I was busily looking for bark mimicking katydids in Cambodia, when I noticed out of the corner of my eye some slow movement on the trunk of a nearby tree. At first I couldn’t quite grasp what I was seeing – a yellow, wormy thing, wiggling its way out of the bark. But after a second I realized that I was seeing the remarkable spectacle of an adult mantidfly emerging from a spider sack after having spent its larval development feasting on spider eggs. I looked more closely and spotted the mother spider near the egg sack, blissfully unaware that she was now guarding the empty coffin of her once future children. The entire process lasted only a few minutes, during which the spider looked right at the monster that had just devoured her eggs, comprehending nothing. Unlike most winged insects, which need to spend some time expanding their wings and pumping them with air in order to be able to fly, this mantidfly emerged from its pupal casing fully formed and, within a few seconds of full emergence, it flew away. The spider kept guarding the egg sack.

Mantidflies are amazing insects. They are not true flies, of course, but rather members of the family Mantispidae (Neuroptera), closely related to antlions and green lacewings. This means that they are not related to praying mantids either, although they could be easily confused with them. Like mantids, mantidflies have large, raptorial front legs on a long, neck-like prothorax. They also share big, bulging eyes, and a similarly excellent vision. But rather than holding their front legs pointing forward, like mantids do, mantidflies hold them in a somewhat contorted, vertical position.

Mantidflies also share with praying mantids their love of live prey – mostly small, flying insects, which they catch with a surprising speed. Like mantids, they are also not above munching on members of their own species, a behavior which sometimes takes place during mating. Unlike mantids, however, the male mantidfly is as likely to eat the female as the other way around.

As holometabolous insects, mantidflies undergo a complete metamorphosis, and the larvae look nothing like the graceful adults. The first instar is tiny and highly mobile, whereas the two subsequent ones are grub-like, with long, piercing mandibles. Many species are predators of spider eggs, others feed on larvae of beetles, flies, and moths. Those species that feed on spider eggs must overcome quite a few obstacles before they can continue their development. After hatching from an egg laid by a female mantidfly, the larva attaches itself with a special “sucker” on its abdomen to the substrate, and waves its outstretched legs back and forth, waiting for a passing spider. But the spider that happens to be passing by is not always a mature female ready to lay eggs. No matter – the larva will attach itself to any spider and bide its time. If the spider is a male, it will try to get on a female’s body when the spiders are mating. If the spider is immature, it will remain on the host as it grows, hiding in the book lungs (breathing organs) of the spider when the animal molts, to avoid being shed with the spider’s old cuticle. Once the female spider is ready to lay eggs, the mantidfly larva allows itself to be wrapped in a silk sack with the eggs, and begins to devour its content.

There are about 400 species of mantidflies worldwide, of which most are found in the tropics, and only about 13 are known from North America N of Mexico. This summer I was pleasantly surprised to find a wasp-mimicking Climaciella brunnea near Boston, and next summer I will be looking for spiders carrying their parasitic larvae.