Although mantises feed primarily on other insects, the largest species can reach around six inches in length and these giants will attack and consume small reptiles, birds, and even mammals. It is likely that this type of predatory behavior is responsible for the common misspelling “preying mantis.”
As a child in the 1960s I was told that there would be a $50 fine for anyone caught killing a praying mantis (and my friends have the same recollections). Since I was unable to uncover a record of any such federal or state law, I can only assume that the story was a scare tactic designed to keep nasty little boys from slaughtering an uncommonly pious insect known to eliminate an array of less religiously inclined pests.
Many people are familiar with the praying mantis’s supposed penchant for cannibalistic sexual encounters, reports of which began showing up in the scientific literature in the late 19th century. Back then, several authors claimed that female mantises regularly bit off the triangular heads of their partners during sex. These same sources also claimed (to many a reader’s astonishment) that the decapitated males continued to copulate, abdomens pulsing away as if nothing of much importance had just happened. According to these references, several hours later the female would stride off, full and fertilized, while the male, having been reduced to a tiny pile of wings and hard bits, stayed put. Similar tales about mantid mating continued into the early 20th century, when members of a new generation of entomologists began investigating the function of this rather puzzling behavior.
One hypothesis reasoned that the male mantis’s brain actually inhibited sexual behavior. With their heads removed, however, males became “disinhibited,” found the rhythm, and eventually pumped out a full load of sperm. Other mantid mavens suggested that getting oneself cannibalized made sense for praying mantis males that might have limited opportunities to mate over their lifetime. It made evolutionary sense, therefore, to fatten up the only female they might ever run into—especially one now carrying their sperm. Furthermore, it would be a plus for both sexes since headless males reportedly pumped out more sperm than those equipped with heads, leading to more fertilized eggs and more offspring. These accounts contributed to an overall impression that the decapitation of male mantises was a normal and perhaps necessary copulatory stage and, soon after, the concept became entrenched in textbooks and the popular literature. Unfortunately, what never quite made it into print was the fact that most observations of mantis cannibalism were made in laboratory settings and only after females had been deprived of food.
In reality, cannibalism varies across this large and diverse group. The behavior has gone unobserved in most species, not necessarily because it doesn’t happen, but because it hasn’t been studied. Researchers now believe that rather than being a required component of mating behavior, the consumption of males is more likely to be a foraging strategy employed by hungry females unable to wrap their raptorial forelegs around an alternate form of nutrition.
Support for this hypothesis comes from studies on a wide variety of mantis species, including those in which worse-for-wear females that cannibalized their mates later exhibited improved body condition, produced larger egg cases (ootheca) and more offspring. Significantly, well-fed female mantises showed no cannibalistic tendencies during mating encounters.
Before we blame mantid cannibalism on captive conditions or starvation, though, the fact remains that both wild and captive males exhibit extreme caution as a normal preamble to copulation. Depending on the species, the males’ initial approach can vary from simple (slow and deliberate movement toward the female, followed by a flying leap onto her back) to complex (the male fixes its stare on the female, goes through a series of stereotypical movements like antennal oscillations and abdominal flexing, then takes a flying leap onto her back). Researchers believe that these movements serve to either circumvent or inhibit the females’ aggressive, predatory response. It is, therefore, extremely unlikely that these forms of cautionary behavior by males would have ever evolved if there weren’t at least some risk of being attacked by females.