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The balloon goes up 

I found this fly on the wall in Lime Kiln recently. The pale colouration and generally soft appearance show that it's recently emerged, as does the 'balloon' protruding from its head, a structure known as a 'ptilinum'. What we think of as the pupa of things like bluebottles and house flies (the hard, dark red capsules that you sometimes find around rubbish bins for instance) is actually the hardened last larval skin, called the 'puparium'. The pupa itself is inside this so the fly needs some way to break through this hard outer layer and this is what the ptilinum is for, its expansion forcing open the puparium and allowing the fly to emerge.

The image below is the same individual a few hours later. The cuticle has hardened up and the ptilinum has been retracted, never to be used again. This is one of the common bluebottles, Calliphora vicina, a species which feeds on decaying organic matter and can be active all through the winter months when the weather is warm enough.

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