One of my friends collected some spider egg cases at Lick Creek Park a few weeks back and was very surprised when instead of baby spiders the mantidfly seen above emerged. These little guys have some of the most fascinating life histories I’ve heard of. Adult mantidflies are predatory, catching small, size appropriate insects in their raptorial forelegs. The larvae of various mantidfly species are generally either predators or parasitoids.
Larvae of the Mantispinae sub-family employ a specially interesting strategy for predating spider eggs. The first instar larvae are mobile, and seek out a spider egg sac to enter. Some species chew through the spider silk, but this is a tough material. Other species get around this trouble by hitching a ride on a female spider, and actually getting themselves spun into to the egg case as the spider lays her eggs. Once inside they spend the rest of their larval development safely ensconced, feeding on the spider eggs. They eventually pupate and emerge as the adult mantidfly seen above.
Mantidflies, also called mantispids, mantisflies and mantid lacewings, are not actually related to preying mantises. They belong to the family Mantispidae in the order Neuroptera, the net-winged insects such as lacewings and antlions. Their similarity to preying mantises is a striking example of convergent evolution–when unrelated organisms faced with a similar ecological challenge independently evolve a similar biological trait. Another good example of this is the case of dolphins and sharks with their similar streamlined bodies and fins, or even bats and birds with their wings for flight. Both preying mantises and mantidflies are predators, and their large raptorial forelegs give them a long reach and a firm hold for a quick snatching of prey.