About 28 species in North America
Siricidae (horntails) in the order Hymenoptera (ants, bees, wasps)

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.

Length: ½ to 2 inches (varies with species).
Habitat and conservation: 
Horntails may be found nearly anyplace with trees but are rarely numerous. Some species lay eggs in hardwood and others in coniferous trees. The pigeon tremex prefers maples, oaks, elms, sycamore, hickories, apple, pear, and hackberry. When laying eggs, horntails tend to select trees that are weakened or dying. The tunneling of the larvae can weaken wood, making it prone to further disease or damage. Sometimes they lay eggs on recently cut wood.
The young of horntails are wood borers, tunneling into the sapwood and heartwood of trees as they eat. Like several other insects that eat wood, larval horntails possess a symbiotic fungus that helps them digest the tough materials wood is made of.
Distribution in Missouri: 
Life cycle: 
After finding a host tree, the mother horntail uses her ovipositor to bore through the bark and inject eggs, one by one, as much as ¾ inch into a tree. After about a month, the eggs hatch into grublike larvae that eat wood, tunneling into the heartwood. They can grow for 1 to 3 years before pupating. Prior to pupation, they tunnel to just beneath the bark. When pupation is complete in late summer, they emerge as winged adults, chewing their way out of the bark, and repeat the cycle.
Human connections: 
Horntails usually do not occur in large enough numbers to be serious pests, though the boring of the larvae does damage trees. Typically, they select declining trees and not healthy ones.
Ecosystem connections: 
Like many other organisms that feed on dying trees, horntails help begin the process the recycles wood back into soil. Larval horntails are not safe in their tunnels; they are eaten by larval ichneumons, whose mothers also possess ovipositors for boring into wood.
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