GENERAL CHARACTERISTICS. 363 



covering a sharp instrument for inflicting wounds, and 

 connecting with it inside of the body is a sac of poison. 

 Insects have three nervous centres, the largest in the 

 head, a smaller in the thorax, and the smallest in the 

 hind body. The breathing-holes, or stigmata, are gen- 

 erally nine in number on each side of the body. The 

 heart consists of a long tube, lying just under the cover- 

 ing of the back, having small holes on each side for the 

 admission of the blood, which is yellow or colorless, and 

 which is prevented from escaping again by means of 

 valves within. The heart is divided into several cham- 

 bers by transverse partitions, in each of which there is a 

 valve, which allows the blood to flow from the hinder 

 part forwards, but not backwards. The blood does not 

 circulate in arteries and veins, as in the higher animals, 

 but is driven from the forward part of the heart into the 

 head, whence it returns to the body and is mixed with 

 the nutritive fluids that filter through the walls of the 

 viscera, and, thus mingled and aerated by contact with 

 the air-tubes, penetrates among the flesh and other inter- 

 nal parts, and nourishes the body and sustains life. 



Insects are produced from eggs, and are never spon- 

 taneously generated. A very few insects do not lay their 

 eggs, but retain them in the body till they are hatched, 

 and thus such insects are ovoviviparous. Others always 

 lay their eggs where .the young, as soon as hatched, 

 will find a plentiful supply of food. Most insects, in 

 passing from the egg to the adult state, undergo great 

 changes of form and habits. These changes are called 

 transformations or metamorphoses, and are so great, 

 that the same insect, at several different ages, may be 

 mistaken for as many different animals. There are three 

 more or less distinctly marked states in the life of an 

 Insect, the larva, the pupa or chrysalis, and the imago 

 or perfect state. In the larva period, which is the one cf 



