The Short Answer: Interesting that I received these two questions within a week of each other, both from the Buffalo, New York area. I’m pretty sure the same, or very similar, insects laid these egg masses on Tricia’s and Susan’s cars. I sent the photos to Dr. Gabor Horvath  and Dr. Gyorgy Kriska , researchers at the Eötvös University  in Budapest, Hungary who have studied why insects sometimes lay their eggs in the wrong place. Based on the photos, Dr. Horvath and Kriska suggested that these are egg masses laid by caddisflies in the family Limnephilidae. Most species in this large family lay their eggs on leaves overhanging water. When the eggs hatch, the caddisfly larvae drop into the water, where they develop in a fairly standard caddisfly way (see this caddisfly article ). They glue together a house, which protects them as they feed on detritus. Eventually, they metamorphose into adults, mate, lay eggs on leaves and start the cycle again.
So why did they lay these eggs on cars, instead of on leaves? How does an insect, with a tiny brain, figure out where to lay its eggs in the first place?
It’s not that a caddisfly female thinks about where to lay. Her brain is coded with a recipe or algorithm for where to lay. For example, the instructions might say, fly till you find yourself over water. Then fly up until you come to a horizontal object. Lay your eggs there. I’m oversimplifying it, but the process is probably something like that. And for many insects, especially those whose eggs and larvae need to be associated with water, automobiles cause confusion. That’s what probably happened in both these cases.
Dr. Horvath  and Dr. Kriska  study insect “mistakes” in order to gain insights into how the vision of insects and other animals works. In the case of insects that lay eggs on cars, the answer seems to be related to polarized light. Light that enters our atmosphere is non-polarized, which means that the light waves are oriented in all directions equally. When light bounces off a horizontal surface, such as the surface of a pond or lake, however, the reflected light is mostly oriented in a single direction. We call that polarized light.
In the natural world, the most common sources of horizontally polarized light are the surface of a body of water, or the surface of leaves. Many insects have evolved to detect that polarized light as an indication of a good place to lay eggs. It’s how they know they are flying above water or leaves and not above dirt. It appears, for example, that Limnephilidae caddisflies use polarized light as one of the signals that suggest a good place to lay their eggs on leaves over water.
Unfortunately, for those caddisflies and other insects that rely on polarized light as an indicator of where to lay eggs, reflective man-made horizontal surfaces, like the dark-colored hood or roof of a car, or even asphalt road surfaces can also reflect light in a way that polarizes it. This can lead to insects laying their eggs on top of cars, which is what Dr. Horvath believes happened in this case.
Thanks: Thanks to Charley Eiseman, co-author of Tracks & Sign of Insects & Other Invertebrates  for confirmation that these might be insect eggs, and to Dr. Horvath and Dr. Kriska for their help in suggesting what kind of insect might have laid them and why.
Polarized Light: If you are curious about how the polarization of light works, you might find this video helpful: https://www.youtube.com/watch?v=gP751qpm4n4