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Horntail or wood wasp is the common name for any of the 150 non-social species of the family Siricidae, of the order Hymenoptera, a type of xylophagous sawfly. This family was, until recently, believed to be the sole living representative of the superfamily Siricoidea, a group well represented in Paleogene and Mesozoic times, but the family Anaxyelidae has recently been linked to this group. The last tergite of the abdomen has a strong, projecting spike, thus giving the group its common name (the ovipositor is typically longer and also projects posteriorly, but it is not the source of the name). A typical adult horntail is brown, blue, or black with yellow parts, and may often reach up to 4 cm long. The pigeon horntail (Tremex columba) can grow up to 5 cm long (not counting the ovipositor), among the longest of all Hymenoptera.
Female horntails lay their eggs in trees. The larvae bore into the wood and live in the tree for up to two years, possibly more. They typically migrate to just under the bark before pupation.
The spiral groove on the ovipositor is visible on the photograph but not easily to the naked eye.
The smaller horntail (Sirex noctilio) is only about half the size of the greater horntail, with pale brown legs and the rest metallic blue-black. It pierces the bark of pines to lay eggs. The smaller horntail ovipositor is very similar to the greater horntail, which is described as:
“as stiff and straight as a needle, polished black, with slight notches in the pointed half. It is hinged, to permit of its being turned at right angles to the body. . . the female selects a tree that is not too healthy, and settles on the bole; then, turning down her boring instrument on its hinge, she drives it through the thick bark to the solid wood."
When we consider the small size of the insect, it seems remarkable that she should have muscular power sufficient to force that slight auger through such resistant material; but that the boring imposes no strain upon her is shown by the fact that she may make several experimental punctures without an egg passing; apparently the wood reached is not quite suited for her purpose, so she tries another spot. Often her operations are upon a tree that has been felled for the builder's use; and by this means the new generation finds its way at times into our homes. Having satisfied herself that at last she has found the proper conditions under which a larva could exist, she passes an egg into the wood; then she repeats the process at some other spot on the same tree or log.
Mr. K. G. Blair has given an interesting account of the proceedings of a female he watched on a felled larch, in which, when discovered, she had her ovipositor embedded deeply; but it was soon withdrawn. "The insect then wandered off, walking rather jerkily over the log, the ovipositor held in its sheath beneath the body, its tip dragging along the bark behind her. As she went her antennae were in constant action, tapping the bark in front of her. About six inches from the spot where we first found her, having apparently discovered another position to her liking, the body was raised as high as possible on her legs, the ovipositor slipped from its sheath and the point inserted in the bark beneath the middle of her body, i.e. some distance, about an inch, away from the spot last explored by her antennae. The ovipositor was then perpendicular to the bark and to the general axis of her body, though this was now somewhat arched, while its sheath remained in its original position. Gradually the ovipositor was driven farther into the log, a slight side-to-side motion of the body being perceptible, until finally it was buried almost to its full length. Though we watched carefully, we saw no sign of the passage of any egg down the ovipositor, but after a few seconds it was seen to be being slowly withdrawn, the withdrawal being considerably more rapid than the entry.From first point of insertion to complete withdrawal occupied ten minutes. The insect then moved off again, but once more the ovipositor was slipped from its sheath and driven for its full length into the wood ; in this case the operation taking a little longer, twelve minutes until complete withdrawal.... Again the insect moved off, the ovipositor dragging along behind her. The terminal spike of the body is not brought into play at all, either when walking over the surface or during the thrusting in of the ovipositor ; neither does the ovipositor sheath appear to afford any support during this operation. This time she wandered further without finding a suitable spot, then suddenly flew away." It has been stated that she lays about a hundred eggs; but Mr. Blair's account may modify this estimate—a boring not being always an egg-laying."
Growth of the egg
The right conditions having been found and an egg discharged through the boring instrument, from this in due course issues a six-legged, whitish larva, which sets to work with capable jaws on the solid wood, beginning the excavation of a long tunnel that will occupy several years before the larva is full-grown. That stage reached, it spins a silken cocoon, and changes into a pupa which has all its limbs of maturity formed and folded beside its body.
Before making its cocoon, however, it takes the precautionary measure of advancing its tunnel close up to the inner bark, so that in its winged state it will have only to bite a way through this softer impediment to its liberty; not that it is now incapable of dealing with anything firmer. Like the notorious caterpillar of the Goat-moth, it does not hesitate, if necessary, to make a way even through soft metal. There is a record of a Sirex-infested tree having been cut into rafters which were used in building a roof and covered with sheet-lead an eighth of an inch thick. One of the rafters contained a Sirex in either the larval or pupal stage; and when the perfect insect sought its freedom, it found the way obstructed by the lead. It went right through, apparently finding lead not much more difficult to deal with than bark. Other similar cases might be cited. It is not improbable that many specimens that issue in houses have been imported in building timber from abroad.’ ”
- Bees, Wasps, Ants and Allied Insects of the British Isles, Edward Step (1932)