Tuesday, November 18, 2008

Water works: Insects master submerged breathing

I remember as an elementary school student watching a classmate attempt to drown a grasshopper by holding its head under water. The grasshopper endured the insult long enough for the childish attention span to turn to some other endeavor, and the insect was released to swim another day.

While the grasshopper was in danger of being squeezed too hard and perhaps getting a bath it neither needed nor wanted, it was in absolutely no danger of drowning with its head under the water.

Insects take in their air by way of a special set of spiracles on the thorax and the abdominal segments. The many spiracular openings lead to a series of very fine, hollow tracheal tubules that repeatedly branch until they are about one micron in diameter (one micron equals 1/1,000,000 of a meter).

These very fine tubules are surrounded by insect hemolymph (insect blood) that can absorb oxygen from the tubules and give back carbon dioxide. In order to refresh the air in these tubules, insects simply stretch or move the abdominal segments. If you watch an insect for a while, it will periodically stretch the abdomen, much like you and I do when we elevate the chest and draw in a fresh breath of air.

So how do the many aquatic insects live completely under water if they use air-filled tracheal tubes for oxygen exchange? There are several interesting solutions to this problem. Many aquatic insects are so small that they are able to simply exchange gases from the water right through their thin exoskeleton.

Another approach can be found in several groups like the mayflies and caddisflies that have special gills as larvae. Some of these gills look like a miniature brush of fine tubes, while others resemble flattened footballs attached to each abdominal segment. These gills exchange gases that are then channeled into the tracheal tubes.

Mosquito larvae and some other dipteran (flies) larvae have a tiny tube that they extend above the water surface so they can take air directly into their tracheal tubes. This is like breathing through a straw while still completely submerged.

(While we now appreciate the damage that was done to our wetlands, this is why years ago oil was spread over swamps in attempts to control mosquitoes. The surface oil would plug up the mosquito respiratory structures and the larvae would die.)

The colder the water is, the more oxygen it can hold. Warm water, on the other hand, can hold much smaller amounts of oxygen. Most streams in Alaska are so cold that it is no hardship for insects to get all the oxygen they need. During the winter months, when the water is at its coldest, the insects are able to get all the oxygen they can use so they are very active feeding and growing.

In warmer waters the oxygen is limited so insects must resort to additional measures. Some insects use hemoglobin in their hemolymph that is the same bright red color as our blood. The insect hemoglobin, just like ours, helps hold onto oxygen and spread it throughout the insect body. In an effort to extract the limited oxygen from warm water, some insects flap their gills in the current to bring them in contact with more water. Yet others can undulate their bodies to create a current or even build little tubes that funnel water past their gills.

When these immature aquatic insects leave the water to become aerial adults, part of the emergence process involves shedding their gills. They then use the normal spiracular openings and tracheal systems for their breathing as adults. When we consider that many insects can fly nonstop for hours, the insect’s simple breathing system apparently works quite efficiently.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the biology of the Kenai River watershed.

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