This question had definitely been answered when I looked in the last version of the Last Word. Some well-informed person had stated it was a male Dobson fly, whose larva is a Hellgrammite. The latter is also a supervillain character from DC comics. From pictues I looked at elsewhere that person was correct, or it is very closely related.

The "Big and Scary" bug from Costa Rica looks like a male Dobson fly, a relative of the lacewings.  For loads of pictures and anecdotes (including its bizarre larvae the hellgrammite), see http://www.whatsthatbug.com/dobs.html

BIG AND SCARY

The formidable insect pictured in the last word in the July 18 issue of NewScientist is a male dobsonfly from the genus Corydalus (Order Megaloptera, Family Corydalidae).  There are 5  species in this genus residing in Costa Rica: C. clauseni, C. flavicornis, C. luteus, C. magnus, and C. peruvianus.  From the picture provided and the indicated size, the specimen is likely Corydalus luteus.

Both male and female dobsonflies can reach lengths up to five inches (12.5 cm).  Although the mandibles of males appear intimidating, they are actually harmless to humans.  Their mandibles are used exclusively during mating, where males show them off and grasp the females during copulation. Female dobsonflies, however, have short powerful pincers and can inflict painful bites, which can draw blood.

Dobsonflies spend most of their life in the larval stage, during which they are called hellgrammites, and are familiar to anglers who like to use the large larvae as bait. Hellgrammites live under rocks at the bottoms of lakes, streams and rivers, and prey on other insect larvae with the short sharp pincers on their heads, with which they can also inflict painful bites on humans. Hellgrammites can reach to 2" to 3" in length, with gills along the sides of their segmented bodies that allow them to breathe underwater.  After a few years of living and growing underwater, the larvae crawl out onto land and pupate. They stay in their cocoons over the winter and emerge only to mate. Upon emerging, they live for only seven days.

Adults can generally be found from late spring into the middle of summer, preferring to remain near the aquatic areas where they grew up. Once they emerge as adults they mate, deposit their eggs on overhanging vegetation near the water and die. They are primarily nocturnal, and like most aquatic insects, are commonly attracted to bright lights.  I collected my first specimen at the light of a gas station restroom in Potomac, Maryland.

The "Big and Scary" bug shown in the July 18-24 issue of New Scientist is a Dobsonfly, and even though the questioner spotted it in Costa Rica, you don't need to go to the tropics to find them. An even larger one, at least 7 inches long, landed on the side of my house a few years ago in Front Royal, Virginia, USA, which is in the Blue Ridge Mountains. Shocked and awed, I peered at him through my dining room window. Lacking a camera, I grabbed a tablet and drew him to scale, as best I could, shading with the side of a pencil to try to portray his eerie, gossamer, translucent, and weirdly long wings. My drawing stumped the pest control rep who routinely checks our house for unwanted bugs: he thought I had made the thing up to freak him out! After a month of showing the picture around, I finally found someone who recognized the Dobsonfly. I was told that my proximity to the Shenandoah River, numerous creeks, and karstic terrain, made the area hospitable to this "fly." You can find pictures on the internet -- some of them much larger than the ones both I and the Last Word questioner have seen.  best regards, julie savell-mccandless, Front Royal, Virginia, USA

This is an insect belonging to the family Corydalidae and Order Megaloptera.  The young ones of this insect are aquatic and prefer fresh water.  Bothe adults and the young ones are predatory - preying on other insects.  The long sickle shaped mandibles are characteristic of this group.