What is a gill?

Sometimes one encounters useful tidbits of insight in unexpected places. Last night, while relaxing in bed with The Insects by Gullan & Cranston* (2005), I came across a characterization of a gill. In their discussion of the caudal lamellae of an aquatic insect larva, they list these criteria for an organ to be considered a gill:

1. Possession of a large surface area.
2. Being moist and vascular.
3. Being able to be ventilated.
4. Being responsible normally for 20-30% of oxygen uptake.

The requirement that a gill be moist is almost redundant, since gills are possessed mostly by aquatic animals. In other words, they are wet anyway. But let us keep it, for as I mentioned in this post, gills and lungs need to be kept wet to keep gas exchange going.

The underlying anatomies of a gill and a lung are the same: a large, wet area overlaying blood vessels and exposed to an oxygen-rich medium. Therefore, these criteria also ably to a lung. Go to this post for more on this topic.


*Yes, my bedtime reading may be unusual by ordinary standards.