Big, slow-moving, amazingly camouflaged to look like leaves and sticks, the Phasmatids are appropriately known as leaf and stick insects. They are so well camouflaged that even though some of them are among the largest living insects, they are seldom seen. They are the sloths of the insect world, designed to eat leaves, big and tasty, and so they mimic their food and remain motionless to fool the birds that are looking for them. If the wind blows, the phasmatids sway slowly so that they continue to resemble leaves and sticks, now swaying in the breezes. Finally, with danger upon them, they may spread their wings to reveal brightly coloured eyespots and fly away or they may fall motionless to the ground and emit some repugnant smell. In the last resort, they can jab an attacker with the spines on their legs. Only when their camo fails them, do they reveal themselves to the world.
Only ten insect orders are known from the fossil record and the phasmatids are one. They date back to the Triassic Period so they have survived all the major catastrophes that eventually wiped out the dinosaurs. There are about 3000 living species of Phasmids, classified in only two families. There are about a hundred species of small, flattened leaf insects in the family Phyllidae, which are found in the Australasian region. The rest, the stick insects, are found world-wide but mainly in tropical and subtropical climates, belong to the family Phasmatidae and include the largest species.
Color is a big part of the phasmid’s protective scheme. Most are mottled brown and green to aid with the illusion of being parts of plants. Even within a single species there is a great deal of variation in mottling between individuals. Solitary insects are usually cryptic but when populations soar, like locusts, the phasmids become brighter and more obvious. Cryptic coloration works best when numbers are low but when there are many, and one is searching for a mate, a bit of brightness can be helpful and is less dangerous because predators have more individuals to choose from. Color changes can also be brought on by changes in environmental conditions such as light, temperature and humidity. Underneath the hard cuticle, skin cells contain pigments that can migrate in response to these conditions. On a bright sunny day, the insect becomes lighter in color, which reflects the heat. As the day goes dark and cool, the pigment granules spread out to make the insect darker, which helps it retain heat.
Because they are solitary and don’t move around much, finding a mate can be problematical. Usually it is the males that can fly while females may emit attractive pheromones to advertise their presence. However if a mate cannot be found, the females resort to parthenogenesis, producing unfertilized eggs. Even the eggs are camouflaged, resembling the seeds of the host plant. The female lays them one at a time by dropping them or catapulting them by flicking her abdomen. She may lay anywhere from a hundred to a thousand eggs this way. The eggs may not hatch for up to three years. When they do, the tiny young resemble their parents. They go through a number of moults before maturing. The females mature quickly after the sixth moult while the males mature slowly after the fifth moult. This coordinates them so that they are ready to mate at about the same time.
These are fascinating insects and you can find out more about them at: http://www.brisbaneinsects.com/brisbane_hoppers/Phasmatidae.htm and http://www.phasmatodea.org/index.html