54 



INSECT LIFE. 



crabs. Crayfishes (Fig. 23) abound in our brooks, 

 and are often improperly called crabs. The lob- 

 sters, the shrimps, and 

 the true crabs live in- 

 salt water. 



The Crustacea are 

 distinguished from 

 all other Arthro- 

 pods by having two 

 pairs of antennae 

 and by their mode 

 of respiration, being 

 the only ones that 

 breathe by true 

 gills. Many insects 

 live in water and are 

 furnished with gill- 

 like organs, but these 

 are tracheal gills. 

 True gills are for the purification of blood, while 

 tracheal gills are for the purification of the air con- 

 tained in the air ves- 

 sels or tracheas of an 

 insect. The former 

 contains a large num- 

 ber of blood vessels. 



Fig. 23. — A crayfish. 



F1G.24.— Crustacea : a, cy/rw,- the latter a large 



Of Cyclops ; c, Dapknia. , ~^ 



number of air vessels. 



Fig. 25. 

 Asow-bug. 



There are minute Crustacea common in ponds 

 and streams. Three of the more abundant of these 



group a species seems to belong, he should verify this determination 

 by a study of the characters of that group given in the detailed discussion 

 of it. 



