Food, Sex, and Cannibalism

A Black and Yellow Argiope (Argiope aurantia) eating a grasshopper

I had been away from Owl’s Knob for almost a month. Upon returning I found that my home was in the care of beneficial arthropods. The two that fascinated me most were the spider and praying mantis. These two species may appear to be quite different, but when it comes to the two most basic primordial instincts, food and sex, they have a lot in common.

Both arachnids and mantids are strictly carnivorous. They both wait for their prey to wander their way instead of hunting it down. Then they attack quickly and devour indiscriminately. The arachnid is on a liquid diet so it uses digestive venom to liquefy its meal before sucking the prey’s inners out. The mantis is the only arthropod that can turn its head 360 degrees. When it sees its prey it moves with extraordinary speed and viciously bites off its prey’s head with giant mandibles. As a gardener, I appreciate both of these arthropods because they eat my garden pests. But if I were an insect, I would fear these predators with every millimeter of my exoskeleton.       

Both of these arthropods identify themselves as predatory carnivores to such as extent that copulation is a problem. In the case of most species of spiders and mantids, the female eats the male either during or after sex. Though this act of sexual cannibalism is simulated by the arthropods’ intense instinct to devour anything that moves, it also has practical purposes for the species.

For arthropods that practice this sexual cannibalism, their short lives are almost over anyway. Once the male has had sex, his purpose in life has been fulfilled and he will die of old age in either a few hours or a few days. Therefore, from a logistical standpoint, it is only proper for his final good deed to be offering himself over as nutrients for his offspring.  Also, if the female eats the male’s head during copulation, the male’s dying body will inject more sperm quicker, which insures a better chance for fertilization.

Still, not every male is completely enthusiastic about becoming his partner’s meal. Many species of spiders have elaborate ways to avoid being eaten. Since spiders have very poor eyesight (even though they have, like, eight eyes), the first step is to be recognized as a male spider and not be mistaken for food. So some spiders do lovely eight legged dances. Others excrete pheromone laced silk to attract a mate. Many males play the strings of the female’s web like a harp and then she plays a rhythm back, creating a romantic song that gets everyone in the mood.

Once the female spider recognizes her mate, the male is not always safe. Some spiders will still play it cautiously by leaping on the famale and then back off before she can eat him. Others sneak under her and because they are so much smaller they can mate with her virtually unnoticed. One species uses his silk to lasso the female’s legs together so he can mate with her and safely escape with his life before she untangles herself.  

The praying mantis is even better known for sexual cannibalism than spiders; however, the wild praying mantis female rarely eats the male. The myth that the female praying mantis always eats the male started when scientist observed them in terrariums. Apparently, when in captivity the female almost always bites the head off her mate during copulation. This might be because she fears her food sources might be limited in her captive environment.

She might also worry more about the fertilization process since in captivity she sees no other available males. When a male praying mantis loses his head he also loses his brain and all inhibitions, while the ganglion in the male’s abdomen, which controls his basic motor functions, is still intact. Therefore, the male has a better chance of fertilizing the female’s eggs without his head because he copulates with wild abandon.

The feeding and sexual habits of arachnids and mantids are fascinating arthropods. Though I would not want to exist in their world, I enjoy their presence in mine. They are welcome guest in my garden and around my yard.

Preying mantises mating (and not she did not bite off his head).