Collembola, also known as springtails, are tiny terrestrial hexapods once considered primitive, wingless insects (based on the fact that they too have six legs, just like insects), but are now known to be distinct from true insects. Most species live in loose surface soil and leaf litter where they feed on decaying organic material. Despite their abundance (up to 200,000/m3) and diversity (more than 7000 species worldwide), they rarely attract much attention because of their small size (0.25-6 mm). Individuals of some of the most diminutive species are smaller than the period at the end of this sentence.
The common name “springtail” is based on the ability of some species to jump large distances when disturbed. A spring-loaded, forked structure called a furcula found on the tip of the abdomen propels the tiny hexapod 15-20 times its body length when released. Putting that into perspective, it is the equivalent of a 6 foot tall man “springing” 90-120 feet in the air. Unfortunately, the springtail has very little control over the direction or landing site of the leap, but if you are a springtail trying to avoid a potential predator, grace and accuracy might not be your most pressing concern!
Collembola are further distinguished by the manner in which the male delivers sperm to the female. Rather than sperm transfer through direct contact (as in most insects), many male springtails produce small packets of sperm, known as spermatophores, which they deposit on short stalks directly on the substrate. To fertilize her eggs, a female must find and take the spermatophore up into her reproductive tract. This method is not unique to Collembola (lots of other invertebrates and even a few vertebrates do it too), but what makes this process so interesting in Collembola are the mating rituals and courtship behaviors which have evolved to increase an individual male’s odds of reproductive success:
- In the most basic process, sort of a “drop and pray” method, the males do not interact directly with the females, choosing instead to simply deposit spermatophores randomly across the environment in the hope that a female will come in contact with one and collect it before it dries out.
- In some species, the male deposits a spermatophore and then uses his antennae (modified to allow him to grasp the female’s antennae) to literally drag the female across it in a gentle “tug-of-war”, a struggle made harder by the fact that males are often smaller than females.
- In the ritual known as the “Love Garden,” males locate a suitable female and then deposit multiple spermatophores. The male then attempts to lure/entice the female to walk through the spermatophore mine-field, fertilizing herself in the process.
- A more ingenious method employed by other species is known as the “Ring of Fire.” It involves a male locating a sedentary female, around which he quickly deposits a “fence” of spermatophores so that she must either spring to escape or is forced to walk across one of the male’s spermatophores.
- Perhaps the most fascinating of these behaviors, called the “Cha-Cha-Cha”, involves males and females engaging in a courtship dance with the male and the female initially standing facing each other, head-to-head and performing a push-and-retreat ritual until a rhythm is established. As the female tries to spin away, the male immediately counters, hoping to woo the female into accepting him as a mate. Near the end, the male will perform 1-2 trial turns (a 180° turn) to test if his dance was successful. If she accepts, the male deposits the spermatophore directly in front of her, and she immediately picks it up with her reproductive tract. If she doesn’t accept, she consumes the spermatophore. Of course, males often eat spermatophores too—their own (if no female has collected them) as well as those of other males (to prevent a female from mating with rivals). This video of the Cha-Cha is from the BBC:
Given that Collembola date back to at least 400 million years ago, one can only conclude that this system of mating must be effective for their ecological niche or else it would not have persisted. It also seems that some male Collembola have learned what many a young male human has yet to learn: If you want to get the girl, you may have to learn to dance.
Hopkin, S.P. 1997. Biology of the Springtails (Insecta: Collembola). Oxford University Press, New York. 334 p.
Kozlowski, M.W. and S. Aoxiang. 2006. Ritual behaviors associated with spermatophore transfer in Deuterosminthurus bicinctus (Collembola: Bourletiellidae). Journal of Ethology 24: 103-109.
Schaller, F. 1971. Indirect sperm transfer by soil arthropods. Annual Review of Entomology 16: 407-446.
Triplehorn, C.A. and N.F. Johnson. 2005. Borror and Delong’s Introduction to the Study of Insects (7th ed). Thomson Brooks/Cole, Belmont, CA. 864 p.
Watch more BBC video of the Springtails in action here: