<P> Carnivorous plants are generally herbs, and their traps the result of primary growth . They generally do not form readily fossilizable structures such as thick bark or wood . As a result, there is no fossil evidence of the steps that might link Dionaea and Aldrovanda, or either genus with their common ancestor, Drosera . Nevertheless, it is possible to infer an evolutionary history based on phylogenetic studies of both genera . Researchers have proposed a series of steps that would ultimately result in the complex snap - trap mechanism: </P> <Ul> <Li> Larger insects usually walk over the plant, instead of flying to it, and are more likely to break free from sticky glands alone . Therefore, a plant with wider leaves, like Drosera falconeri, must have adapted to move the trap and its stalks in directions that maximized its chance of capturing and retaining such prey--in this particular case, longitudinally . Once adequately "wrapped", escape would be more difficult . </Li> <Li> Evolutionary pressure then selected for plants with shorter response time, in a manner similar to Drosera burmannii or Drosera glanduligera . The faster the closing, the less reliant on the flypaper model the plant would be . </Li> <Li> As the trap became more and more active, the energy required to "wrap" the prey increased . Plants that could somehow differentiate between actual insects and random detritus / rain droplets would have an advantage, thus explaining the specialization of inner tentacles into trigger hairs . </Li> <Li> Ultimately, as the plant relied more on closing around the insect rather than gluing them to the leaf surface, the tentacles so evident in Drosera would lose their original function altogether, becoming the "teeth" and trigger hairs--an example of natural selection utilizing pre-existing structures for new functions . </Li> <Li> Completing the transition, the plant eventually developed the depressed digestive glands found inside the trap, rather than using the dews in the stalks, further differentiating it from genus Drosera . </Li> </Ul> <Li> Larger insects usually walk over the plant, instead of flying to it, and are more likely to break free from sticky glands alone . Therefore, a plant with wider leaves, like Drosera falconeri, must have adapted to move the trap and its stalks in directions that maximized its chance of capturing and retaining such prey--in this particular case, longitudinally . Once adequately "wrapped", escape would be more difficult . </Li> <Li> Evolutionary pressure then selected for plants with shorter response time, in a manner similar to Drosera burmannii or Drosera glanduligera . The faster the closing, the less reliant on the flypaper model the plant would be . </Li>

How big is the biggest venus fly trap