322 INSECT TRANSFORMATIONS. 



of great distention. When the insect, therefore, 

 becomes desirous of escaping from its prison, it 

 blows out the extensile part of its head like a bladder, 

 alternately pushing it forward in the form of a muzzle, 

 and swelling it out at the sides in the form of a ball, 

 till it succeeds in rupturing the pupa case. As this 

 envelope is too opaque to see the process distinctly 

 on the outside, it is necessary to open the pupa just 

 before its transformation, when the movements be- 

 come obvious. The same mechanism occurs in the 

 pupae of some of the fibrous gall-flies (Tephrites), 

 for the purpose of dissevering the woody fibres 

 which imprison the insects. In the instance of the 

 thistle gall-fly (Tephritis Carclui), Reaumur found 

 that those kept in his study often became too rigid 

 for the insects to force their passage, and after 

 making repeated efforts they gave up the task in 

 despair and died.* In the open air this accident is 

 prevented by the rain moistening the galls. We 

 have more than once had occasion to make the same 

 remark in the woody galls, such as the bedeguar of 

 the rose, in which the flies have to gnaw for them- 

 selves a passage, but which they cannot always 

 effect when the galls are kept through the winter in a 

 dry room. 'I' 



In another genus of flies, the pupa does not make 

 use of its head, but turns round and employs its tail 

 to force a passage. This may be observed in the 

 pupa formed from the rat-tailed maggot of the com- 

 mon-sewer fly (Eristalis tenax, Fabricius), w^hich 

 was observed by Reaumur to push off the lid of its 

 pupa case by means of its tail. 



The caterpillar of the clear-wing hawk-moth 

 {Mgeria asiliformis, Stephens), before going into 

 pupa, gnaws away the wood of the poplar tree, 

 where it is lodged, till it leaves only a plate of it as 



* """ r, iv., Mem. S. ' f J. I{. 



