The presence of the dorsal wing cases indicates the arrival of the insect at the pupa state.
"The morning is the time they generally choose for completing this change, because I am pretty certain it is in their power to retard this regeneration considerably, if I can judge from concurrent circumstances attending those I have kept in glass bowls, on purpose to observe their nature and behaviour. In one of these I have seen two libella-caterpillars, that were fully grown, for three days successively creep up the straws out of the water, in order to undergo their transformation; but finding themselves deprived of the sun-shine, (a circumstance generally attending this performance,) or at least that agreeable warmth of air so necessary for their purpose, they retired into the water: in an hour's time they made another essay, but finding the same circumstances subsisting as before, they again retired under water, and this they continued doing for three mornings; till at length one of them, wearied out by those frequent efforts, the period of nature not being to be totally avoided or suppressed, although it might be retarded, perished in the water, being at length so weak as to be unable to creep above its surface. The other, on removing the vessel into the sunshine, yielded its proper insect."