Larva of Chaoborus Crystallinus (de Geer). 
351 
hooks almost jumped out of the pocket, then were slowly with- 
drawn. This occurred at least half-a-dozen times. 
The remaining specimens all showed the hooks internal. These 
were dropped into a killing mixture ; three of these we examined 
when dead and found the hooks external. The fourth we took out 
of the fluid and examined before it was dead, and gentle pressure 
applied by the cover of the live box caused the hooks to pop out 
of the pocket and expand ; on releasing the pressure they slowly 
withdrew. From these observations we were satisfied that the 
hooks are movable, their normal position being inside the pocket, 
and not external and fixed as suggested by most of the accounts 
referred to. Is this a degenerate retractile hooked foot ? 
“ All insects were primarily terrestrial ; that some have become 
aquatic is no doubt due to an acquired habit. Many remain air- 
breathers, coming to the surface of the water for supplies ; others 
completely changed the tracheal system, extracting supplies of air 
from the water. 
“ The transformation of the Corethra larva has been complete. 
It is the only insect larva that has become a free swimmer, and 
remains so during the whole period of its larval existence.” 
We have already referred to the vestigial legs, and we will 
now carry our minds back to the time when the larva first became 
