20 



ENTOMOLOGY 



parasitic species; hind wings represented by a pair of knobbed 

 threads, or balancers. Larvae usually cruciform, with the head fre- 

 quently reduced to a mere vestige with or without a pair of mandibles, 

 and usually without true legs, though pseudopods may be present. 

 Pupa naked, or enclosed in a puparium. The flies. Example, crane- 

 fly, Tipula (Fig. 31). About forty thousand described species. 



FIG. 31. Tipula. A, larva; B, cast pupal skin; C, imago. Slightly reduced. 



20. Siphonaptera. Metamorphosis indirect. Head small, not 

 sharply separated from the thorax. Eyes minute and simple, or 



absent. Antennas short and 

 stout, situated in depressions. 

 Mouth parts suctorial. Body 

 laterally compressed. Tho- 

 racic segments subequal, free; 

 coxae large; tarsi five-segment- 

 ed. Wings absent or at most 

 quite rudimentary. Larva 

 with a head, mandibulate, 

 apodous, vermiform. Adults 



FIG. 32. Cat and dog flea, Ctenocephalus cams. 

 A, larva (after KUNCKEL D'HERCULAIS); B, adult. Saltatonal, parasitic On warm- 



blooded animals. The well 



known fleas. Example, Ctenocephalus (Fig. 32). One hundred and 

 fifty species. 



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. 



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

 tracting their special, or adaptive, peculiarities, their remaining charac- 

 ters may properly be regarded as inheritances from some vanished 



