All Sarracenia have a lid, commonly referred to as a "hood", which is part of the prey-trapping mechanism. There are, however, some hybrids that are lidless, or have greatly reduced lids, such as S. leucophylla 'Bris'. Even S. purpurea has a hood - it's just at a 90 degree angle to the ground and cannot block rain; instead it collects it: A Sarr's hood is referred to as the first "Hooker Zone", or zone 1, which is part of a set of zones of a pitcher named after Joseph Hooker, who was a botanist known partly for his studies on Nepenthes in the 1870's. These zones can be used to describe the trapping and digestion mechanisms of all carnivorous pitcher plants. Rather than go into detail on the Hooker Zones, I'll just post a link to the Sarracenia-specific write-up regarding these zones, written by Barry Rice:
Purpurea hybrids like S. x catesbaei (S. flava x purpurea) have hoods that block some rain, but their purpurea heritage causes a marginally-effective hood that lets more rain though than, say, S. flava.
Catesbaei hood: Rain collected in this very same catesbaei pitcher after 1 hard rain: Even though some Sarr's do not collect rain water, they do often collect morning dew.
Pitcher shown below is the inner wall of a S. leucophylla x flava rugelii, commonly known as S. 'Judith Hindle'.
These dew droplets were gone by noon:
Sarracenia have bristle hairs on the underside of their hoods (these bristle hairs exist in other zones as well, especially near the bottom where prey reside), which usher in prey and make it more difficult to escape. These bristle hairs are slippery, as is the lip of the pitcher, so when they land on the underside of a hood or on the pitcher's lip to investigate the enticing scent the Sarr produces, or to collect a nectar reward, they often slip on the bristle hairs or the lip, and fall in (photos inside another Judith Hindle). When insects fall into the pitcher, even winged insects can't escape - the flapping of their wings can create a vacuum, pulling them deeper into the pitcher. Even winged insects near the tops of pitchers often can't escape.
The wasp in the photo below did not escape:
Winged insects climbing futilely, knowing full well by this point after multiple failed attempts, that flying out is not an option:
The inner walls of (some/all? I'm not sure) Sarracenia pitchers also contain microscopic scales that point downward, adding to the effectiveness of their trapping mechanism:
http://www.psmicrographs.co.uk/pitcher- ... e/80018477
To segway into the digestion notes below, the inner walls of Sarracenia pitchers lack the waxy coating that envelops most plant leaves (the "cuticle"); this allows for easier absorption of nutrients from trapped prey.
A. Plants like S. flava, S. rubra, S. minor, etc. (hooded pitchers whose hoods block rain):
The insect will die in here, either from starvation or drowning (if enough rainwater has made it in from, say, a windy torrential downpour). The plant releases enzimes from inside the walls on the lower portion of the pitcher, which envelops prey and starts breaking it down. The enzimes absorb the nutrients (e.g. nitrogen) from the insect back into the inner wall of the pitcher. Below is a cross-section of one of these pitchers. In some cases a Sarracenia will have so many digested insects inside it will fall over from the weight. Sarracenia are gluttonous pigs. The above cross-section photo was not taken by me. It is property of Barry Rice, sarracenia.com
B. Plants like S. purpurea/S. rosea (Sarrs that collect rainwater):
Purpurea, like the pitchers above, also secrete the standard Sarracenia digestive enzymes, so part of the digestive process here is the same as those with hoods that block rain. If prey falls into a newly-opened S. purpurea pitcher and it has not rained since the pitcher opened, the plant can still absorb nutrients from it via process A.
In addition to this, insects that fall into these Sarr pitchers generally drown in the collected rainwater. Bacteria and other microorganisms, which reside in the water in these pitchers, break apart prey and help dissolve their bodies, which aids the digestive process.
For Sarracenia, the absorbed nutrients from insects consist primarily of nitrogen and phosphorous. In both of the above cases (A and B), to complete the digestion process, the now-nutriated enzimes/water that were absorbed back into the inner walls of the pitcher travel via absorption pathways throughout the plant, all the way down to the rhizome to "feed" the plant. It stores these nutrients and uses them, in conjunction with light-energy, for everything from generating flower stalks/producing seed, to root development, to making additional pitchers.