This Ant is an "Aphid Rancher." (SE Costa Rica)
To the student of nature the vegetation of the tropics will ever be of surpassing interest, whether for the variety of forms and structures which it presents, for the boundless energy with which the life of plants is therein manifested, or for the help which it gives us in our search after the laws which have determined the production of such infinitely varied organisms
- Alfred Russel Wallace (British Naturalist) 1895
As is apparent above in the quote by renowned British naturalist Alfred Wallace, the ecosystems of the neotropics, such as those of Costa Rica, have fascinated scientists for over 100 years. The sheer number of organisms that call the neotropics home is astounding. Equally as fascinating as the biodiversity that exists in this region of the world is the ability of all these living things to interact with one another to create one of the most complex and successfully inhabited ecosystems in the world. However, attempting to understand how this ecosystem functions by analyzing the relationships between all the organisms, from the microbes to the towering trees, is simply unrealistic. As opposed to this universalistic approach to understanding ecosystems like those of Costa Rica, it is more manageable and easier to understand from a specific perspective. In this way, one or a small group of organisms, through their interactions serves as an example of the diversity and interdependence that exists in the neotropics. For this purpose, the wasps of Costa Rica serve as a model organism to represent the biodiversity, interdependence, coevolution, and evolution of neotropical organisms. By studying Costa Rican wasps and their interaction with others and their environment, many of the key principles that contribute to the complexity of the Costa Rican ecosystems are illustrated.
Throughout the world there exist two general types of wasps: solitary and eusocial. Unlike solitary wasps which live alone and have no contact with their offspring after laying their eggs, eusocial wasps consist of those that function in a stratified community with one or more queens. Eusocial wasp communities, much like the honeybee communities of the United States, consist of several different castes of wasps, all within the same species, that perform very distinct jobs inside and outside the nest. In order for the wasp colony to be truly eusocial, some of the castes must be sterile, working their entire life without the ability to fertilize the queen. Some of the 900 different species of both eusocial and solitary wasps that call Costa Rica home are very similar, while others have developed very different methods of interacting with each other and their environment; from the way they communicate with each other using chemical signals (usually secreted from either the sternal or mandibular glands) to the materials and methods they use to build their communal or solitary nests. Typically the nests of Costa Rican eusocial wasps number anywhere from 100 to 1000 individuals, smaller than the colonies of wasps that can be found in the more temperate environments of North America. The reason for the smaller nest size could be due to the inability of a nest, primarily made from chewed wood pulp and attached to buildings or trees, to support a very large colony in a humid environment like that of the neotropics. While different species of eusocial wasps have differing diets, prey generally consists in large part of other insects (grasshoppers, katydids, butterflies, spiders, larvae). These insects are killed or paralyzed (depending on the wasp species) with very precise stings that target the nerve centers of the prey and then are dragged back to the nest. At the nest, the legs and antennae are often removed and the hemolymph is sucked from within the paralyzed or dead insect. The most common prey of insectivorous wasps is insect larvae presumably because they put forth little resistance. And while insects are often wasp prey, wasps themselves are most often preyed upon by birds, monkeys, spiders, and bats. In fact, at the beginning of the Costa Rican wet season in May, after many nests have been attacked and damaged by birds, eusocial wasp colonies are often seen in swarms, attempting to find a site at which to establish their new nest.
One of the most striking differences between the life cycles of eusocial and solitary wasps, aside from solitary wasps living and surviving as individuals rather than in a colony, is that while eusocial wasp larvae are nurtured in the nest by a caste of wasps, many solitary wasps are parasitic. Some solitary wasps have potent venom that rather than killing their prey, serves as an anesthesia of sorts, paralyzing their prey for as many as 4 months (Spider wasps, which often prey upon spiders as large as tarantulas, have been observed to paralyze their tarantula prey for up to 5 months while the spider remains alive and immobile). While the prey is immobilized by the venom, many solitary wasps inject their eggs into the still living prey (only female wasps can sting, the stinger being used also as an ovipositor). After the venom is metabolized, the victim regains movement and continues to live until it is eaten alive by the developing wasp larvae inside it (depending on the solitary wasp species, the prey may die before the larvae actually develop or it may continue to live). Exhibiting their sophistication and intelligence, many solitary wasps sting their victims (especially the more menacing ones like spiders) in their mouth and leg parts first, rendering the victim unable to fight back while the wasp administers several more precisely placed stings into the nerve centers of the future egg host. The predation upon many different insects by wasps plays a large part in the control of many insect populations within the neotropics. Also, another striking difference between social and solitary wasps is that while social wasps build conspicuous nests, solitary wasp nests are much harder to locate, often in a small underground burrow or attached to the underside of a leaf or branch.
As seen above, even by looking within a single order of insects, the differences in biology can be astoundingly complex. However, not only are wasps a good example of the biodiversity within the neotropics, but they also serve as great examples of how starkly different living entities can be closely related and dependent upon each other for survival. For instance, Chalcid wasps and fig trees in Costa Rica are 100% dependent upon one another for their survival, having coevolved so closely that if either was removed, the other would be unable to live. The relationship involves the wasp laying her eggs in the fig, the eggs hatching, and the new wasps fertilizing the fig flower. It begins with a female Chalcid wasp burrowing into the underdeveloped fig of a fig tree (at this point the fig is really an enclosed flower with the male portion of the flower being near the skin of the fig and the female portion being the innermost portion). As the wasp burrows through the not-yet-developed, inactive male portion of the flower, her wings are scraped off. Upon reaching the center, she fertilizes the female portion of the flower with the pollen from the flower she was born in, lays her eggs and then dies. When the eggs later hatch, a chemical signal from the wasps signals the plant to surround the larvae with nutrient rich plant tissue which serves as an ideal environment for growth and also offers a food supply. The male wasps hatch and develop first in order to seek out the female wasps, still in their larval stage, and mate with them. The male’s fun is then over as their only other job is to enlarge the original entrance to the fig made by the now deceased mother before they die. When the females develop, they are able to leave the fig while retaining their wings thanks to the enlarged hole. As they leave the fig, they are dusted with pollen from the now mature male portion of the fig. They then go in search of another fig tree and begin the process anew. Obviously these two living things, one a plant and the other an insect, have evolved life cycles that depend entirely upon the other, illustrating how intertwined life in the neotropics can be.
Although the neotropics represents an amalgam of infinitely complex ecosystems, they can be understood through small, isolated examples of organisms that, through their relationships with other organisms, help to simplify the many interactions that make the neotropics so diverse. Rather than the universalistic approach, using the species of both eusocial and solitary wasps that reside in Costa Rica and exploring how they develop and how they interact with both other wasps and their environment makes it easier to understand the evolutionary pressures that have helped mold the many intraspecies’ and interspecies’ interactions that are commonplace in the ecosystems of Costa Rica. From playing a vital part in controlling insect populations to being the sole pollinators of a common plant, the wasps of Costa Rica have developed a biology that is intertwined with that of many other organisms. And while they may not be the most glamorous predators or insects in the neotropics, these wasps will forever sing their part in the chorus of the neotropical ecosystems.
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