Naturalist’s Notebook: Snail Shells and Praying Mantises

A few last apples hang in the orchards, wet with dew. It is November, and it came fast. Flickers now come to pick at the soft fruits, too soft even for applesauce, the nectar gaining strange notes of vinegar. The branches shudder, and moss replaces leaves.

Elsewhere, robins are all over the huckleberries. I walk through young forest and watch them dash among the bushes. In areas with maples, I find bands of varied thrushes down from the mountains. They pick through banks of leaves for treats. When they startle, they fly silently to a branch and peer down to see if they have been tracked.

Almost winter. There was a period there in September, nominally past summer but with the apples just ripening, where I got to thinking that maybe my mood in winter isn’t so different from that in summer after all, that long work hours and sun and freeform life would march on. That thought’s gone, banished as much by the chill each morning as by the deaths that have come hunting my circle in this year’s version of autumn. At this point, it’s a relief to admit the time has come to contract into shelters and shells.

How do snails build their shells?

A reader recently asked how snail shells are made.

To begin, let’s return to those words: shell and shelter. They seem to have the same root, but they are actually unrelated; like the biological convergence between eggshells and mollusc shells, they represent a linguistic convergence from different sources. The word shell comes from Old English and German words meaning scale and eggshell, and carries a more ancient undertone of splitting and separating: an eggshell must eventually split to fulfill its purpose. The word shelter is a more recent invention, about 500 years old, that comes from the word shield.

A snail shell, unlike a bird eggshell, exists to shield a snail through every stage of its growth. The answer to how a snail shell is made is that a snail excretes it, layer by layer, from its mantle, so that as a snail gets bigger, its shell does too.

(As a side note, snails also hatch from eggs. Their eggs are gelatinous or rubbery. Marine snails often have colorful eggs with intricate shapes.)

Look closely at a snail shell’s spiral. At its tiny center, you can imagine how small the snail must have been when it first hatched. The spiral grows by accretion, a word that gets my geology brain thinking, and is made of calcium carbonate, the same stuff that makes limestone and coral reefs. In my experience, hard things break down over time, so it is strange to think of shelters and rocks that grow, layer by layer.

Snails do stop growing at a certain point in life. Their internal organs rest safely inside the widest part of the shell’s spiral.

To excrete a shell that functions well for the organism in every stage of growth is actually quite a tricky geometry problem. It’s especially tricky for clams and other bivalves, where two shells must always meet at a continuous seam. Look closely at a clam shell. It also follows spiral growth, but its opening flares out much faster as it grows. The parameters that guide spiral growth include the shape of the generating curve, its position relative to the axis of coiling, the rate of increase of the generating curve, and the curve’s rate of translation along the axis. Most settings for these parameters result in shells that eventually block their own functioning.

In the case of the snail shell’s tight single spiral, the mathematical form has been studied for centuries. Albrecht Dürer in the 1500s called it the eternal line. Jacob Bernoulli in the 1600s called it spira mirabilis, the marvelous spiral. Now it is known as a logarithmic spiral. Its pitch angle is constant, and its ratios remain unchanged as it increases in size.

Bernoulli famously wanted a logarithmic spiral on his headstone, engraved with the phrase eadem mutata resurgo: “Although changed, I arise the same.”

Our most common snail is the Pacific sideband snail. Well, on second thought, that’s on land. The most abundant snail on the Key Peninsula, surely, is the small marine snail that covers rocks near the high tide line, called the checkered periwinkle.

The egg cases of praying mantises

Birds aren’t the only ones to use a shell to protect their developing young. Insects do too. For many insects, such shells are rightfully called a shelter. The long dark stretch of winter poses a significant survival challenge to such small, cold-blooded creatures, and most of our insects do not overwinter as adults. Brown marmorated stinkbugs are one exception, as are conifer seed bugs and mourning cloak butterflies. It is much easier to put a tiny egg or pupa into suspended animation and hide it away.

Lately I’ve found several oothecae — there’s a word for you — under fence rails on our farm. An ootheca is the trilobite-shaped case that protects the eggs of a praying mantis. Like spray foam, it is made of a froth, extruded by the female along with 100 or so eggs, that dries and toughens into a strange light material that will give you the willies if you try to scrape through it.

To make an ootheca is a big investment of energy and nutrients on the part of the mother mantis. If she has not had enough prey before mating, she will often turn around and eat her mate to secure her investment. For his part, the male mantis tends to seek a female that looks well-fed before mating with her.

Both adults soon die anyway. The oothecae wait out the dark, damp months.

When spring warmth returns, baby mantises emerge as wormlike prelarvae, which soon molt into nymphs that look like tiny adults. Insects have exoskeletons — yet another form of protective shell — and an exoskeleton cannot stretch or grow, so the mantises must shed their exoskeletons and harden larger ones eight times before it is late summer and they are adults ready to breed.

Our species is Mantis religiosa. It is native to Eurasia and Africa. In my experience, it is common in certain scraggly fields but absent from forests and manicured yards.