The ancient stick insects that have thrived without sex for one million years

Nature · Published 3 October 2026 Swipe more like this →

In short: Timema are the most primitive stick insects on Earth, representing the earliest branch of their evolutionary tree. While most species reproduce sexually, some have abandoned males entirely. These asexual lineages have persisted for over a million years, setting a record for the longest known period of asexuality in any insect.

Native to the mountainous regions of western North America, the genus Timema consists of twenty-one described species of stout, wingless insects. As the sole extant members of the family Timematidae, they occupy a unique position as the sister group to all other stick insects, which are collectively termed Euphasmatodea. Unlike their relatives, Timema possess tarsi with three segments rather than five, giving them a physical appearance more akin to earwigs than typical walking sticks.

The genus is a masterclass in adaptation and camouflage. These night-feeders rely on crypsis, with body colors and patterns—stripes, scales, or dots—that mirror their host plants. Research indicates that specific traits, such as the white dorsal stripe, have evolved independently multiple times to match needle-like foliage. In species like Timema cristinae, color morphs remain in flux, potentially shifting in predictable waves based on predator preferences or thermal tolerance. This ongoing evolution suggests that the genetic mechanisms underlying their appearance are still actively changing.

Perhaps most striking is their reproductive strategy. While sexual species engage in elaborate courtship—where males guard females for up to five days after mating—five species have transitioned to parthenogenesis. Research led by Tanja Schwander confirms that two of these, T. tahoe and T. genevievae, have reproduced asexually for over one million years. While a 2023 genetic analysis hinted that rare males might occasionally engage in sexual reproduction, these insects remain the oldest known example of long-term asexual persistence in the insect world.

Beyond their reproductive quirks, Timema provide a window into the early stages of speciation. Some populations are currently diverging into distinct ecotypes based on host plant preference, such as those favoring Adenostoma versus Ceanothus. These groups exhibit physical differences, including body size and leg length, which are genetically inherited. Though they can still interbreed, these ecotypes represent a snapshot of evolution in progress, illustrating the mechanisms that eventually lead to the formation of entirely new species.

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