Fossils Shine Light On the History of Gall-Making Wasps

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We can learn a lot about life on Earth from the fossil record. I am always amazed by the degree of scrutiny involved in collecting data from these preserved remains. Take, for instance, the case of gall-making wasp fossils found in western North America. A small collection of fossilized oak leaves is giving researchers insights into the evolutionary history of oaks and the gall-making wasps they host.

Oaks interact with a bewildering array of insects. Many of these are gall-making wasps in the family Cynipidae. Dozens of different wasp species can be found on a single oak tree. Female wasps lay their eggs inside developing oak tissues and the larvae release hormones and other chemicals that cause galls to form. Galls are essentially edible nursery chambers. Other than their bizarre shapes and colors, the compounds released by the wasp larvae reduce the chemical defenses of the oak and increase the relative nutrition of the tissues themselves. Often, these relationships are precise, with specific wasp species preferring specific oak species. But when did these relationships arise? Why are oaks so popular? What can fossil evidence tell us about this incredible relationship?

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Though scant, the little fossil evidence of oak galls can tell us a lot. For starters, we know that gall-making wasps whose larvae produce structures similar to that of the Cynipids have been around since at least the late Cretaceous, some 100 million years ago. However, it is hard to say for sure exactly who made these galls and exactly what taxonomic affinity the host plant belongs to. More conclusive Cynipid gall fossils appear again in the Eocene and continue to pop up in the fossil record throughout the Oligocene and well into the Miocene (33 - 23 million years ago).

Miocene aged fossils are where things get a little bit more conclusive. Fossil beds located in the western United States have turned up fossilized oak leaves complete with Cynipid galls. The similarity of these galls to those of some present day species is incredible. It demonstrates that these relationships arose early on and have continued to diversify ever since. What's more, thanks to the degree of preservation in these fossil beds, researchers are able to make some greater conclusions about why gall-making wasps and oaks seem to be so intertwined.

 Holotype of Antronoides cyanomontanus galls on fossilized leaves of  Quercus simulata . 1) Impression of the abaxial surface of the leaf, showing the galls extending into the matrix. 2) Galls showing close association with secondary veins. 3) Gall showing the impression of rim-like base partially straddling the secondary vein. 4) Close-up of gall attached at margin extending down into the matrix. 5) Gall uncovered revealing spindle-shaped morphology.

Holotype of Antronoides cyanomontanus galls on fossilized leaves of Quercus simulata. 1) Impression of the abaxial surface of the leaf, showing the galls extending into the matrix. 2) Galls showing close association with secondary veins. 3) Gall showing the impression of rim-like base partially straddling the secondary vein. 4) Close-up of gall attached at margin extending down into the matrix. 5) Gall uncovered revealing spindle-shaped morphology.

 1)  Xanthoteras clavuloides  galls on fossilized  Quercus lobata , showing gall attached to secondary vein. Specimen in California Academy of Sciences Entomology collection, San Francisco. 2) Two galls of attached to a secondary vein showing overlap of their bases. Specimen in California Academy of Sciences Entomology Collection, San Francisco. 3) Three galls collected from leaf of California  Quercus lobata  showing clavate shape and expanded, ring-like base. 4) Gall showing the annulate or ribbed aspect of the base, which is similar to bases of  Antronoides cyanomontanus  and  A. polygonalis . 5) Galls showing clavate shape, pilose and nonpilose surfaces, and bases.

1) Xanthoteras clavuloides galls on fossilized Quercus lobata, showing gall attached to secondary vein. Specimen in California Academy of Sciences Entomology collection, San Francisco. 2) Two galls of attached to a secondary vein showing overlap of their bases. Specimen in California Academy of Sciences Entomology Collection, San Francisco. 3) Three galls collected from leaf of California Quercus lobata showing clavate shape and expanded, ring-like base. 4) Gall showing the annulate or ribbed aspect of the base, which is similar to bases of Antronoides cyanomontanus and A. polygonalis. 5) Galls showing clavate shape, pilose and nonpilose surfaces, and bases.

Gall-making wasps seem to diversify at a much faster rate in xeric climates. The fossil records during this time show that mesic tree speciess were gradually being replaced by more xeric species like oaks. Wasps seem to prefer these drier environments and the thought is that such preferences have to do with disease and parasite loads.

Again, galls a large collections of nutrient-rich tissues that are low in defense compounds. Coupled with the juicy grub at their center, it stands to reason that galls make excellent sites of infection for fungi and other parasites. By living in drier habitats, it is believed that gall-making wasps are able to escape these environmental pressures that would otherwise plague them in wetter habitats. The fossil evidence appears to support this hypothesis and today we see similar patterns. White oaks are especially drought tolerant and its this group of oaks that host the highest diversity of gall-making wasps.

Photo Credits: [1] [2] [3] [4]

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

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]

The Trumpet Creeper

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With its impressive bulk and those stunning tubular red flowers, one would be excused for thinking that the trumpet creeper (Campsis radicans) was a tropical vine. Indeed, the family to which it belongs, Bignoniaceae, is largely tropical in its distribution. There are a handful of temperate representatives, however, and the trumpet creeper is one of the most popular. Its beauty aside, this plant is absolutely fascinating.

As many of you probably know, the trumpet creeper can reach massive proportions. In the garden, this can often result in collapsed structures as its weight and speed of growth is something few adequately prepare for. In the wild, I most often see this vine in somewhat disturbed forests, usually near a floodplain. As such, it is supremely adapted to take a hit and keep on growing year after year.

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One of the many reasons this plant performs so well both where it is native and where it is not is that it recruits body guards. This is easy to witness in a garden setting as the branches and especially the flowers are frequently crawling with ants. Trumpet creepers trade food for protection via specialized organs called extrafloral nectaries. These structures secrete sugary nectar that is readily sucked up by tenacious ants. When a worker ant finds a vine, more workers are soon to follow. 

Amazingly for a temperate plant, trumpet creepers produce more extrafloral nectaries of all four categories - petiole, calyx, corolla, and fruit. What this means is that all of the important organs are covered in insects that viciously attack anything that might threaten this sugary food supply. Hassle one of these vines at your own peril. With its photosynthetic and reproductive structures protected, trumpet creepers make a nice living once established.

Reproduction is another fascinating aspect of trumpet creeper biology. A closer inspection of the floral anatomy will reveal a bilobed stigma. Amazingly, this stigma has the ability to open and close as potential pollinators visit the flowers. Stigmatic movement in the trumpet creeper has attracted a bit of attention from researchers over the years. What is its function?

Evidence suggests that the opening and closing of the lobed stigma is way of increasing the chances of pollination. Touch alone is not enough to trigger the movement. However, when researchers dusted pollen onto the stigma, then it began to close. What's more, this action happens within 15 to 60 seconds. Amazingly, there appears to be a threshold to whether the stigma stays closed or reopens after 3 hours or so.

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It turns out, the threshold seems to depend on the amount of pollen being deposited. Only after 350 grains found their way onto the stigma did it close permanently. Experts feel that this a means by which the plant ensured ample seed set. If too few pollen grains end up on the stigma, the plant risks not having all of its ovules fertilized. By permanently closing after enough pollen grains are present, the plant can ensure that the pollen grains can germinate and fertilize the ovules without being brushed off.

It is interesting to note that the flowers frequently remain on the plant after they have been fertilized. This likely serves to maintain a largely floral display that continues to attract pollinators until most of the flowers have been pollinated. Speaking of pollinators, observations have revealed that the trumpet creeper is pollinated primarily by ruby-throated hummingbirds. Although insects like bumblebees frequently visit these blooms, bringing pollen with them in the process, hummingbirds, on average, bring and deposit 10 times as much pollen as any other visitor. And, considering the threshold on pollen mentioned above, trumpet creeper appears to have evolved a pollination syndrome with these lovely little birds.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4]

The Ant-Farming Tillandsias

Tillandsias are all the rage. Their relative ease of care has found them included in seemingly every terrarium sold these days; often in very inappropriate circumstances that result in their death. There is no denying that these epiphytic relatives of the pineapple are unique and beautiful plants but I would argue that their ecology is probably the coolest aspect about them. I am particularly fond of the bulbous species because of their relationship with ants.

That's right, there are upwards of 13 species of bulbous Tillandsia that offer up housing for ants. If you look closely at the leaves of these species, you will notice that they roll up to form tubes that lead down into the bulb at the base. The space between the leaves forms a hollow chamber, functioning as a perfect microclimate for ants to nest. In many habitats, these Tillandsia offer better housing than the surrounding environment. One would be surprised at how many ants can fit in there too. Colonies containing anywhere between 100 - 300 ants are not unheard of.

The rewards for the plant are obvious. Ants provide nutrients as well as protection. In return the ants get a relatively safe and dry place to live. Ant domatia have been recorded in roughly 13 different species, many of which are some of the most commonly sold Tillandsias on the market such as T. baileyi, T. balbisiana, T. bulbosa, and T. caput-medusae. If this doesn't make your hanging glass Tillandsia orb even cooler then I don't know what will.

Photo Credits: scott.zona (http://bit.ly/16kZ1RR) and Alex Popovkin (http://bit.ly/1BXMEUH)

Further Reading: [1] [2]

Snuffing the Fire

Few childhood memories are more fond to me than catching fireflies on summer evenings. These little beetles are famous the world over for their dazzling light displays. Using chemical means, they are some of the most efficient light producers ever discovered. Their displays are for the purpose of mating and there are as many variations on the theme as there are species. Sadly, like so many natural wonders that we take for granted, fireflies are disappearing from our wild places. Future generations may never know the joys of these natural fireworks. 

Exactly why we are seeing a decline in fireflies is not certain. Researchers are only just beginning to uncover the secret world of the firefly. The answer is undoubtedly complex, however, evidence is beginning to pour in that we should look no farther than ourselves for the cause.

Fireflies require a few things to get by. The first is some sort of standing water. They seem to love ponds, creeks, rivers, and vernal pools. Second is tall grass and a lot of forest litter. Their larvae live and hunt in and amongst fallen logs and plant litter. Though we aren't entirely sure what their larvae eat, they are certainly hunting things like snails, slugs, and small insects, which also require moist areas with a lot of debris. Fireflies also need taller plants like grasses. They will climb up the stems to begin their aerial light displays. Finally, fireflies need darkness. They communicate by light and any surplus light sources are likely to mess them up. 

With increasing human development, former firefly habitat is giving way to paved roads and chemical laden lawns. Mowers run endlessly during the summer, eliminating fireflies and their habitat. People are needlessly clearing land of brush piles and fallen logs, which their larvae as well as their prey need. Light pollution is only getting worse too. As with many other insects, the wanton use of insecticides are undoubtedly taking their toll as well. Areas that once harbored huge populations of fireflies are quickly becoming overrun with human traffic as new housing, commercial and other forms of development garble up what free land remains. 

At this point you may be wondering what you can do to help. If you are a land owner, please consider the following:

- Turn off outside lights at night when they aren't needed
- Let logs and other plant debris accumulate in places around your property
- Consider creating a water feature of some sort
- Avoid the use of pesticides and fertilizers on your lawns
- Plant native plants
- Don't over-mow your lawn and leave some areas un-mowed

The best part about these solutions is that they benefit so much more than just fireflies. Native wildlife will be all the better if you take these steps to making your property more ecologically friendly. We are lucky to be aware of this issue but we must take matters into our own hands. Get out there and enjoy nature and try to be a bit of steward at the same time. 

Photo Credit: Peilun Hsu (http://bit.ly/1rJkufG)

Further Reading:
http://www.firefly.org/why-are-fireflies-disappearing.html