and found one under a wooden shelf by a large cotoneaster, about 2 m away from the first one. It looked very fresh and translucent, and still contained a caterpillar.

The large oval cocoon and large caterpillar inside points to another female will emerge from this one too.