Breakthrough Genomic Study Resolves Century-Old Enigma of Angel Insects

Small and elusive, angel insects resemble termites and often dwell beneath bark or inside decomposing wood. Despite their unassuming nature, their classification within insect evolution has puzzled scientists for over a century.

Since their initial description in 1913, the insect order Zoraptera has baffled researchers who have struggled to determine its exact place on the evolutionary tree using both anatomical and genetic data. This enduring puzzle is famously known as the “Zoraptera problem.”

A recent comprehensive genomic study, published in Proceedings of the Royal Society B, resolves this longstanding debate. The analysis reveals that Zoraptera represents the earliest branch to diverge from other extant members of Polyneoptera—a major lineage of winged insects that includes grasshoppers, cockroaches, mantises, stick insects, earwigs, and stoneflies.

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This conclusion was reached only after applying a variety of evolutionary models to address unusual molecular data patterns found within these insects.

Reasons Behind Zoraptera’s Shifting Evolutionary Placement

With just a few dozen identified living species, Zoraptera is one of the smallest and least studied insect orders. Members of this group look strikingly alike and lack distinctive anatomical traits that would clearly link them to other insect orders.

This similarity complicated evolutionary studies based on morphology. Subsequent molecular analyses failed to provide a definitive answer as well.

Two main hypotheses have dominated discussions. The Haplocercata-first hypothesis suggests that Zoraptera is closely related to Dermaptera (earwigs), placing both near the base of the Polyneoptera tree. Conversely, the alternative model—the Zoraptera-first hypothesis—posits that angel insects form an independent early branch that separated before the diversification of other polyneopteran groups.

Previous studies supported one or the other hypothesis, depending on the data and methods employed.

This contradiction inspired the recent research. Chenyang Cai and Yehao Wang told Phys.org that persistent disagreements among morphological, transcriptomic, and earlier phylogenomic data motivated their investigation.

They suspected that statistical biases within the molecular data might have caused the misleading signals that grouped Zoraptera with earwigs.

Analyzing Over 1,300 Genes to Rigorously Test Competing Theories

To broaden available data, the team sequenced and assembled genomes from two new Zoraptera species and integrated these with three existing transcriptomes. Their dataset encompassed 89 polyneopteran species and 23 outgroups, providing a wide comparative framework for exploring deep evolutionary relationships.

Sampling accounted for both major families and most subfamilies within Zoraptera.

From this collection, researchers extracted 1,367 single-copy genes—genes present as a single copy in each genome, tracing back to a common ancestor. They prepared six amino acid datasets using varied filtering approaches and examined them using multiple evolutionary models rather than relying on any single method.

This comprehensive approach is a hallmark of phylogenomics, which leverages vast amounts of genetic data to reconstruct evolutionary histories.

Not all models yielded identical trees.

While simpler site-homogeneous models—similar to those used in older studies—suggested a Zoraptera and earwig pairing, more sophisticated site-heterogeneous models that consider different evolutionary patterns at various protein sites pointed to a different relationship.

With these complexities accounted for, the Haplocercata-first hypothesis was rejected, and Zoraptera emerged as the earliest diverging lineage among polyneopterans.

Statistical Biases Fueled Conflicting Evolutionary Signals

A significant challenge was the effect of long-branch attraction, an error in phylogenetic analyses where rapidly evolving lineages appear artificially related due to their many molecular changes.

Zoraptera exhibited both fast evolutionary rates and uneven amino acid compositions, factors that likely produced the misleading signals.

The team conducted multiple tests to validate their findings, including statistical comparisons of tree topologies, random gene removal (known as gene jackknifing), filtering genes based on evolutionary speed, and employing the ASTRAL multispecies coalescent method to reconcile differing gene histories with species trees.

Genes evolving more slowly consistently disproved the false grouping of earwigs and Zoraptera, whereas independent analyses repeatedly supported the Zoraptera-first scenario.

Under the best-fitting models, the results were stable. Wang and Cai concluded succinctly: “Zoraptera forms the very first split from all other Polyneoptera.”

This outcome was confirmed through multiple evolutionary trees, model comparisons, coalescent analyses, topology tests, and repeated gene removal procedures.

Earwigs Form a Clade with Stoneflies Instead

Extracting Zoraptera from its traditional pairing with earwigs also revised the phylogenetic position of other groups.

Dermaptera and Plecoptera—the earwigs and stoneflies respectively—were recovered as a clade called Dermoplectopterida. This grouping appeared as the sister lineage to the remaining polyneopterans after Zoraptera’s divergence. Both multispecies coalescent and gene jackknifing analyses reinforced this arrangement.

The next step in this research will introduce a temporal aspect to the evolutionary tree. Cai and Wang intend to integrate these phylogenetic relationships with fossils of Zoraptera preserved in amber alongside other polyneopteran fossils to better estimate the timeline for major insect divergences.

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