Title text: I feel bad for Earth 2 and their shadowflies.
Fireflies are varieties of beetles that emit flashes of light, a process known as bioluminescence. Megan explains that while this ability is not uncommon in ocean-dwelling species, it's pretty rare on land. Presumably, this is because sunlight doesn't penetrate very far under water, so there's evolutionary pressure there to develop a process to generate one's own light. Land animals, on the other hand, have much less need to generate their own light -- even at night there's light from the moon and stars, so nocturnal animals tend to have good eyesight or other enhanced sensory abilities (echolocation, olfaction, etc). It's thought that bioluminescence in fireflies originally served aposematically to ward off predators from larvae, but that it was co-opted as an adult mating signal. There are many varieties of bioluminescent fungus species, and the ecological benefit of that effect is uncertain. Some ocean-dwelling species, such as anglerfish, use their bioluminescence as lures for prey, whilst other creatures use it as camouflage. Yet others may employ such light as warning and/or as mating signals to deliberately reveal (not conceal) their presence. Often creatures with sufficient control of their phosphorescence can even fulfil multiple purposes according to need. These functions are similar to how organisms in more well-lit environments might normally rely upon surface hues (of skin, fur, feathers, scales, etc.) that work more (or less) passively under external illumination. Megan also mentions fatty acid enzymes, presumably a reference to luciferase, a class of enzymes used by fireflies and many other bioluminescent organisms.
While Megan and Cueball are walking towards a field with fireflies, she points all this out and suggests that it was just a fluke that fireflies developed this ability. When they see the firefly display, the two of them agree that we're lucky to be on a planet where this happened. Sadly, the moment may be fleeting as fireflies are going extinct, at least in North America.
The title text suggests that there's an alternate Earth that has "shadowflies", which are presumably like fireflies but cast shadows instead of light, and that this isn't as nice for the people there. Since everything that's opaque casts a shadow, this isn't really much of an ability. Possibly Randall's shadowflies have some sort of vantablack-like coating, or somehow create darkness near them beyond merely absorbing incident light (in violation of physics as we understand it). "Earth 2" is the name of an alternate world in the DC Comics universe, but it's unclear if this is the specific world being referred to. A presumably different alternate earth has been mentioned before. Earth 2 may also be a reference to an actual exoplanet which appears very similar to Earth.
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- [Cueball and Megan are seen in silhouette (drawn in white against a black background) throughout the strip. They are walking to the right.]
- Cueball: So where is this spot?
- Megan: It's just up ahead.
- [Cueball and Megan continue walking.]
- Megan: You know,
- Megan: fireflies didn't have to exist.
- [Closeup on Megan.]
- Megan: The ocean has lots of bioluminescence, but it's less common on land.
- Megan: Creatures that glow are pretty rare here.
- [Cueball and Megan continue walking. The light of one firefly is seen to their right.]
- Megan: So it's not some niche whose exploitation was inevitable.
- Megan: If insects hadn't stumbled on their fatty acid enzyme trick, Earth just wouldn't have fireflies.
- Megan: Ooh, look!
- Megan: They're starting!
- [A panel as wide as the first four combined. Cueball and Megan stand in the tall grass, at night, and many stars can be seen in the sky above them. Above and below them, to their left and right, the lights of dozens of fireflies can be seen.]
- Cueball: I'm glad we got a planet that has these.
- Megan: Yeah, it's a good one.
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