134 



NATURAL HISTORY OF ARTHROPODS. 



tracheal system has a different distribution, and equally important changes occur in 

 the circulatory system. Our space is not sufficient to detail these changes, but their 

 extent can be seen by a comparison of the adjacent figures, which represent the 

 nervous system in the larva of a bee, and also in the adult insect. These changes 

 are not sudden, but are accomplished by constant modifications of the organs con- 

 cerned. 



Besides the common sexual reproduction, some insects can produce young without 

 the intervention of males. In some of these cases of agamogenesis or parthenogenesis 



FIG. 194. Nervous system of a larval bee. 



FIG. 195. Nervous system of adult bee. 



the eggs are laid and undergo the regular development of fertilized eggs, but in 

 others the eggs are developed inside of the parent, and the young are born in an active 

 condition. 



The systems of classification of the Hexapoda are very numerous and very differ- 

 ent, though now it is pretty universally conceded, both in this country and in Europe, 

 that the Thysanura are the lowest and the Hymenoptera the highest orders. But 

 within these extremes the sequence of orders varies greatly. We shall here follow 

 Dr. Packard's latest scheme, without, however, adopting all the features of his arrange- 

 ment. Dr. Packard groups the hexapods under five super-orders and ten orders, but 

 for our purposes the former may be omitted. 



