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 the colors insects can see is based on a combination of behavioral studies and understanding the morphology and physiology of the insect’s eye. The compound eye found in most insects is made up of a few thousand individual units called ommatidia, and each ommatidium includes a rhabdom, a transparent structure beneath the lens, which stores photosensitive pigments called opsins used to detect the colors. The more types and combinations of opsins an insect has in its rhabdom, the more colors it can see. Most insects can only see a narrow spectrum of colors, if at all. Insects with true color vision, such as 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 demand a simple majority of seven votes, or a unanimous vote of all 13?
Norman P. Briggs | Bonita, California

The main concern was that a majority of states and a majority of the population accept the new governing framework. The population of nine states would represent the majority of the population. One of the largest states, Pennsylvania, ratified the Constitution early, in December 1787. The ninth state, New Hampshire, ratified it in June 1788, which technically satisfied the requirements. But the hope was that every state would ratify it, and it was especially important to the success of the union that populous Virginia and New York join. George Washington encouraged ratification in Virginia, while Alexander Hamilton, John Jay, and James Madison published compelling essays (known as the Federalist Papers) in New York newspapers under the name “Publius.” Virginia and New York both ratified it in the summer of 1788. After that, it was really only a matter of time before the latter two states signed, as it was not realistic for them to stay away from their neighbors who had joined a powerful union. . Despite this, North Carolina did not 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 not?
Chris Aument | Itasca, IL

Most water is at least somewhat salty, even in freshwater rivers and lakes. Salts dissolve in water as water moves through soils and rocks. But water from lakes and rivers is constantly coming and going, with precipitation, snowmelt and groundwater keeping the supply relatively fresh. This fresh water (and any salt it contains) eventually flows into the ocean, but the only way the 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 is warm and evaporation is high. Salt water is also found in inland lakes where there are no outlets, such as the Great Salt Lake of Utah and the Caspian Sea. —Rebecca Hale, senior scientist, Smithsonian Environmental Research Center