Can Insects See Color? And More Questions From Our Readers

How do scientists know which insects can see color? 
John Jay Rouse | St. Cloud, Minnesota

Much of what we know about which colors insects can see is based on a combination of behavioral studies and understanding the morphology and physiology of the insect eye. The compound eye found in most insects is made up of a few to thousands of individual units called ommatidia, and each ommatidium includes a rhabdom, a transparent structure under the lens, which stores photosensitive pigments known as opsins that are used for detecting colors. The more kinds and combinations of opsins an insect’s rhabdom has, the more colors it can see. Most insects can only see a narrow spectrum of colors, if any at all. Insects with true color vision—like many bees, butterflies and dragonflies—can see not only many colors, but also ultraviolet wavelengths beyond the visible spectrum of humans.  —Floyd Shockley, curator of entomology, National Museum of Natural History

The Constitutional Convention required nine states to ratify the Constitution. Why didn’t they require a simple majority of seven, or a unanimous vote of all 13?
Norman P. Briggs | Bonita, California

The chief concern was that both a majority of the states and a majority of the people agree to the new frame of government. The population of any nine states would amount to a majority of the people. One of the largest states, Pennsylvania, ratified the Constitution early on, in December 1787. The ninth state, New Hampshire, ratified in June 1788, which technically met the requirement. But the hope was for every state to ratify, and it was especially important to the success of the union that populous Virginia and New York join. George Washington promoted ratification in Virginia, while Alexander Hamilton, John Jay and James Madison published persuasive essays (known as the Federalist Papers) in New York newspapers under the name “Publius.” Virginia and New York both ratified in the summer of 1788. After that, it was really just a matter of time until the last two states signed on, since it wasn’t realistic for them to remain apart from neighbors that had joined a powerful union. Even so, North Carolina didn’t ratify until November 1789. The last holdout, Rhode Island, joined in May 1790. —Barbara Clark Smith, curator of political history, National Museum of American History

Why are oceans salty but lakes are not? 
Chris Aument | Itasca, Illinois

Most water is at least a little bit salty, even in freshwater rivers and lakes. Salts dissolve into the water as water moves through soils and rocks. But the water in lakes and rivers is constantly coming and going, with rainfall, snowmelt and groundwater keeping the supply relatively fresh. This freshwater (and any salt it contains) eventually pours into the ocean, but the only way that water leaves the ocean is through evaporation. This leaves the salts behind, making the salt concentration much higher. The ocean is saltier in places where it’s warm and evaporation is high. We also find salty water in inland lakes where there is no outlet, such as the Great Salt Lake in Utah and the Caspian Sea. —Rebecca Hale, senior scientist, Smithsonian Environmental Research Center