Ant-Mimicking Spiders

Have you ever noticed a tiny spider moving around inside your home? These spiders are usually small, rarely exceeding one centimeter in body length, and they often move by hopping rather than walking. If you have seen such a spider, there is a good chance it belongs to one of the most popular pet spiders today: the jumping spiders.

“Jumping spider” is actually a common name for a large family of spiders known as Salticidae. Because many members of this family move primarily by jumping, they are widely referred to as jumping spiders. The family itself is enormous. More than six thousand species have already been described, and forty-one species are known from Taiwan alone.

Although jumping is a characteristic form of locomotion for many salticids, the family is so diverse that exceptions exist. In this article, we focus on a jumping spider that does not rely on jumping as its primary way of moving.

Mimicry

Like countershading discussed in previous articles, mimicry is a biological strategy of disguise. Countershading allows animals to blend into the lighting conditions of their environment, whereas mimicry involves imitating another organism. Many toxic animals use bright colors to signal to predators that they are dangerous to eat.

As a result, some harmless species have evolved similar bright colors, effectively deceiving predators into believing they are also toxic.

Mimicry can be divided into three major categories.

Batesian mimicry occurs when a harmless species imitates a harmful one, thereby gaining protection from predators.

Müllerian mimicry involves multiple harmful species evolving similar appearances. When a predator encounters any one of them, it learns to associate that shared appearance with danger.

Aggressive mimicry occurs when predators imitate harmless organisms in order to lure prey closer.

The spider discussed in this article uses Batesian mimicry by resembling ants in order to avoid predators.

The Cast of Characters

Siler collingwoodi。 圖片來源：57Andrew，採用 CC BY-NC-ND 2.0 授權。

Mimic (the protagonist)

Siler collingwoodi

Models (the organisms being mimicked)

Meranoplus bicolor

Crematogaster egidyi

Technomyrmex sp.

Polyrhachis jianghuaensis

Polyrhachis dives

Control species (a jumping spider without mimicry)

Phintelloides versicolor

Major predators

Portia labiata (also a jumping spider)

Gonypeta brunneri

Host plants where interactions occur

Ixora chinensis

Carmona microphylla

Main Text

Siler collingwoodi is a brightly colored jumping spider distributed in Myanmar, Vietnam, and Hainan in southern China. One remarkable feature of this species is that it imitates ants while moving.

When walking, it raises its front legs so that they resemble the antennae of ants. It also lifts its abdomen in a manner similar to Crematogaster egidyi. Unlike most jumping spiders, which frequently leap, this species moves entirely by walking.

To understand the purpose of this unusual behavior, researchers collected individuals of S. collingwoodi from Hainan along with five species of ants living in the same habitat, two predator species, a non-mimetic jumping spider for comparison, and two common host plants.

Body coloration was examined first. Although the locomotion of S. collingwoodi appeared to be the main focus of the study, its vivid coloration raised additional questions. Why does this spider possess such striking colors? Are they used during courtship, or do they serve another function?

To investigate this question, researchers analyzed how visible S. collingwoodi and P. versicolor appeared from the perspective of two predators.

One of these predators, Portia labiata, has color vision. Researchers discovered that when S. collingwoodi rests on the flowers of Ixora chinensis, it becomes difficult for P. labiata to detect. In contrast, the plant Carmona microphylla does not provide the same level of concealment. Meanwhile, P. versicolor remained conspicuous regardless of which plant it was on.

The mantis Gonypeta brunneri perceives the world differently. Its vision is essentially monochromatic, meaning it mainly detects differences in brightness rather than color. Even under this visual system, S. collingwoodi showed smaller brightness contrasts against Ixora chinensis compared with P. versicolor, giving it better concealment.

These observations strongly suggest that the body coloration of S. collingwoodi helps it avoid detection by predators.

The next question concerned locomotion. Researchers performed behavioral experiments to test whether the spider's movement also contributed to predator avoidance.

The results showed that Portia labiata clearly avoided ants. In all experimental trials, this spider never attempted to attack ants. When encountering S. collingwoodi, it sometimes inspected the spider but spent significantly less time doing so than when examining P. versicolor. Across all experiments, P. labiata launched five attacks, and every one of them targeted P. versicolor rather than the ant-mimicking spider.

The mantis predator behaved very differently. Gonypeta brunneri attacked ants, S. collingwoodi, and P. versicolor alike without hesitation.

Why do these two predators respond so differently? Researchers believe body size plays an important role. Portia labiata is only slightly larger than its potential prey and therefore risks injury from ant mandibles and defensive chemicals such as formic acid. The mantis, on the other hand, is much larger and does not face the same danger when attacking ants.

The experiments also revealed an intriguing detail about the spider's mimicry. S. collingwoodi does not imitate a single ant species. Instead, its movements resemble an intermediate gait shared by multiple ants. Researchers suggest that this strategy may allow the spider to display several possible warning patterns simultaneously, increasing its ability to adapt to different environments.

The researchers also evaluated how difficult each ant species might be for predators to consume. The results showed that the larger and heavily spined ants, Polyrhachis jianghuaensis and Polyrhachis dives, were the least palatable prey. In contrast, Meranoplus bicolor and Crematogaster egidyi were easier for predators to handle.

Interestingly, the movements of S. collingwoodi more closely resemble the smaller ants such as M. bicolor, C. egidyi, and Technomyrmex sp. The researchers suggest this may occur because these ants are closer in body size to the spider, making them easier models to imitate. Another factor may be distribution: ant species that occur more widely are encountered more frequently, increasing the likelihood that the spider evolves to mimic them.

Conclusion

The experiments demonstrate that although the mimicry of Siler collingwoodi does not protect it from every predator, it significantly reduces the risk of being attacked. This advantage likely improves the spider's chances of survival in the wild.

The researchers also suggest that future studies should employ experimental setups that more closely resemble natural field conditions in order to obtain even more accurate results. Another important question for future work is whether the striking coloration of S. collingwoodi is shaped jointly by natural selection and sexual selection.

Author: Bai Leng

Reference:

Zeng, H., Zhao, D., Zhang, Z., Gao, H., Zhang, W., (2023). Imperfect ant mimicry contributes to local adaptation in a jumping spider. iScience.