Trail Blazing #16 - September 2017 - "Egg Roles"
by John Lampkin
What do insect eggs have to do with Sir Walter Raleigh and his cannonballs?
When a visitor to the SCC Nature Trails examines foliage very carefully, tiny engineering miracles abound--insect eggs that come in an astonishing array of shapes and sizes. Some, like the Spicebush Swallowtail butterfly, lay a single cannonball-shaped egg, one per leaf. Why just one per leaf? Because when the egg hatches, the caterpillar needs the whole piece of real estate. It soon folds it over, seals it and then hides in the protective cubby during the day and emerges to feed at night. Other species like the Monarch can be voracious mini-cannibals and will eat their siblings if they share a leaf. The savvy ovipositing female knows this and keeps her Hannibal wannabes widely separated.
The Io Moth lays a small cluster of patterned eggs, as shown top right. Io caterpillars are not cannibals and in fact are gregarious, feeding in groups and marching single file to new leaves so there is no problem with eggs being laid in close proximity.
Some species take egg laying a step further and use a safety-in-numbers strategy. The densely packed cylindrical clutch of stink bug eggs on the lower left looks like a store of miniature 55-gallon drums, cemented to the leaf with yellowish resin.
The pic bottom right show part of a clutch of 1000 half-millimeter-sized owlet moth eggs stacked in layers bound together with tiny fiber threads. The black critters are parasitic platygastrid wasps that inject their own even tinier eggs inside the white moth eggs in the stack. Presumably, they lay as many as possible and possibly this is why the moth eggs are densely stacked the way they are. The outer layer could be sacrificed to predators while protecting the inner layers. Only two of the 1000 moth eggs need to survive to maturity to maintain a stable population. As an aside, Platygastrid wasps have strong maternal instincts and will defend both their own eggs and those of the host moth against other predators and possibly that is what these in the image are doing.
In 1587, Sir Walter Raleigh posed a question to mathematician Thomas Harriot: “What is the most efficient way to stack cannonballs on ship?” Tom responded in math lingo with an equation, but in practical terms, all Walt had to do was to look at an array of Owlet moth eggs. Moths know the answer! They have to because their survival depends on it. To learn more, Google “Close-packing of equal spheres” or click here and you will be as smart as a moth on the SCC Nature Trails.