546 
DIPTERA. 
protecting case, the pupa and its case together being called a c 4 pupa¬ 
rium.” In the Orthorhapha, with few exceptions ( e.g ., the family 
Stratiomyidce), we find, 
on the other hand, that 
the last larval skin is 
completely shed and cast 
aside, leaving the pupa 
bare and unprotected; 
in such pupae the limbs Fi ^ 349 “ a p f ^it-fly (dacus) emeiiging fhom 
and general shape of the 
future imago can be seen, while in the Cyclorhaphous puparium all these 
details are completely concealed by the hardened and contracted larval 
skin which encloses the true pupa. This difference in the type of pupa 
is associated with a difference in the way the fly gets out of it : in the 
Orthorhapha the splitting of the comparatively weak pupa-skin is 
effected by what we may call 44 hunching the shoulders,” and the top 
of the thorax is the part of the fly’s body which is first exposed. The 
Cyclorhapha employ a different method, perhaps owing to the less yield¬ 
ing nature of the hardened skin which surrounds the true pupa. In order 
to escape, the fly, instead of trying to crack this skin along the thorax, 
pushes out the end of it and emerges head foremost. To push out the 
end it cannot use its feet and limbs, since these are confined mummy-like 
in the puparium, but it gets over the difficulty by the help of a very 
remarkable structure, in the shape of an expansible balloon arising from 
the head. When inflated with liquid from the body, this balloon (called 
a 44 ptilinum ”) pushes off the end of the puparium and releases the fly, 
whose escape is rendered easier from the shrinking of the body due to 
the absence therefrom of the liquid used to inflate the balloon. (Figs. 
349, 350.) The ptilinum is afterwards deflated and disappears into the 
head, leaving the scar called the 44 frontal suture ’ ’ as evidence of its 
existence (fig. 407). All flies with this frontal suture are Cyclorhapha. 
It should be noted that three families ( Platypezidce , Pipunculidce and 
the big family Syrphidce) are included in the Cyclorhapha although 
they do not possess a frontal suture : they are therefore known as Cyclor¬ 
hapha 44 Aschiza ” (i.e., without suture), being classed with the Cyclor¬ 
hapha because of the mode of splitting of the pupa-case as well as 
by the form of the antennae and by their possessing a small but sys- 
