8 Outline of Life-history 



to the fly ; for the moment they are shrouded in a deli- 

 cate, transparent envelope, the pupa-skin. The pupa 

 commonly lies within its burrow, or half in and half out, 

 until the time of extrication of the fly is at hand ; it 

 neither feeds nor swims about. Sometimes it lies with 

 its tail buried in mud, the head and tracheal gills sticking 

 out, or it may excavate a little basin in the mud by the 

 movements of the tail, and lie in it. The tail or abdomen 

 is always the part which bends to and fro. When kept in 

 a saucer of muddy water, the pupa lies on the surface of 

 the mud, and being insufficiently supported by the mud. 

 takes an unnatural position, lying on its side. 



The red colour of the fresh-emerged pupa soon darkens, 

 and two bunches of silvery filaments just behind the head 

 show out with great distinctness. In two or three days 

 the pupa becomes buoyant, and rises to the surface, where 

 it remains until the fly escapes. The process of extrica- 

 tion from the pupa-skin is accomplished so quickly that 

 it is hard to see in detail what happens. The cast skin 

 floating on the water tells us that the back of the thorax 

 splits lengthwise, as at an ordinary larval moult, and 

 that the fly emerges through the cleft. Considering that 

 the long and slender legs, the antennae, the new mouth- 

 parts, the wings, and the abdomen have all to be drawn 

 out from their sheaths, it is startling to find the fly 

 taking wing before one is able to focus the eye upon it. 

 In the case of a fly which escaped more slowly than 

 usual, we estimated that the whole process occupied ten 

 seconds. Now and then something catches, and the fly 

 extricates itself with great effort, or not at all. 



Most of the larvae which we find in winter are destined 

 to pupate and turn to flies in early spring. These lay 

 eggs, and produce a fresh crop of young larvae. There is 

 a rapid succession of broods until late autumn. A live 

 fly is occasionally seen on the window-pane even in the 



