450 Observations on Habits and Parasites of Common Flies 
In the case to be described the puparium ruptured at 12.15 p.m., 
and immediately the dilated, transparent ptilinum, or distensible frontal 
sac of the fly, appeared. It remained distended for periods of about 
9 seconds, then collapsed for 2 or 3 seconds and was again distended. 
Very shortly afterwards the eyes appeared. The whole head had 
emerged in 2 minutes, and the thorax and anterior legs were released 
in 2| minutes. The whole fly was free of the puparium in 3 minutes. 
All this time the ptilinum was being alternately distended and retracted 1 . 
At this stage the fly is a curious looking object, which the writer has 
been accustomed to call a “ walker.” It is very soft, dark grey in colour, 
with well-formed legs, and all its bristles and hairs are black and well 
developed. The lines of the bristles on its thorax are marked by 
darker stripes. The wings, however, are represented by opaque, narrow 
processes with undulating longitudinal folds, extending backwards from 
the thorax to the middle third of the abdomen. There is a very 
marked semicircular recess in the inner margin situated in the distal third 
of the wing. The “walker” walks about with very great rapidity, 
occasionally distending and withdrawing its ptilinum and presenting 
1 If the ptilinum is pricked clear fluid in large quantities is discharged from it as the 
fly tries to distend it. 
Plate XXIII. Figs. 2-5. Puparia of C. erythrocepliala after the emergence of the flies. 
(Nat. size.) 
Fig. 6. The puparium illustrated in Fig. 5 enlarged. 
Figs. 7-27. Photographs of living specimens of C. erythrocephala (nat. size) in 
various stages from the emergence of the head from the puparium to the mature 
insect. Figs. 7, 8. Head protruded from puparium and ptilinum partially distended. 
Fig. 9. Head and part of thorax protruded, and ptilinum much distended. 
Figs. 10, 11. Thorax further protruded. Figs. 12, 13. Thorax completely pro¬ 
truded, and folded wings visible. Figs. 14, 15, 16. The fly in the act of extricating 
its legs and abdomen from the puparium. Fig. 17. Early “walker” with ptilinum 
distended. Figs. 18, 19. Early “walkers” (ptilinum retracted). Figs. 20, 21, 22, 23. 
Stages in the expansion of the wings. Fig. 24. Soft grey fly with fully expanded 
wings, and ptilinum partially distended. Up to this stage the abdomen is markedly 
pointed. Fig. 25. Soft, grey, normally shaped fly. Fig. 26. Fully developed male. 
Fig. 27. Fully developed female. The dark longitudinal stripes marking the lines 
of the dorso-central bristles are distinctly visible from the moment the thorax is 
protruded in the puparium. 
Fig. 28. Cluster of blow-flies ( x J) in large outdoor cage sheltering from wind 
in the evening. 
Fig. 29. Two blow-flies ( x |) sunning themselves after emerging from crevices 
in a paling. 
Fig. 30. Blow-fly ( x J) sunning itself after emerging from a crevice under loose- 
bark. Flies often select situations of the kind illustrated where they are very difficult 
to distinguish. 
