This is a pretty cool question:

Hi,

I killed a large green grasshopper-like bug, only to find several reddish- brown rice-shaped things inside. Are they eggs? Can they still hatch?

I’m not sure what type of insect it was, but my best guess after an Internet search is… conehead/katydid.

Also, why might I find these insects in my home? Surely bright green bugs feed on vegetation?

In this case, it sounds like a Katydid which got inside a house by accident. It’s common for bugs to get inside a house, and katydids just kind of keep looking for a way to get out when this happens. Here, the katydid was smashed and some eggs popped out…which led to the above question.

If you crush a bug, and eggs come out, they’re not going to hatch. The eggs need to be activated before they can develop, and taking the eggs out of the female without them being laid bypasses that process.

…but how are the eggs activated?

What, exactly, has to happen while the eggs being laid for them to start to hatch?

Fertilization

Most insects have two sexes, which makes things a bit weird when we talk about laying eggs. For most animals, the egg needs to be fertilized before it begins to develop. In most animals who have two sexes, it’s assumed that fertilization kicks off egg development.

Um, do ladybugs mate or is this just one cannibalizing another? @BugQuestions pic.twitter.com/z5z7SEZRy8

— Reg Reader (@RegReader) 9 de septiembre de 2017

For insects like the Tsetse, or any livebearing cockroach, fertilization is probably what causes the egg to start developing. The eggs are held inside the body before hatching, so they’re not really exposed to any environmental factors. There may be some stuff from the female which kicks off egg development…but in most animals the stimulus is fertilization. I can’t really think of a reason to not think this isn’t the case for these animals.

In humans, fertilization lets calcium into the egg…which sets off the Phosophlipase C pathway. It’s a really long biochemical pathway, and the details aren’t super important here, but it eventually results in the cell dividing and the embryo growing. How, exactly, fertilization activates the eggs in insects isn’t super well understood on the other hand. Drosophila, which is one of the best understood insects, doesn’t require mating to activate it’s eggs and the exact requirements to begin development aren’t actually known.

But, not all insects need both sexes for egg development. And for many who have two sexes, fertilization alone isn’t sufficient to kick off the process of baby-making.

So in animals which don’t need males to make babies, how does this work?

Pressure

The eggs of wasps don’t harden until they’re outside, allowing them to deform while activating. Image Credit: Horner & Wolfner, 2008.

Bees and wasps are kind of a cool system to study, because they have two sexes…but don’t actually need to mate to make babies. They have a system called haplodiploidy (explained further here), where males come from unfertilized eggs. Because they don’t need to be fertilized to develop, there has to be something which kicks off egg development.

It turns out that pressure is what causes the eggs to start developing for bees and wasps. As the eggs are laid, they pass through the stinger. The stinger is slightly smaller in diameter than the egg, and this smushes it just a little bit. Not enough for it to crack, but enough for it to temporarily change shape. Simply passing through a tube, whether it’s a glass capillary or a stinger, will cause it to start to developing into a larva.

Water

A lot of insects have ovipositors, including sawflies and crickets. However, passage through the ovipositor isn’t the thing which kicks off development in cricket, sawfly, and drain fly eggs. Even though they need to mate for the eggs to develop, water is what kicks off this process in crickets and drain flies.

Sawflies are the most primitive relatives to bees and wasps, so you’d think that they would activate their eggs in a way like honeybees. Crickets, with their long ovipositors, would seem to be in a similar situation. Neither live in aquatic environments, so on the surface this doesn’t make a whole lot of sense.

Cricket eggs. The eggs on the left have been fertilized, but remain unactivated. The eggs in the center have been fertilized and activated by submerging them in water. Image credit: Sarashina et. al 2003

Despite this, the environment their eggs end up in isn’t that different than the environment in which drain flies lay their eggs in. Sawflies lay their eggs inside plants, and crickets lay their eggs in moist soil. Drain flies lay their eggs in water. In all the environments these animals lay their eggs in, there’s water that can cause the eggs to develop.

…something else?

This is where things get a bit weird. Parasitoid wasps can have their eggs activated by passing through the ovipositor. Scientists can watch the eggs develop a little bit, but they die before they hatch. There’s something else that’s needed.

Parasitoids live inside other insects, and inside those insects there are a lot of proteases, which are proteins which slice up other proteins. It appears that the parasitoid Venturia needs to be activated by passing through the ovipositor. Once it’s in the caterpillar host, those proteases coax the egg to develop into a larva and hatch.

Without the proteases, the egg starts to develop but dies halfway through. The proteases are needed to keep the egg development on track, although it’s not clear why or how.

The Bottom Line

Egg activation is kind of neat, because there’s no actual practical use. The eggs are always activated as soon as the female lays them, but it pays to know why they begin to develop. Dissecting eggs from the female and incubating them isn’t really that practical for insect rearing on any appreciable scale. Because they’re already developing when they’re laid, we also know that you can’t use this process for a pesticide target of any sort.

It does, however, tell us some important things. Many pest insects have some degree of parthenogenesis, and we’d really like to know how that evolved. Knowing that tells us a lot about how these insects reproduce…and that information is something we can use if we understand it well enough.

Interestingly, while doing research for this article, I stumbled upon a paper where a group of scientists managed to breed a parthenogenic line of Drosophila flies. We don’t know exactly why they’re different down to the biochemical details…but these eggs need to start developing somehow…and knowing how and why that happens is an important part of this process.

Works Cited