Have you ever wondered why insects are so small? Or why there aren't any insects the size of a dog or a cat? Well, a long time ago, insects were much bigger than they are today. During the Carboniferous period, around 350-300 million years ago, the skies were home to dragonflies with wingspans measuring almost 75 centimeters. Meanwhile, another giant arthropod, a type of millipede known as arthropleura, grew up to 2.6 meters long. But why don't such large terrestrial arthropods exist anymore? Most scientists hypothesize that the lower oxygen content in the atmosphere today is the main thing restricting insect body size. This is because an insect's gas exchange system operates via simple diffusion. An insects body has tiny tubes that allow air to enter. This air then flows through even smaller branches, that lead to individual cells. This is called the tracheal system. The tubes are referred to as trachea, and the smaller branching tubes that exchange gases with the cells are the tracheoles. Higher oxygen content in the carboniferous atmosphere would have allowed oxygen to diffuse more easily through the longer tracheal tube of large arthropods. Of course, being small isn't necessarily a bad thing from an evolutionary point of view, it minimizes the resources needed for survival, and allow insects to make use of micro habitats. For example, a whole colony of aphids can survive on a single plant. Meanwhile, many species of insects find refuge in small spaces under leaf litter, or even between grains and sand. As few resources are needed for development, insect generation times are fast, and therefore, insects have rapid rates of evolution. Insects are ectotherms, that rely on external sources of heat to regulate body temperature. Having small bodies means that insects have a large surface area to volume ratio, enabling them to quickly absorb heat from their environment for thermoregulation. Another advantage to being small is that muscular action is extremely efficient. For example, ants can lift more than 50 times their own body weight. The details of why this is possible will be covered in a later module. Being physically small also means that insects can take advantage of passive modes of dispersal. Passive dispersal occurs when animals use modes of locomotion other than their own muscles, to move within or between habitats. A good example of this is wind dispersal. Some immature insects take advantage of the wind by releasing silk threads to catch a breeze. This can carry them to new locations without expending energy. Although there's small body size provides insects with many benefits, it comes with disadvantages as well. Being small means that insects may be at greater risk of being eaten by larger predators like birds and mammals. There is also greater vulnerability to damage from natural forces, such as being blown away in the wind, or getting trapped in water. Most importantly, the high surface area to volume ratio of insect bodies puts them at risk of water loss through evaporation. As a result, insects have various adaptations such as the waxy outer layer of the exoskeleton, the epicuticle, to help prevent drying out. Remember from our previous lesson that the exoskeleton not only protects insects from drying out and predation, it also provides sites for muscular attachment. It can be further modified to serve other functions such as camouflage and mimicry. While the exoskeleton provides many benefits, it requires substantial modification for insects to sense their environment. Imagine you are wearing a suit of armor. Now, think of how difficult it would be for you to touch, smell, or see your surroundings through the armor. To solve this problem, insects have special sensilla, that perceive and transmit signals across the exoskeleton to the central nervous system. This allows insects to successfully navigate in their environment. As a result, insects have developed a sophisticated nervous system. We will introduce you to the nervous system of insects in a later module. So the small size of insects partly explains some of their success and diversity, but what other traits are important to their success? In the next video, we'll examine how metamorphosis, dispersal abilities, and reproductive capabilities have all contributed to the success of insects
