G. S. Graham-Smith 
451 
a ludicrous appearance. At this stage the ptilinum is used to help the 
creature to pass through earth or sand, if emergence has taken place 
under the soil, or through a cotton wool plug if it is confined in a test 
tube. The mode of procedure is to thrust the head, with the ptilinum 
retracted, as far forward as possible and then to distend the ptilinum 
so as to obtain a purchase against neighbouring particles. The body 
is then drawn up to the head, the ptilinum retracted, the head again 
thrust forward and the process repeated. 
The “walker” stage lasted in this case 90 minutes, during which 
time the creature became much lighter in colour, assuming a light grey 
tint and semi-transparent appearance, and the abdomen gradually 
increased in size till the somewhat attenuated abdomen of the early 
stage had become pointed at the distal end and rounded and fully 
distended in its proximal parts. Then very rapidly the wings unfolded 
and were almost completely expanded in two or three minutes. 
Shortly after the unfolding commences the wing has a crumpled 
appearance and the last part to expand is the tip. When first fully 
expanded the wings are convex when looked at from above, and have a 
milky-grey, opaque appearance. Within ten minutes the wings become 
firm and flat, though still grey and opaque, and the fly looks a soft 
and grey, but otherwise normal, specimen. Even at this stage the 
ptilinum is distended at intervals. 
Three-quarters of an hour later the normal blue coloration of the 
fly began to appear, and the upper surface had nearly assumed the 
normal tint in another half hour, though the under surface was still 
much lighter in colour. Another hour and a half elapsed before the 
fly showed the typical coloration all over, though its chit-in was still 
soft. 
At 49° F. flies complete the stages described in four to five hours. 
After this the chitin becomes hard and no further change or growth 
takes place. At higher temperatures, and especially when the flies 
are exposed to the sun, the stages are completed very rapidly, some¬ 
times within an hour. At lower temperatures, on the other hand, 
the time is very much prolonged. Occasionally the chitin may harden 
before the wings are expanded, leaving the fly a cripple. By removing 
a fly in one of the earlier stages from a warm to a cold room it may be 
caused to remain at the same stage of development for some hours. 
In the spring months under natural conditions blow-flies usually 
emerge early in the morning, about 7 a.m. in April and still earlier in the 
warmer months. On fine days development is complete by 10 a.m.. 
