Growing Camouflage

 A garden on the back of a weevil living a humid Chilean rainforest.

A garden on the back of a weevil living a humid Chilean rainforest.

Lots of us will be familiar with organisms like decorator crabs that utilize bits and pieces of their environment, especially living sea anemones, as a form of camouflage and protection. Examples of terrestrial insects attaching bits and pieces of lichens to their body are not unheard of either. However, there are at least two groups of arthropods that take their camouflage to a whole new level by actively growing miniature gardens on their bodies.

Little is known about these garden-growing arthropods. To date, these miniature gardens have only been reported on a few species of weevil in the genus Gymnopholus as well as a species of millipede called Psammodesmus bryophorus. Coined epizoic symbiosis, it is thought that these gardens serve as a form of protection by camouflaging the gardeners against the backdrop of their environment.

 Bryophytes on a  Psammodesmus bryophorus  male.

Bryophytes on a Psammodesmus bryophorus male.

Indeed, both groups of arthropods frequent exposed areas. What is most remarkable about this relationship is that these plants were not placed on the carapace from elsewhere in the environment. Instead, they have been actively growing there from the beginning. Closer inspection of the cuticle of these arthropods reveals unique structural adaptations like pits and hairs that provide favorable microclimates for spores to germinate and grow.

The plant communities largely consist of mosses and liverworts. At least 5 different liverwort families are represented and at least one family of moss. Even more remarkable is the fact that even these small botanical communities are enough to support a miniature ecosystem of their own. Researchers have found numerous algae such as diatoms, lichens, and a variety of fungi growing amidst the mosses and liverworts. These in turn support small communities of mites. It appears that an entire unknown ecosystem lives on the backs of these mysterious arthropods.

 FIGURE 39. Elytral base of Gymnopholus (Niphetoscapha) nitidus with exudates. FIGURES 40a–b. Gymnopholus (Niphetoscapha) inexspectatus sp. n., live specimen with incrustrations of algae and lichens; photographs M. Wild, Mokndoma.  [SOURCE]

FIGURE 39. Elytral base of Gymnopholus (Niphetoscapha) nitidus with exudates. FIGURES 40a–b. Gymnopholus (Niphetoscapha) inexspectatus sp. n., live specimen with incrustrations of algae and lichens; photographs M. Wild, Mokndoma. [SOURCE]

There is still much to be learned about this symbiotic relationship. Although camouflage is the leading hypothesis, no work has been done to actually investigate the benefits these arthropods receive from actively growing these miniature gardens on their backs. Mysteries still abound. For instance, in the case of the millipede, gardens are found more frequently on the backs of males than on the backs of females. Could it be that males spend more time searching their environment and thus benefit from the added camouflage? Only further research will tell.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

Grasses That Feign Infestation

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Given the option, most of us would rather avoid a salad riddled with insects or an apple chock full of worms. Much as we prefer to avoid insect-infested fruits and vegetables, so too do many herbivores. Some plants seem to be taking advantage of this. In response to strong herbivore pressure, some plant species have evolved insect mimicry. One such case involves grass and aphids. 

Paspalum paspaloides can be found growing in tropical regions around the globe. In many ways they are similar to other C4 grasses. When they flower, however, one may notice something interesting. All of the flowers appear to be covered in aphids. Close inspection would reveal that this is not the case. Those clusters of dark specks swaying the breeze are simply the numerous dark anthers of the inflorescence. This has led some to hypothesize that these plants may be mimicking an aphid infestation.

This observation begs the question: "what benefit is there in mimicking aphids?" There are two major hypotheses that have been proposed in order to explain this phenomenon. The first is defense against herbivory. As stated above, herbivores often avoid plant material that has been infested with insects. Aside from any potential palatability issues, large populations of insect pests can signal a decrease in the nutritional value of a potential food source. Why waste time eating something that is already being eaten? Evidence in support of this hypothesis has come from other systems. A wide array of herbivores, both mammalian and insect, have been shown to avoid aphid-infested plant material.

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The second hypothesis is one of avoiding future infestations. Aphids are clonal organisms with a short generation time. It does not take long for a few aphids to become many, and many to become an infestation. As such, aphids looking for a new plant to colonize habitually avoid plants that already have aphids on them. It could very well be that such aphid mimicry is a means by which the grass keeps actual aphids at bay.

If this is a form of true mimicry then the question is not a matter of which hypothesis but the relative influence of each. It seems that it very well could be driven by a mixture of both strategies. Still, all of this is speculative until actual experiments are carried out. Those who originally put forth these ideas have identified similar potential mimicry systems in other plants as well. The idea is ripe for the testing!

Photo Credits: [1]

Further Reading: [1] [2] [3]