example of this. When stationary, a single zebra stands out because of its large size. To reduce the risk of attack, zebras often travel in herds. The striped patterns of all the zebras in the herd may confuse the predator, making it harder for the predator to focus in on an individual zebra. Furthermore, when moving rapidly, the zebra stripes create a confusing, flickering motion dazzle effect in the eye of the predator. Fighting back Defensive structures such as spines may be used both to ward off attack as already mentioned, and if need be to fight back against a predator. Methods of fighting back include chemical defences, mobbing, defensive regurgitation, and suicidal altruism. Chemical defences Many prey animals, and to defend against seed predation also seeds of plants, make use of poisonous chemicals for self-defence. These may be concentrated in surface structures such as spines or glands, giving an attacker a taste of the chemicals before it actually bites or swallows the prey animal: many toxins are bitter-tasting. A last-ditch defence is for the animal's flesh itself to be toxic, as in the puffer fish, danaid butterflies and burnet moths. Many insects acquire toxins from their food plants; Danaus caterpillars accumulate toxic cardenolides from milkweeds (Asclepiadaceae). Some prey animals are able to eject noxious materials to deter predators actively. The bombardier beetle has specialized glands on the tip of its abdomen that allows it to direct a toxic spray towards predators. The spray is generated explosively through oxidation of hydroquinones and is sprayed at a temperature of 100 °C. Armoured crickets similarly release blood at their joints when threatened (autohaemorrhaging). Several species of grasshopper including Poecilocerus pictus, Parasanaa donovani, Aularches miliaris, and Tegra novaehollandiae secrete noxious liquids when threatened, sometimes ejecting these forcefully. Spitting cobras accurately squirt venom from their fangs at the eyes of potential predators, striking their target eight times out of ten, and causing severe pain. Termite soldiers in the Nasutitermitinae have a fontanellar gun, a gland on the front of their head which can secrete and shoot an accurate jet of resinous terpenes "many centimeters". The material is sticky and toxic to other insects. One of the terpenes in the secretion, pinene, functions as an alarm pheromone. Seeds deter predation with combinations of toxic non-protein amino acids, cyanogenic glycosides, protease and amylase inhibitors, and phytohemagglutinins. A few vertebrate species such as the Texas horned lizard