The amazing world of the small.
Arthropods have existed for more than 350 million years on earth, an exorbitant amount of time when compared with humans. Fossil records for hominids only exist for about three million years. It is reasonable to think that the strategies arthropods have developed allowed them to have the success they have.
Arthropods are divided into four major groups: insects; myriapods (including centipedes and millipedes); arachnids (including spiders, mites, and scorpions); crustaceans (including slaters, prawn, and crabs).
Insects comprise the most diverse group of land animals – about 900,000 named species, while arthropods represent about 80% of all animal species. The great evolutionary potential of arthropods is linked to the development of a cuticle that formed the exoskeleton that characterizes them, allowing them to leave the water and colonize the land avoiding desiccation. Insects are the first animals to develop flight – 150 million years before reptiles, birds, and flying mammals. The ability to fly allowed insects not only to colonize the land but also airspace.
Arthropods appeared in fossil records dating back 542 – 488 million years ago to the Cambrian period. Evidences were found that in the Carboniferous (359-299 million years ago) they have developed large sizes, like Odonata (dragonflies) with a wingspan of over 75 cm, or the case of millipedes 50 cm wide and almost two meters long. Such colossal dimensions of arthropods during the Carboniferous Period was probably due to increased oxygen concentration in the atmosphere (35%) than there is today (21%).
Arthropods have been able to inhabit most all ecological niches on earth and feed on a wide variety of plants and animals for their food. Over millions of years, arthropod species have acquired an extraordinary ability to survive in ever changing and often adverse environmental conditions.
The diversity and the most varied relationships of forms are the result of their interactions with the environment and their various situations. The ability to adapt to constant environmental changes over millions of years has enabled arthropods to evolve a near infinite number of species. The astronomical number of individuals allows an unimaginable opportunity to evolve new species to adapt to a changing world. An infinite laboratory led by virtually minute animals allows the creation of many adaptation and strategies to survive.
That is why arthropods were able to build condominiums with air conditioning, build acoustic equipment, make paper, take slaves, cultivate gardens, breed other animals, create their own oxygen tanks to breathe under water and cause explosions. Arthropods created many life sustaining adaptations well before humans appeared on earth.
In arthropods, body temperature varies with ambient temperature, and referred to as ectotherms, ie, they can only rely on external sources of energy, generally the sun, to regulate their temperature. Some insects can generate some heat by muscular activity. While other insects have acquired the ability to survive extreme cold temperatures by storing ethylene glycol in their tissues, a chemical compound that prevents ice crystals from forming in their tissues.
Most arthropods have some degree of vision with their compound eyes. Some can see in color and some can see ultraviolet light. The compound eyes are geometrically perfect and are the primary sensory organ in insects. These compound eyes are formed by bundles of cells called ommatidia. Ants have only a few ommatidia, up to 10,000 while dragonflies have up to 10,000 ommatida in their compound eyes. The more ommatidia, the greater the visual acuity.
Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many arachnids have book lungs. Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and arachnids.
The stiff cuticles of arthropods normally would block out information about the outside environment, except that the cuticles are penetrated by many sensors or connections from sensors to the nervous system. In fact, arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae. Pressure sensors often take the form of membranes that function as eardrums but are connected directly to nerves rather than to auditory ossicles. The antennae of most hexapods include sensor packages that monitor humidity, moisture, and temperature.
Insects are the most diverse and abundant group of animals within arthropods. In addition, there are more than 65,000 named species of arachnids and 67,000 named species of crustacean, and about 16,000 named species of myriapods. However, scientists estimates that the numbers of arthropods are even greater.
Another peculiarity in this small world, which possibly nurtures fiction, is that of metamorphosis. During this amazing process, caterpillars transform themselves into adults – such as butterflies, moths, wasp, bees and beetles.
The amazing aspect of metamorphosis, beyond the adaptive advantages, which creates the possibility of avoiding competition between youth and adults, since each one has a different lifestyle, where even their food and habitat often differ, is that once the caterpillar is locked in its cocoon, there is a chemical rearrangement to create an adult animal.
In the process of metamorphosis, major structural and physiological changes occur, there are changes in size and an increase in cell number, but also there occur changes of cell differentiation, i.e. cells acquire a shape and a specific function to fulfill in this new stage.
Conservation situation
Arthropod are declining in abundance, biomass, and biodiversity worldwide. There is data and anecdotal evidence reflecting the declining of several species or group of species, but the global decline is still unquantified.
There are several reasons why arthropods are declining:
A planet without arthropods…
The numerous, abundant and wide variety of arthropod species play a significant role in the world’s ecosystems.
Insects are the food source for many amphibians, reptiles, birds, mammals, and fish making their roles in food chains and food webs extremely important.
Insects pollinate a vast majority of plants that produce seeds, and that includes our crops and the food we eat. The honeybee pollinates most of our agricultural crops.
Arthropods enrich the soil by recycling nutrients and disposing of dead organisms and waste – essential ecosystem services.
Arthropods also provide various products such as silk, honey, wax, medicine, and dyes.
Small in size, but with a number of individuals estimated in the billions, arthropods have an extraordinary impact, contributing to the functioning of the earth’s ecosystems.
It is not only their vast numbers, but the dependency of ecosystems, and humanity on them, that makes the conservation of insect diversity critical for future generations. Insects are irreplaceable components in our survival as are other invertebrates and biodiversity in general.
Reflections
Conservation of arthropods is essential to ensure the existence of an intricate network supporting the earth’s living ecosystems by complex relationships, which allow the fragile balance of life to continue. The world’s human populations depend on healthy arthropod populations and healthy ecosystems.
Protections of healthy arthropod populations, especially insects are essential to sustain life on earth. In addition, the intrinsic value of life as we know it on earth now is important to maintain and protect.
Appreciating and knowing this world, often unknown, the world of arthropods, discovering its beauty, its complexity and some attractive aspects of their lives, as also some of their strategies for survival and reproduction, can help us to see the sense of their behavior and reflect on their importance as living beings, and allowing us to extend this appreciation to the rest of the life forms that inhabit every corner of our vast and wonderful planet.
