Why some insect eggs are spherical while others look like hot dogs

Look at the nail of your pinky finger. That’s about the width of the biggest known insect egg, which belongs to the earth-borer beetle Bolboleaus hiaticollis. The smallest egg, from the wasp Platygaster vernalis, is only half the width of the thinnest recorded human hair.

Insect eggs range across eight orders of magnitude in size, and come in a stunning variety of shapes, a new database of almost 10,500 descriptions of eggs from about 6,700 insect species shows. The Harvard University team behind the database thinks it’s figured out one reason why. In a separate analysis, the researchers determined that where insects lay their eggs — for example, in water or in the bodies of other critters — helps to explain some of the diversity that’s evolved over time. The database and study were both published July 3 in Scientific Data and Nature, respectively.

“Eggs provide a wonderful window into the evolutionary and ecological forces involved in animal reproduction,” says Mary Stoddard, an evolutionary biologist at Princeton University not involved in the new work. Stoddard and her colleagues analyzed over 47,000 photos of eggs of 1,400 bird species in a 2017 study, which found a link between a bird’s egg shape and the animal's ability to fly. “Compared with bird eggs, insect eggs are truly wild,” she says. “Some insect eggs are spherical or elliptical, but others resemble arrowheads or hot dogs.”

To compile the database of insect eggs, researchers developed computer programs that extracted egg measurements from text and photos in 1,756 digitized publications, and then used the measurements to estimate egg sizes and shapes. Representatives of over 500 families from all insect orders were included.

Harvard University evolutionary and developmental biologist Cassandra Extavour, an author of both papers, says that eggs, being just single cells without complex features that might complicate comparisons, make a “great starting point” to study how insects develop.

Analyzing the egg data revealed an astounding diversity. Still, within that variety, many insect groups have converged upon similar designs, such as spherical or elongated, says Samuel Church, an evolutionary biologist at Harvard University.

All sizes

These drawings represent the range of sizes found in insect eggs. The largest, from an earth borer beetle (left), is about 800 million times as big as the smallest egg, from a parasitoid wasp (right). An analysis of thousands of insect species’ eggs suggests larger eggs tend to be laid in soil or under leaf litter, while smaller ones are laid in water or within another animal’s body.

Scientists previously had proposed reasons for those similarities; larger eggs, for example, might be more elongated because it’s easier for females to lay them. But using phylogenetic and statistical analyses, Church and his colleagues found no support for that idea. Even the seemingly straightforward hypothesis that larger eggs would be linked to species with larger adult body sizes was rejected for many species, the researchers say.

Instead, an egg’s form is best explained by where it is laid, the team reports. Specifically, eggs laid in or on water tend to be smaller and rounder, while those laid inside another animal tend to be smaller and asymmetrical, with one end coming to more of a point than the other. Eggs laid in soil or leaf litter tend to be larger.

“The level of technical rigor [in these publications] is extremely high, and they did everything carefully and thoughtfully” to construct an impressive database, says entomologist Jay Rosenheim at the University of California, Davis, who was not part of the studies. But Church’s team might have applied “unduly stringent criterion” for rejecting certain hypotheses, Rosenheim says. He sees in the data “a general pattern that egg size increases with body size in all of the taxa,” though the correlation isn’t always statistically significant.

“I'm convinced that the patterns we have described are broad patterns,” says Church, but emphasizes that an egg’s form is likely explained by more than just its immediate environment.