Stick insect eggs
Photo credit: Piotr Naskrecki/Minden Pictures/Corbis | Source: Science
Here's a lovely example of evolved mimicry, this time in stick insects. Stick insects are noted for their mimicry of, well... sticks really.
By studying the Australian stick insect, Eurycnema goliath, Australian scientists from Macquarie University, North Ryde, New South Wales and The University of Melbourne, have discovered that they also employ mimicry to have their eggs dispersed and protected - by ants.
Their eggs have a strange knob on one end known as a 'capitulum' which seems to have no purpose, but study has shown that ants gather these eggs and take them back to the safety of their nests. They do this because the knobs are coated in a fatty acid which the ants find irresistible. But clearly, since they don't eat the eggs and get no benefit from taking them back to their nests, this inability to resist the fatty acid on stick insect eggs is unlikely to have evolved just because it's there.
So why would ants find this fatty acid irresistible, so creating an environment in which there was an advantage to these stick insect in evolving eggs with fatty acid-coated knobs on them?
The answer to that is that many plants, which DO benefit from being picked up and taken into their nest by ants, induce this behaviour by giving ants a food reward, in particular, a food reward consisting of a coating of - you've guessed it - fatty acid. This is known as chemical signalling and is used by some plants to attract the attention of ants to their seeds and so get them protected and dispersed.
This created an environment in which there was a benefit to stick insects in mimicking this chemical signal - a case, like other forms of mimicry, of convergent evolution.
Numerous tree species’ seeds contain an ‘elaiosome’ that acts as a food reward for ants and thus induces dispersal of the seeds. Many stick and leaf insect species appear to have evolved a convergent adaptation for dispersal whereby the egg ‘capitulum’ serves to induce ants to pick up and carry their eggs. Here, we investigated whether the capitulum facilitates egg dispersal by ants in the Australian stick insect Eurycnema goliath. The total fatty acid composition of E. goliath egg capsules and egg capitula were characterized to identify potential signaling compounds. Removing capitula from E. goliath eggs significantly reduced the likelihood of eggs being carried into the nests of Rhytidoponera metallica ants. Furthermore, attaching capitula to inert objects (polystyrene balls) resulted in these objects being carried into nests by R. metallica. Several fatty acids were present on the egg capsule surface in only trace amounts, whereas they made up over 10 % of the dry weight of egg capitula. The fatty acid composition of egg capitula consisted mostly of palmitic acid (C16:0), linoleic acid (C18: 2n6c), oleic acid (C18:1n9c), linolenic acid (C18:3n3), and stearic acid (C18:0). Previously reported research has found that a diglyceride lipid species of oleic acid induces carrying behavior in R. metallica when added to inert artificial stimuli. Therefore, we propose that the dispersal mechanism of E. goliath eggs has converged upon the same chemical signaling pathway used by plants to exploit ant behavior.*
Egg Dispersal in the Phasmatodea: Convergence in Chemical Signaling Strategies Between Plants and Animals?
Anthony O. Stanton, Daniel A. Dias, James C. O’Hanlon
Journal of Chemical Ecology, Volume 41, Issue 8, pp 689-695 DOI: 10.1007/s10886-015-0604-8
*Copyright © 2015, Springer Science+Business Media New York. Reprinted with permission under licence #3735300694796.
So here we have, yet again, an explanation which fits into scientific evolutionary theory like a hand into a glove, and which is entirely inexplicable in terms of magic design because an intelligent designer would have designed a far simpler method for getting stick insect eggs protected, even if it had made the mistake of creating something the eggs needed to be protected from in the first place.
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