Invertebrate animals survive in the desert in two ways. Those that rely on water like the single-celled aquatic animals, shrimps, water fleas and crabs must either rely on a constantly wet environment or must lay resistant eggs which can hatch the next time water is available. Other invertebrate animals like spiders, insects, centipedes and scorpions have a hard waxy coating (the cuticle) on the outside of their bodies through which moisture escapes only slowly.
Centipedes and millipedes
These two groups of lower animals have representative species that live under rocks in the desert. They have bodies made of many segments ╨ centipedes have a single pair of legs on each segment and millipedes have two pairs of legs on each segment. Millipedes are usually seen in deserts after rain storms, so desert people often call them ╘rain worms╒. Desert millipedes feed on both living and decomposing leaves and the bark of shrubs and cacti.
Desert centipedes may be up to 8 inches long, and use poisonous claws to catch and kill insects and other small animals.
Insects
Desert insects include species which are land dwelling or which fly only short distances. These insects have learned to cope with the harshness of desert conditions from day to day. Other insects like the locusts avoid the worst desert conditions by migrating long distances in search of food.
Desert locusts are found in North Africa and Asia. In dry periods there are few locusts, but when it rains more young survive because there is more vegetation to feed on. Thousands of young grow during a three week period, and then develop into winged adults capable of flight. When the food runs out, they take off in swarms ╨ sometimes the swarms are so large that they can blot out the sun╒s rays. Wind may sweep them to more fertile areas outside the desert, where millions can descend like a plague and strip whole areas of crops.
Non-flying insects include beetles, and the best known group are probably the scarab or dung beetles. These small insects break off pieces of animal dung and roll the pieces into balls which they roll into balls and bury with an egg in each ball. Another group of beetles called darkling or tenebrionid beetles lives in the deserts of the American southwest and the Namib desert and ╘drink╒ by condensing moisture on their bodies.
A number of species of ant are found in the world╒s deserts. The Honeypot ant has an abdomen that becomes so swollen with nectar and other sugary substances that it cannot move and stays anchored to the ceiling of its nest. These ants are found in North America, southern Africa, and Australia where they are one of the traditional foods of some of the Aborigines of the central Australian deserts. Harvester ants collect seeds to store in their nests. When a scout finds a rich supply of seeds, it then trails a special odor from the tip of its abdomen all the way back to its nest. The other ants in the colony then follow the trail to the seeds. The ants eat most of the seeds but they reject or drop a few and therefore help the plants by dispersing the seeds.
Spiders
Spiders are the commonest of the invertebrate desert predators, and a number of different families of spiders are found in the world╒s deserts. These range from the hairy desert tarantula to various types of wolf spider. Because flying insects are not as common in desert conditions as elsewhere, web-building spiders are relatively rare in desert regions. The trapdoor spider digs a hole in the desert and then uses its silk to spin a roof over the top. When an insect crawls over the roof, the silk shakes and the spider emerges to catch and eat the insect.
About 20 different species of tarantula are found in the southwestern deserts of the US. Females may live a long time - some are known to have survived for 30 years or more. Like other tarantulas, the desert tarantula spends most of its time in an underground burrow. When the cold weather comes, it plugs up the burrow╒s opening and waits for the return of warm weather. Desert tarantulas use poison to kill their insect prey, but the poison is not strong enough to injure humans seriously. The greatest enemy of the desert tarantula is the tarantula hawk wasp. The sting of this wast paralyzes the tarantula, which is then carried to a prepared grave. The wasp lays an egg on the tarantula's abdomen thus providing a fresh food supply for the wasp╒s larva when it emerges from the egg.
Scorpions
Only two kinds of desert scorpion may be seriously venomous ╨ these are usually harmful only to young children and elderly people. Scorpions will usually run away and hides when disturbed. When a scorpion hunts its prey it seizes the victim in its pincers. It kills the prey by squeezing, or by using the sharp stinger on the end of its tail, which can whip forward with lightning speed. Scorpions are close relative of spiders and have four pairs of legs rather than three like insects. Scorpions hatch from eggs, and after they are born, they climb immediately onto their mother's back. The young scorpions remain clinging to their mother until they shed their first skin ╨ they then wander off by themselves.
Whip scorpions are close relatives of the main group of scorpions and they live in the southwestern desert states of the USA across to Florida. They are not venomous and have a flatter abdomen than other scorpions. Their first pair of legs are modified into long feelers.
