598 EVOLUTION OF ANIMALS Part V 



developing as internal pads; in the immature stages the young are called larvae 

 and pupae. Bees, wasps and ants, moths and butterflies, beetles, and caddis- 

 flies develop in this way. The larvae eat ravenously and increase greatly in size. 

 At the end of several molts, the number depending upon the species, they 

 transform into pupae. The pupa does not eat and moves little or none. It is a 

 stage of transformation in which the outer form and the internal structures are 

 changed; the digestive tube is reshaped; the reproductive organs are developed. 

 Even the tissues are reorganized, and muscle is literally made over. The adult 

 is the final mature stage. The larvae of moths and butterflies are caterpillars, 

 strikingly different from the adult even in appetite. "Cabbage worms" have 

 insatiable appetites for cabbage leaves; cabbage butterflies follow the scent 

 of cabbage plants but only to lay their eggs on them, never to eat them. 



Foods Habits and Mouth Parts. Insects of one sort or another eat all kinds 

 of food. Many are very special but altogether they fall into four general groups, 

 plant feeders, predators, scavengers, and parasites. 



Nearly half of all insects feed upon living plants, the most reliable food there 

 is. Most plant feeders prefer one group of plants or they may feed upon only 

 one part of the plant, the leaf, stem, root, bud, flower and fruit. Plant lice 

 (aphids) insert their slender, piercing mouth parts into the tissues of ten- 

 der leaves and stems, dissolve the tissues with saliva and suck out the juices 

 (Fig. 30.8). In spring, the garden cut-worms (larva of noctuid moths) are 

 roused from hibernation in the soil and begin biting off the stems of seedling 

 plants — tomatoes, cucumbers and others at the surface level of the soil. Gipsy- 

 moth caterpillars eat oak leaves, veins and all; larvae of elm-leaf beetles take 

 only one layer of the leaf. Most plant feeders take their meals in daylight, but 

 there are some evening diners. 



Predacious insects are less abundant than the plant eaters. Predators have 

 dash and go, or stealth. Dragonflies with their arrowy flight, clutching fore- 

 legs, and chewing jaws were built for predation 100 million years ago. The 

 larva of the ant lion (Myrmeleon) digs a trap, an inverted cone in loose dry 

 sand. Ants roll down the slopes of the cone and as they struggle, they are 

 showered with sand by a twist of the ant lion's head whose jaws await them at 

 the bottom. The majority of predatory insects depend upon less active vege- 

 tarian insects for food. 



Certain insects, especially the larvae, are scavengers that eat dead and 

 decaying animal matter. Two familiar ones are houseflies and clothes moths. 

 Both are typical scavengers but the clothes moth larvae have an insatiable 

 craving for keratin, the hornlike substances in hair (fur, wool) and feathers. 

 Many insects are parasites, living on other animals, and gradually consuming 

 them while they are in the living state. Among them are the blood-sucking fleas, 

 biting lice of birds, and the parasites of other insects. 



The mouth parts of insects are often specialized and elaborate. The main 



