THK FORMATION OF THR PUPA. 121 



being entirely checked in their development, undergo hy- 

 bernation. Their dormant vitality remains, however, to be 

 awakened by the warmth of spring. 



When the imago is ready to burst from the pupa case, the 

 anterior extremity is forced off by the dilatation of the frontal 

 sac. This is effected by the contraction of all the muscles then 

 existing, which forces the greater part of the circulating fluid 

 into the frontal sac, and distends it until it far exceeds the 

 head in size. At this period a pair of large muscles extend 

 from the edges of the frontal sac to the margin of the occipital 

 foramen. They assist in the dilatation of the sac by diminish- 

 ing the cavity of the head by their contraction. They after- 

 wards undergo degeneration, and may be looked for in vain in 

 the mature insect. 



FINI3. 



THOMSON AND FINDER, PRINTERS, BOLT COURT, FLEET STREET. 



