AQUATIC INSECTS 



edge which cuts the air during flight. There may be many 

 small cross veins in the wings as there are in mayflies and their 

 branching is often very complicated but the ground plan is 

 simple (Fig. 140). There are five veins — costa (C), 

 subcosta (Sc), radius (R), media (M), and cubitus (Cu), that 

 strengthen the front and middle section, and three anal (A) 

 veins stiffening the hindpart of it. Cross veins connect these 

 main veins and their branches (Fig. 141). 



An insect's legs are all built on the same plan. Beginning 

 where it is attached to the body the leg has two short segments, 

 the coxa and trochanter, followed by two long ones, the femur 

 and tibia, which make the greater part of the length of the 

 leg, and finally there are several (1-5) small segments which 

 form the tarsus or foot, usually tipped by one or more claws 

 (Fig. 138). 



The insect abdomen is divided into segments, there being 

 ten in the stonefly; it never bears any true legs though it 

 may have short fleshy props or prolegs like those on cater- 

 pillars and the larvae of midges (Fig. 228). Stoneflies and 

 many other insects have tails or cerci which are many-jointed 

 like antennas ; in adult mayflies these are a couple of times the 

 length of the body and seem to help steer during flight (p. 205). 



The inside of an insect. — The food tube or alimentary canal 

 runs from one end of the body to the other; it includes a crop 

 and a gizzard whose functions are similar to those of birds 

 and behind which open the Malpighian tubules, the kidney- 

 like organs of the insect (PI, VIII). Beneath this tube is 

 the nerve chain and above it is the long tubular heart whose 

 pulsations can sometimes be seen along the midline of the 

 backs of newly molted mayfly nymphs. 



Insects do not breathe through their mouths as is so often 

 supposed. All through their bodies there runs an intricate 

 network of air tubes or tracheas which carry air. In adult 

 insects like the dragonfly the tracheae open by spiracles along 

 the sides of the body; in whirligig beetles their openings are 



183 



