The fontanellar gun is a defense mechanism in the form of a hornlike frontal projection (nasus) on the heads of certain specialized soldier termites (nasutes) in the subfamily Nasutiterminae, used to ward off predators. The "gun" is a gland with a duct on the front of the head. It operates by a chemical reaction triggered when the termite contracts its mandibular muscles. The termite is "able to eject the frontal gland material over a distance of many centimeters". The gland material is forced out through the front of the internal nose and its pores, which covers much of the termite's head. Termites have been shown to be extremely accurate with the gun, even though they are blind. The exact nature of how termites retain such accuracy and are able to orient themselves toward their enemy is currently unknown, but it is believed that the ability is "olfactory or auditory" in nature. In some subspecies, when the glandular secretion is fired, it rips off the termite's mandibles, forcing them to retreat back into the nest. However, some species have evolved as such that their mandibles in and of themselves offer little protection or defense, so the loss of them does not hinder the termites. It is also possible for the termites to fire their gun more than once, if the threat that caused them initially to fire continues to exist.
The fired material is glue-like and sticks to objects that it hits, causing the legs of an attacking enemy to become stuck to the ground. Most often, though, a number of termite soldiers will fire upon the enemy and the combined force of the "bullets" will kill the enemy along with covering it in the glue-like substance. It has also been observed that, for those that survive the force of the material, it also causes varying negative effects, likely because of the terpenes contained in the compound. Eventually, if not killed from other effects, the secretion of the termite will kill any enemy insects after 24–48 hours. In comparison, the effect of the secretion on termites of the same species was considerably worse, with the secretion causing death within 5–6 hours.
The secretion from Tenuirostritemes tenuirostris consists of a mixture of three terpenes, namely 62% pinene, 27% myrcene and 11% limonene. These form a resinous glue resembling pine resin. The secretion contains an alarm pheromone that alerts other soldier termites of an enemy attack and causes them to fire their fontanellar gun. It was discovered that the pinene was also acting as an alarm pheromone while it was forming the composition of the terpenes. Because later arriving termites did not also fire their gun, it is believed that the pinene pheromone lasts for only a brief period of time before dissipating.
In Cubitermes and Crenetermes, the frontal gland secretes a mixture of terpenes, including unusual diterpenes, which is species-specific.
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