Every now and then I devote a column to a different group of flying animals, the insects. We’ll recognize the insects as honorary birds today.

This column was precipitated by an interaction I saw last weekend while mowing the lawn. A spider wasp was interacting in the grass with what I thought was a second wasp. Going closer, I realized the wasp was attacking a large wolf spider. This interaction was the beginning of a fascinating but macabre relationship.

The spider, at least five times the size of the spider wasp (family Pompilidae), tried to escape as the wasp injected a neurotoxin that quickly took effect, causing paralysis. The wasp then quickly dragged the spider to the side of our house, up a granite foundation stone and underneath the lowest cedar shingles.

The rest of this tale will take place sight unseen. The wasp will lay one inside the paralyzed spider. Though paralyzed, the spider will live. The wasp egg will hatch inside the spider and the larva will eat the internal organs. The wasp larva will pupate, emerging as an adult. Maybe I should have saved this story for Halloween!

Entomologists classify such wasps as a parasitoid. Unlike a true parasite, a parasitoid either kills or sterilizes its host.

Many wasps, in dozens of families, are parasitoids. Some lay eggs on other insects, but wolf spiders are commonly used. Wolf spiders typically do not build webs. A spider’s web is used primarily to capture prey but offers a secondary benefit of protection from wasps. The free-ranging wolf spiders are therefore at risk to parasitoid wasps. I may have unwittingly caused the spider’s demise by cutting the grass and making it easier to see.

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MEANWHILE, BIRD MIGRATION is near its peak. We can predict the order of departure in the fall: First the swallows and flycatchers, then the warblers, then the sparrows and hawks. Migration implies a predictable, seasonal movement. Animals may engage in nomadic wanderings that are not predictable enough to warrant as migration. For instance, white-winged crossbills wander widely to find bumper crops of conifer cones.

Although birds are the best migrants, they do not have a monopoly on migration. You have seen videos of wildebeests, zebras and other mammals migrating to and from the Serengeti Desert.

Some insects migrate as well. A dragonfly, the wandering glider, occurs on six continents. Populations in Africa migrate to India after the monsoon season starts. The abundant rains provide ample opportunities for the females to lay eggs in aquatic habitats.

Some butterflies migrate. In North America, the best-known migrant is, of course, the monarch. Any monarchs you see this fall will attempt to fly south to a small pine forest in mountains in northwestern Mexico. Even they make it, they will not return. The complete migration requires five or six generations.

The painted lady butterfly occurs in both the New and Old World. British scientists used radar to track the spring arrival of painted ladies into the United Kingdom and their fall departure to Africa. These butterflies flew at heights up to 3,000 feet, explaining why humans do not typically see them migrating. With favorable winds they were able to fly 30 mph. In 2009, the radar revealed that 11 million painted ladies arrived in the U.K. and 26 million departed in the fall.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at

whwilson@colby.edu


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