I found a little surprise this morning when I headed towards the compost bin. Its lid and sides were covered with maggots.
It was a reminder that flies lay eggs if the right smell triggers them to do so, even if there’s an obstacle between the potential food source and eggs. The latter eventually turn into larvae, which are capable of moving. On other occasions, I have occasionally found them at the bottom of the bin. They probably slipped under the loose lid and crawled down the inner sides of the bin or sneaked through one of the two small holes at the corners.
This is an inadvertent variation of a classic Francesco Redi experiment, which revealed that flies do not spontaneously generate out of rotting organic matter.
The lack of maggots in the sealed flask was not evidence enough to disprove the hypothesis that the decomposing meat was the source of the maggots. (Redi, of course, used more than one flask for each of the conditions.) One could have argued that fresh air was needed for the maggot-forming process to start. Without access to it, the maggots could not develop. Thus had Redi stopped after the first set of observations, his experiments would not have been conclusive.
By covering the flask with a screen or gauze, air was able to penetrate and also compounds from the decomposition could attract flies, signalling females to lay eggs. Maggots would eventually appear on the gauze with no way to enter. The lid on my bin, with its imperfections, was more like the one with the gauze than the one with the seal. Without gaps the flies would never have smelled the compost.
The experiments were conducted in the 1600s. A couple of centuries later, Pasteur demonstrated that microorganisms involved in the souring of wine do not spontaneously generate from a nutrient mixture. What is analogous to the gauze (or imperfect lid) in his experiment was the S-trap, something that Pasteur himself devised. ( Or did his clever technician think of it? 🙂
I returned the bin a few hours later with the intention of hosing down the maggots. Most were gone from the surface. Had birds eaten them? When I opened the lid, I realized how quickly they crawl. There were scores of them on the bin’s inner walls, journeying down towards their food source.
Sciences in the Mural of Life 