The Squirming Mass.
From a distance the food looks like some indeterminate lump of something green, but on closer inspection it is possible to see that the lump is, in fact, covered with a mass of constantly moving tiny brown hairy caterpillars. Underneath I can see that they are each eating away a small furrow and this is giving the mass a spongy texture.
As I move in closer it seems like the larvae are hovering in mid-air as if already they are spinning silk. Perhaps they are. They already move as their parents moved when searching for a place to spin: their hind quarters locked into position and their black heads, outlined by a paler thorax, circling the air, presumably sniffing for food. Perhaps they have chemical receptors that will react to only one thing: mulberry leaf.
Anyway, the whole effect is slightly revolting, so I just had to share it...
Looking at the photograph I can see now why the eggs grow darker. I'd thought it was just that the egg-case hardens and darkens as a consequence. But on closer study I think the darkening is due to the black caterpillar forming inside. The indentation in the surface is because it is curled around itself in a ring. The case of the egg, I think, remains translucent, and as the caterpillar grows it becomes more darkly pigmented.