Friday, July 20, 2007
I have been interested in insects since childhood, but had never seen such a strange-looking creature. It was much smaller than a praying mantis, being only about 0.7 inch from head to tip of wings. The wings have a wide, brown outside border, which is not obvious in my photograph; better photos showing different perspectives can be found at the web site http://bugguide.net.
After consulting a field guide (1), I concluded it was a mantidfly, probably a brown mantidfly, Climaciella brunnea. Although unrelated to the much larger and more familiar praying mantis, mantidflies have evolved a very similar body form and hunting style in order to capture small insects. They perch quietly on flowers and wait for small insects to approach, and then sieze them with a scissors-like motion of the front legs. I was able to capture this specimen (it was rather slow-moving) and keep it in a jar and provide it with small insects, including ants and aphids.
Life style. If the appearance of the mantidfly seems bizarre, its life-style is even more so. My field guide did not provide much information so I searched on Google.com, and found several excellent papers describing the biology of the Mantispidae (mantidflies). According to Redborg (2), who has studied these insects for many years, the larvae feed on the eggs of spiders. Depending on the mantidfly species, the larvae enter the spider egg case either while the female spider is laying eggs, or penetrate the egg case once the eggs are encased in its silk cover.
The brown mantidfly larva spends much of its early life riding the back of the spider, staying on those parts of the spider that the female cannot reach with her legs. During this stage, the larva stays alive by sucking spider blood. The larva eventually enters the spider egg case while the female spider is laying eggs. It must enter the egg case before the spider spins a protective silk cover over the eggs, because it cannot penetrate the silk cover once it is formed. After entering the spider egg case, the larva uses its piercing and sucking mouthparts to drain the contents of the eggs. After feeding and developing on spider eggs, the mantidfly larva spins a cocoon within the spider egg case, and later emerges as an adult. Host spiders for the brown mantidfly are the Lycosidae, or wolf spiders (1). These spiders hunt on the ground and do not build webs. I frequently see wolf spiders around our house, sometimes on the cellar floor or stairwell.
Laying eggs. Mantidflys lay their eggs in places the host spiders are likely to frequent. Eggs of the brown mantidfly are laid on or near the ground. When the eggs hatch, the larva adopts a vertical posture that allows them to grasp a passing spider and then "hitch a ride" on its host. Different mantidflies are usually associated with different types of spiders, such as wolf spiders, crab spiders and jumping spiders.
Mantidflies were once thought to be uncommon, although Redborg states: "They can in fact be incredibly common, even in temperate North America, and are major predators of spiders."
1. L. Milne and M. Milne, National Aububon Society Field Guide to Insects and Spiders, A. A. Knopf, New York, 1980.
2. K. E. Redborg, "Biology of the Mantispidae," Annual Reviews of Entomology 43, pp. 175-94 (1998).
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