8 ENTOMOLOGY 



Handlirsch has made the most exhaustive investigation of the phylo- 

 geny of the major groups of insects. His revolutionary system, which 

 is based upon fossil as well as recent forms, is of the kind to which one 

 applies the term "epoch-making," but is unfortunately so erratic and 

 fantastic in some respects that it has not been generally adopted. 



As the orders of insects have evolved from one another in many 

 different directions, like the branches of a tree, their natural relation- 

 ships can not be expressed correctly in any linear sequence, like that 

 of this book. Here the orders are listed approximately according to 

 the degree of specialization, beginning with the most primitive insects; 



INSECTA 

 CRUSTACEA \ ,CHIIOPODA 



mum 



ARACHNIDA" 



FIG. 9. Diagram to indicate the origin of classes of Arthropoda. 



and the attempt is made to group together orders that are nearly 

 related to one another. 



In the course of the following synopsis of the orders of insects it is 

 necessary to use some terms, as metamorphosis and thysanuriform, in 

 anticipation of their subsequent definition. 



i. Thysanura. No metamorphosis. Eyes aggregate, compound 

 or absent. Antennae long, filiform, multiarticulate. Mouth parts 

 mandibulate, either free (ectognathous) or enclosed in the head (ento- 

 gnathous). Wings invariably absent. Thoracic segments simple 

 and similar; pro thorax well developed. Abdomen usually elongate, 

 with ten evident segments and often traces of an eleventh segment; 

 with two to eight pairs of rudimentary limbs, or styli, often accompanied 

 by eversible ventral sacs. Cerci usually long, filiform, multiarticulate, 

 with frequently a similar median pseudocercus; but sometimes with 



