Acarodomatia
In the plant world, domatia are tiny chambers produced by plants that houses arthropods. From domus, latin for home, domatia are cavities or structures on plants that house insects or other small arthropods. One of the more dramatic cases of a plant providing a home to a helpful insect is that of the acacia and the ant. Acacias provide the ants a hollow structure at the base of the leaf in which to live, and in return, the ants defend the acacia from caterpillars and other predators. Other plants have similar, if more subtle structures. Acarodomatia are small leaf structures, hidden in plain site on the back of a leaf, that shelter mites. Mites are tiny, spider-like animals that can help plants by feeding on fungi and other parasites.
Swollen stipules at the base of leaves that provide shelter for ants in acacia trees. Photo by Vilseskogen, creative commons license
I am always interested cooperative relationships in nature. We have a certain narrative about nature that everything is a relentless competition and fight to the death, but that’s not the only story. Many relationships are mutually beneficial. In the case of plants and mites, mites may have a role in the longevity and health of the leaves and plants. Some mites are predators, feeding on other arthropods that might attack leaves, and some mites feed on fungi, scavenge dead plant material, and graze on green algae. The habits of mites are not well understood due to their small size and difficulty in studying them in the field, but there may be as many as 10,000 mites per square meter of leaves living in the tropical canopy. Amazing!
I became enchanted with these mite houses when I took tropical plant class at the University of Michigan. Dr. Robyn Burnham taught the course, and she had investigated acarodomatia in tropical vines in the Bignoneaceae family. We know the Bignoneaceae family in the or eastern temperate zone from trumpet vine and catalpa trees. Dr. Burnham found and defined three types of domatia: pocket, tuft, and pit. The pocket was an overhang created at the axis of two leaf veins where the tissue created a sort of lean-to. The tuft was a patch of hairs at a similar vein axis, and the pit was a depression on the leaf.
Dr. Burnham had only looked at the vines, so as a class project I looked for acarodomatia in the tree species of the Bignoniaceae family. To research tropical plants, my destination was (naturally!) Saint Louis, Missouri. Saint Louis is home to the Missouri Botanical Garden. The Botanical Garden has an incredible display collection, but is also the site of botanical research and home to one of the world’s most important herbaria. An herbarium is a library of dried plant specimens that can be consulted for various research purposes. The Missouri Botanical Garden herbarium has over 7 million specimens. While a trip to the tropics would have been great, a trip to MOBOT was also thrilling for new student of botany.
At the herbarium, I took out dried Bignoneaceae specimens to examine under a microscope for the presence of acarodomatia. Herbarium specimens are records of centuries of exploration and can be quite old. The specimens and their documentation can be time capsules of ecology and human history. For example, some of the specimens I looked at had tags from “British Honduras,” which has been called Belize since 1973.
Using the Bignoneaceae specimens in the herbarium collection, I documented the frequency and type of domatia that in each genus and species. I have lost the data in the intervening years, but the gist of my findings was that domatia are common in Bignoneaceae trees, most often as tufts and pockets. The pictures that I took through the microscope remain fascinating, hinting at a complex ecosystem on the surface of a leaf and worlds of existing taking place below the scale of our notice.
