By Arthur V. Evans
A few years back, on a warm muggy evening in September at the Savage Neck Natural Area Preserve on the Eastern Shore of Virginia, I was checking a light trap designed specifically to lure night-flying insects. As the beam of my headlamp swept over nearby shrubbery, my eye caught a pale green insect just over half an inch long perched on a leaf.
With four clear wings folded rooflike over its body and a pair of grabbing forelegs held tightly against its long, slender thorax, this animal looked like a cross between a green lacewing and a praying mantid. In fact, it was the green mantidfly, Zeugomantispa minuta.
Mantidflies belong to the order Neuroptera and are only distantly related to mantids. Instead, they are related to antlions (whose larvae are known as doodlebugs), lacewings, and owlflies. Praying mantids are in the order Mantodea and are actually cousins of cockroaches.
Adult mantidflies capture and eat all kinds of small insects in captivity, but little is known of their food preferences in the wild. The larvae are also predators. Some are known to attack the pupae of moths, or the larvae of beetles, flies, bees, and wasps, while others prey on the eggs of spiders.
There are two basic strategies for mantidfly larvae to successfully dine on spider eggs; they are either egg sac penetrators or spider-boarders. Egg sac penetrators, such as those of green mantidflies, seek out their food directly.
When they find an egg sac they chew their way through the silk casing with highly modified jaws. The grooved mandibles and maxillae fit snugly together to form a pair of piercing/sucking tubes through which they draw out the embryonic fluids of the eggs.
Spider boarders are unable to chew their way inside spider egg sacs. Instead, they seek out a female spider, climb onto her body, and wait. What are they waiting for? Egg-laying day. The larvae disembark just in time to be wrapped up with the eggs in the egg sac.
Those mantidlfy larvae that initially hitched their fortunes to a male must eventually switch to the egg-producing sex. They switch hosts either while the male is mating or being cannibalized by a female.
The life cycle of mantidflies is a type of complete metamorphosis known as hypermetamorphosis. The first larval stage is quite active and resembles a slender, leggy silverfish, a body type that serves them well as they seek out egg sacs or spiders. Once inside the egg sac, the larva switch into feeding mode and go through two grublike stages before completing their development into adulthood. These fat, short-legged larvae are decidedly more sedentary. Afterall, they are thorax deep in food and have no need to go anywhere.
One of the most amazing things to me about mantidfly natural history is that scientists figured it out at all. The seemingly unlikely relationship between mantidflies and spiders was first described in 1869 and, with careful and patient study, continues to unfold today. It is this unrelenting promise of discovery that keeps me and my colleagues forever enamored with the world of insects and spiders and dreaming of an endless summer.
© 2010, A.V. Evans