INSECTA 67 



for both biting and sucking. The walking legs have five 

 parts: a proximal coxa, often fixed immovably to the 

 sternum to which it is attached; a- short trochanter; a long 

 femur; a slender tibia; and a jointed tarsus which is usually 

 provided with little hooks or pads at its free end. The legs 

 may be adapted in various ways for grasping, swimming, 

 digging, leaping, or other purposes. The wings arise as 

 outgrowths from the two posterior thoracic segments. 

 They are chiefly of two types: broad for sailing, as on a 

 butterfly; and narrow for rapid propulsion, as on a house 

 fly. They often bear scales or hairs. They may be thin 

 and membranous, thick and heavy for protection, or vary 

 in other ways. The little " veins" which traverse insects' 

 wings are not primarily for carrying blood, but are thicken- 

 ings serving as supporting or skeletal structures. 



The inside of an insect's body is filled with digestive, 

 reproductive, respiratory, circulatory, and excretory organs. 

 The alimentary canal varies in its structure and extent 

 according to food habits. Vegetarians have longer ali- 

 mentary canals than other insects. The parts of the 

 digestive system are: a mouth, or buccal cavity; a slender 

 esophagus; a thin- walled crop; a glandular stomach from 

 which little pouches, or caeca, branch out; and a long 

 slender intestine. At the junction of the stomach and 

 intestine the slender Malpighian tubules discharge their 

 excretions into the alimentary canal. 



The respiration of insects takes place through the tracheal 

 system. This is a very much ramified network of tubes 

 which carries oxygen to all parts of the body. The air 

 enters the tracheae through little openings, the spiracles, 

 along the sides of the abdomen and thorax. An insect 

 cannot be drowned by sticking his head in water; the 

 spiracles must be covered before it will die. Good flyers 

 have their tracheae expanded into air sacs, which make the 

 body light, but insects. which fly little have no such ex- 

 pansions (Fig. 36). 



Insects usually hatch from eggs. As they grow the 



