Horntails are wasplike insects with a taillike spine that projects from the tip of the abdomen. They have cylindrical bodies and lack the narrow waist so common in wasps. They are usually black or brown, sometimes with rust, orange, or yellow markings. Some species are fairly large. Males and females both have a hornlike spine at the abdomen tip; females have an additional projection below, which is the ovipositor, used for laying eggs. Horntails do not sting or bite.
Larvae are pale yellow or white segmented grubs with 6 tiny legs. They are rarely seen because they bore in the wood of trees.
One of the commonly seen horntails is the pigeon tremex, or pigeon horntail (Tremex columba), which has yellow and black bands on its abdomen and dark or amber-colored wings. It can be 2 inches long.