238 



THE RISE OF ANIMAL LIFE 



Fig. n-32. The larval dragonfly is equipped with mouth 

 parts that are adapted for catching other insects in 

 the water, where it lives until it becomes an adult. 

 The upper picture shows the parts thrust out in the 

 striking position; in the lower picture they are re- 

 tracted where they are held except when in use. 



hopper and bee, there are wide modifica- 

 tions in these parts in different species. 

 Starting at the anterior end of the insect 

 and working posteriorly, some of the modifi- 

 cations are as follows: The antennae may 

 be very short, as in the dragonfly (Fig. 11- 

 31) or they may be very long, as in the 

 long-horned grasshoppers, in each case per- 

 forming a specific function that requires the 

 ^articular type of antennae in question. 

 The eyes may be extremely large, as in the 

 dragonfly (Fig. 11-31), where they detect 

 the flying mosquitoes which the airplane- 

 like insect pursues. Or they may be absent, 

 as in the termites which work in the dark. 

 The mouth parts vary even more widely 

 than the differences between the grasshop- 

 per and the bee would indicate. The larval 

 dragonfly has a formidable weapon for 

 catching its prey (Fig. 11-32). The butterfly 



has a long tube which is carried in a coil 

 under its "chin" when not in use, but, when 

 stretched out during the process of taking 

 nectar from a flower, it may be as long as 

 the animal itself. The cicada possesses a 

 stiff beak which is used in penetrating plant 

 tissues to obtain the juices on which it 

 feeds (Fig. 11-33). The deerfly has fierce, 

 biting mouth parts which make a deep inci- 

 sion in the skin when it obtains a meal. In 

 fact, when disturbed, it often departs with 

 a small fragment of the skin between its 

 mandibles. There are also the thin dart-like 

 mandibles of the mosquito which can pierce 

 the skin very delicately and withdraw its 

 meal of blood, at the same time injecting a 

 small amount of saliva to prevent the blood 

 from clotting. The mouth parts of all insects 

 are homologous, yet witness the variety of 

 functions they perform. 



The thorax bears two pairs of wings and 

 three pairs of legs, all of which are variously 

 modified in different insects. The wings are 

 formed as thin sacs, by evaginations from 

 the thoracic wall, through which trachea 

 make their way. Eventually the sacs col- 

 lapse and the walls unite and harden, the 

 "veins" being formed by the trachea. Some 



Fig. 11-33. The mouth parts of the cicada (Alagicicado) 

 are modified to form a stiff beak, which it uses in 

 piercing twigs in order to obtain the sap, its chief 

 source of food. Note the three simple eyes; the one 

 directed anteriorly is particularly conspicuous. 



