At first glance, this moder-day dragonfly and its fossilized ancestor probably don't look very different. Both seem to have long, slender bodies, two sets of wings, and large eyes. Would it surprise you to know that the dragonfly ancestor, Meganeura, lived about 300 milion years age and had a wingspan of 75 cm? 1ts the largest known flying insect! In comparison, the largest modern dragonfly has a wingspan of only 16 cm. Think about why we don't see such large insects anymore. List your ideas below.

There are several factors that could explain why we don't see such large insects anymore, despite the existence of their fossilized ancestors like Meganeura. Here are some possible reasons:

1. Oxygen Levels: During the time when Meganeura lived, oxygen levels in the Earth's atmosphere were significantly higher than they are today. Insects have a passive respiratory system, meaning that oxygen is delivered to their tissues through tiny tubes called tracheae. Higher oxygen levels would have allowed insects to extract oxygen more efficiently, facilitating the growth of larger bodies. As oxygen levels decreased over time, it would have become more challenging for insects to sustain larger body sizes.

2. Predation Pressure: Larger insects like Meganeura would have been more noticeable and potentially vulnerable to predation. In response, natural selection might have favored smaller body sizes as a means of avoiding predators. Today, insects face a wide range of predators, including birds, bats, and other insects. Smaller body sizes can provide advantages in terms of maneuverability and hiding from predators.

3. Metabolic Constraints: Insects have an open circulatory system, which limits their ability to deliver oxygen and nutrients to their tissues efficiently. Larger size would require a more complex circulatory system to meet the metabolic demands of the body, which may have imposed physiological constraints on reaching larger sizes.

4. Ecological Niches: Over time, ecological niches and available resources for insects may have changed. Larger insects might have occupied specific ecological roles or utilized resources that are no longer available or have been filled by other organisms. Changes in plant species, for example, could have influenced the availability of suitable food sources for large insects.

5. Evolutionary Trade-offs: Evolution often involves trade-offs between different traits. While larger body size might offer certain advantages, such as enhanced flight capabilities or reproductive success, there may have been associated costs. These costs could include increased energy requirements, longer development times, or reduced reproductive output. If the costs outweighed the benefits, natural selection would favor smaller body sizes.