THE HOLLY-FLY 



legs) to the surface of the ground, shortly before emergence, 

 and when the fly has become free, the empty case is often 

 seen half-protruded from the burrow. 



This difficulty is no sooner overcome than another appears. 

 The last larval skin in this, as in all the more specialised 

 flies, is not cast at pupation, but retained as an outer 

 defence during the resting-stage ; it 

 usually hardens to a shining brown 

 or black cylinder, tapering at both 

 ends, and somewhat resembling a 

 seed (Fig. i). Inside this the fly 

 develops, shrouded within a soft 

 white covering, which is the proper 



FIG. 3. Holly-fly, emerging from the leaf. In the lower fig. the 

 pupa-case is shown within the leaf. Both figs, show the bladder on 

 the forehead, the compound eyes, antennas, &c. 



pupal skin (Fig. 2). All goes well until the moment for 

 emergence arrives. The fly is then so tightly packed within 

 its double case that it cannot move a limb ; its skin is soft 

 and flexible ; how is it to force its way out ? You might 

 study this problem for a long time without hitting on the 

 expedient which nature has provided an expedient which 

 has this chief merit, that it hardly ever fails of its purpose. 

 Upon the head of the newly formed fly is a bladder-like 

 expansion (Fig. 3), which can be so greatly distended with 

 liquid as to exceed in size the head itself. The liquid is 

 the transparent, colourless blood of the fly. So great is the 



