Biology 342 Fall 2012
Emily Agan, Christina Barrett
Adaptive Value of Trap and Lure Behavior
Bioluminescence has been observed to serve several different functions in the species of arthropods that utilize it, including in defense, aggression, communication, mate and prey attraction. The glowworm uses bioluminescence as a lure for prey species that exhibit positive phototaxis, and set traps of mucus laden silk lines in which to ensnare the entranced insects.
Although there are many trap-building, ‘sit and wait’ arthropods, and many bioluminescent luring predators, glowworms are somewhat unique in their combination of the two techniques, in what can be called the trap-building, ‘sit and lure’ feeding strategy. This has been shown, experimentally, to be a more successful strategy in the dark habitats that glowworms inhabit, such as caves and dense forests as compared to traps set without bioluminescent lures. In this environment traps with light lures were about 8.09 times more likes to capture prey than lure-less traps in caves, and 6.14 times more likely in the worms’ bush dwellings as shown in the figure below(Broadley, 2001).
Figure 1. Photograph of fly caught in larval glowworm's mucus laiden lines hanging from the the Waitomo cave roof. Sourced from Willis, R. E., C. R. White, et al. (2011)
The ‘sit and lure’ strategy also tends to be more energy efficient than either prey foraging behavior or ‘sit and wait’ behavior. The average metabolic rate, as determined by carbon dioxide production, of a glowworm in full glow was only found to be about 1.26 times the metabolic rate of a non-bioluminescing worm. The energy cost of this ‘lure’ is much lower than measured costs of foraging behaviors (Boadley, 2001).
Figure 2. Graph of prey caught by live, glowing larva, as compared to non-glowing controls in different habitats. Sourced from Boadley, 2001.
Although the ‘sit and wait’ strategy comes with no extra cost of the actual capture of animals, the energy expended on trap building tends to be much larger for these animals then for the glowworm, which has a trap building metabolic rate of 2.56 times the resting rate. This is probably due to the worms web recycling strategy, which allows it to detach prey from it’s silk lines without completely destroying the trap, and laying it out again with minimal energy output, while many arthropods, such as more spiders, rebuild their webs from scratch every time a section is damaged. The snares of glowworm also tend to be less extensive than those traps that do not have lures to increase their effectiveness. The combination, then, or lure and trap building, expending a relatively small amount of energy on each, represents an incredibly efficient way to capture prey that can be maintained constantly, in almost any set of conditions (Willis andWhite, 2011).