Carnivorous plants do not need all of the insect. Most of what you see is the chitinous exoskeleton which the Venus Flytrap cannot digest. Fat, which is virtually non-existant in flies, is also indigestible. If you dissected the flies, they would be minus most of their internal protein rich organs. If the plant took 5-14 days to digest the flies, there is no problem. I also have noted that a Flytrap might not take long to digest insects when it is catching a large number in a short time period as the plant simply does not need that much nitrogen. After a long time of several weeks without prey, a Flytrap will close for a longer time on one prey item to derive as much nutrient out of it as possible.

Nepenthes are capable of partially digesting the chitin in insect exoskeletons, however; even spiders will show up as crumpled, thin (almost transparent) legs and thorax parts after a few weeks in a Nepenthes pitcher.