CLASSIFICATION 17 



or cutting. Larvae with complex mouth parts and frequently abdominal 

 legs. Phytophagous. Example, Tremex (Fig. 31). 



Suborder Aculeata (Heterophaga, Petiolata). Abdomen petio- 

 late or subpetiolate; first abdominal segment transferred to the thorax. 

 Ovipositor often modified to form a sting. Larvae apodous. Example, 

 Apis (Fig. 281). 



Interrelations of the Orders. The modern classification aims to 

 express relationships, and these are most clearly to be ascertained by a 

 comparative study of the facts of anatomy and development. 



FIG. 30. Cat and dog flea, Ctenocephalus canis. FIG. 31. Tremex columba. A, 



A, Larva (after KUNCKEL D'HERCULAIS); B, adult. Imago; B, larva (with parasitic larva 

 Length of adult, 2 mm. of Thalessa attached). Natural size. 



After RILEY. 



The most generalized, or primitive, insects are the Thysanura. Sub- 

 tracting their special, or adaptive, peculiarities, their remaining characters 

 may properly be regarded as inheritances from some vanished ancestral 

 type of arthropod. This primordial type, then, probably had three 

 simple and equal thoracic segments differing but slightly from the ten 

 abdominal segments; three pairs of legs and no wings; three pairs of 

 exposed biting mouth parts; a pair of long, many-jointed antennae and a 

 pair of cerci of the same description ; a thin naked integument ; a simple 

 straight alimentary canal distinctly divided into three primary regions.; 

 a ganglion and a pair of spiracles for each of the three thoracic and the 

 first eight abdominal segments, if not all the latter; no metamorphosis; 

 functional abdominal legs and active terrestrial habits. 



The existing form that best meets these requirements is Scolopendrella, 

 which is not an insect, however, but belongs among or near the diplopods. 

 The most primitive of known insects are Anajapyx and Campodea, 



3 



