Friday 5+1: The Brief Life of a Lethocerus Egg

In case anyone reading this doesn’t know already, a large part of the research I do deals with giant water bug eggs.  I spend a huge amount of my time staring at eggs with an electron microscope, rearing eggs, doing experiments on eggs, grinding eggs up to do chemical analyses, counting eggs…  Perhaps I spend a little too much time with eggs, though I’ll leave you all to decide that on your own.  Giant water bugs have a lot of interesting features to recommend them (including some really beautiful structures on the egg-shell), but I think one feature in particular is especially worth mentioning.  If you know much about eggs in general, such as bird eggs, reptile eggs, or other insect eggs, you probably know that most animals lay their eggs and the embryos develop within the confined space inside.  This isn’t what happens with giant water bug eggs!  Instead, they absorb water (a lot of water!) and puff up the eggshell from the inside so they get bigger over time.  In fact, giant water bug eggs, as big as they are to begin with, nearly double in size between the time they are laid to the time the nymphs hatch and swim away.  Their eggs GROW!  Simply spectacular.

For today’s special Friday 5+1, I’m going to share a series of photos I took of the eggs of the giant water bug Lethocerus medius a few years ago that show how they grow as they develop.  It may a little difficult to see if you don’t spend as much time around these things as I do, but compare the Day 1 eggs to Day 6 eggs and you should be able to see the change clearly.  I’m also going to give you a bit of commentary so you know what to look for.  Let’s start at the obvious place…

Day 1.  Lethocerus medius eggs start off just shy of 3 mm long and about 6 mg, a substantial insect egg.  This species is an emergent brooding giant water bug (see my post about giant water bug child care for more information), so it lays its eggs on vegetation out of water.  As you can see, the eggs are very tightly packed so that most of each egg is touching the others with only a small part of the top free:

day 1 eggs

Day 2. On the second day, things look rather similar from the outside, though the eggs get a little taller and a little heavier:

day 2 eggs

Day 3.  By day 3, the eggs have gained almost half a millimeter in height and 0.2 mm in width.  The weight has gone up too, nearly 2 mg.  You can start to see the eggs bulging at bit at the top:

day 3 eggs

Day 4. The eggs are growing more noticeably now, gaining another 0.5 mm and 2-3 mg overnight!  You can see how the eggs start to crowd each other a bit.  They’re fixed in place at the bottom, but they start to spread out at the top so that they can all fit:

day 4 eggs

Day 5. By day 5, the eggs have stopped growing up and begin to grow out a bit, adding 1/10th of a millimeter and another 2-3 mg in weight.  The eggs are now over 4 mm tall and 2 mm wide and weigh nearly 13 mg! The eggs continue to spread apart at the top end as they increase in size so that you begin to see gaps between the eggs and can start to see the sides of the eggs as well as the tops:

day 5 eggs

Day 6. During their last day in the egg stage, the eggs have topped an enormous 5 mm (that’s HALF A CENTIMETER!  Huge!) in height, nearly 2.3 mm in width near the top of the eggs, and reached 14+ mg!  These are truly big eggs now, and have nearly doubled in height in 6 days.  You can see nearly all the way down to the stick in some of the gaps between the eggs and the eggs themselves look like they’re ready to pop:

If they make it this far, you’ll usually see the following events the same night.  Hatching:

hatching eggs

… and then the newly hatched nymphs swim away, leaving behind only a stick and some empty shells:

hatched, empty eggs

And there you have it!  A wonderful set of growing insect eggs! Lethocerus medius isn’t the only water bug that exhibits this amazing growth either.  Other giant water bugs have shown similar patterns, including a mix of emergent brooders and back brooders.  Growing eggs seem to be quite common, if not universal, within the family to which the giant water bugs belong, the Belostomatidae.  Just one more way that giant water bugs are among the most amazing insects ever!

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