Adult alderflies are usually black, dark brown, or gray. They look a lot like stoneflies but are more closely related to fishflies and dobsonflies. They are generally poor fliers. At rest, the 2 pairs of heavily veined wings are held folded over the back in a rooflike (usually not flat) shape. Antennae are about half the length of the body. Alderflies lack ocelli (small, simple eyes), while fishflies and dobsonflies both have ocelli on their heads, between the compound eyes. The fourth tarsal segment (one of 5 the beadlike components of the foot) is inflated and 2-lobed.
Alderfly larvae are aquatic and look a lot like fishfly larvae, but they are usually much smaller. They lack gill tufts below the abdomen, and the abdomen tip has only 1 tail filament that points straight back from the body. They are usually fairly light-colored.
- Adult stoneflies have a pair of cerci (similar to antennae) at the abdomen tip; alderflies lack cerci.
- Adult fishflies have ocelli (simple eyes) and tend to hold their wings flat, not rooflike; they are usually larger; they lack the inflated fourth segment on each foot.
- Dobsonflies are larger yet, and their forewings are usually more than 2 inches long, while those of alderflies are much shorter.