The Acorn Beetle
During August and the early part of September 2000, I began to find significant numbers of a curious looking bug in my insect traps. This little fellow appeared light brown to the naked eye, had a long, narrow beak that seemed almost as long as its body, a pair of hardened forewings covering a delicate set of hindwings that allowed it to fly, and spiny legs with sharp, pointed claws (see micrograph below) that helped it to cling tenaciously to my collection net. Note the two hairy pads, one on each side of the claw.
Its body was covered with tiny scales. Some, on the forewings, were whitish gray. Others, especially on the pronotum between the head and the forewings, were a darker brown. The patches of whitish gray scales gave it a mottled appearance. Its long beak was very narrow, about the size of a thick brush bristle. This was fitted with a pair of antennae, each attached to the beak some distance down its length. These were elbowed and segmented beyond the elbow to their tips. At the tips the last three segments were thickened to form a club. A large compound eye was located on each side of its head.
The micrograph on the right is of the tip of the snout, showing the rostrum. It looks a lot like the tips of drills used to bore holes in the earth for oil exploration. The word "rostrum" comes from the Latin rod(ere), meaning "to gnaw", which is the source of our word "rodent", and the suffix -trum, which designates a kind of instrument.
Since several oak trees were found nearby, I suspect this bug to be a long-snouted acorn beetle (the short-snouted species has a beak that is less than half the length of its body). The female of this species bores tiny holes into the hard shell that protects the nutmeat inside the acorn. Once this hole has been bored, the beetle eats her fill of the nutmeat, then (if she is in the egg-laying stage) takes a single egg and deposits it in the hole, sealing the hole with a fecal plug to protect the egg from predators. The egg hatches a tiny worm, or larva, that lives on the meat of the acorn, storing up food for its non-eating phase, or pupa. Eventually, the acorn falls to the ground, and the larva leaves the acorn shell to take up residence in the soil. Here it stops eating and begins to pupate.
The word pupa comes from a Latin root meaning girl, doll or puppet. Interestingly, the word pupil comes from this same root, and we might think of the pupa as a stage in life where the larva "learns" how to be an adult. In the case of the acorn beetle, big changes take place during this phase that transform the larva from a worm into an adult snout beetle. For some species of curculionidae, including the long-snouted acorn beetle, this process may take as much as three years or more. When the pupal metamorphosis is complete, a fully mature snout beetle adult emerges.
It Is a small insect, not quite the size of a pea. The body, beak and all, is about 0.36 in. (9.16mm) long. Its long beak measures 0.16 in. (3.94mm), or 43% of its total length. Note the slight swelling of the tip of the beak in the photo above. This suggests that some sort of boring or chewing apparatus might be found there on closer examination.
This bug "played dead" when it was disturbed. When I pulled its claws away from the collection net and placed it on a flat surface, it just lay where it was placed, without moving, for several minutes.
The hardened forewings,
or elytra, meant that this insect belonged to the order Coleoptera. That word has a Greek origin and means "sheath wing", which was the expression used by Aristotle (384-322 B.C.) to describe beetles.
My identification of this beetle uses purely circumstantial evidence (the oak trees nearby). Most of the beetles in this genus are very particular about the food material they eat and place their eggs into. An informative paper on acorn insects, written by Charles E. Williams, has been placed on the web by the Michigan Entomology Society, that includes instructions on how to experiment with these insects to learn more about them.
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