268 THE SCHOOL OF REAUMUR 



found, on opening the shell, a white pupa, in which the 

 parts of the fly could be distinctly recognised. By a 

 brief immersion in boiling water the pupa, in its later 

 stages at least, could be readily extracted without injury, 

 and he was thus able to study its growth with facility. 

 Two or three days after the maggot became motionless 

 three pairs of short legs appeared on its thorax ; next 

 day the wings could be distinguished, while the legs had 

 apparently grown longer ; a little later the proboscis 

 became visible, and so on. E^aumur next discovered 

 that the new organs begin to form some time before 

 they appear externally, for they are at first telescoped 

 into the body in such a way that only their extremities 

 are free. By gentle pressure he succeeded in causing 

 them to protrude from the deep infoldings in which 

 they were at first concealed. " In fact," says he, "I 

 was able myself to complete the development of the 

 pupa, effecting in a moment a transformation which 

 ought to have occupied several days." He made the 

 acute suggestion that the protrusion may be accom- 

 plished in the living insect by blood-pressure, and this 

 suggestion has now been confirmed by actual observation. 

 Keaumur next explains how the fly makes its escape 

 from the shell (hardened larval skin). At the head-end 

 is a horizontal cleft, which divides the thoracic and first 

 abdominal segments of the larval skin into dorsal and 

 ventral halves ; this cleft is a line of weakness, prepared 

 in advance to facilitate the emergence of the fly. By 

 pressing a late pupa between the finger and thumb the 

 cleft can be made to gape ; the dorsal and ventral flaps 

 part, and may fall off", for they are easily detached along 



and that the milky pulp which Reaumur observed is largely made up of gorged 

 phagocytes. Something of the same kind has been observed in many other 

 insects, but it is a peculiarity of Muscid larvae that they are almost entirely 

 dissolved in this way. 



