444 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1931 



example, if we should wish to know something about some par- 

 ticular person, we might find on an interview with him that he had 

 forgotten many things of his past, which his relatives, with better 

 memories in such matters, would readily divulge, if approached in 

 the right way. So it is in the study of insects — their relatives in 

 many cases will reveal things about them that we should never be 

 able to get from the insects themselves. One reason why our under- 

 standing of insects is not all that it might be is that entomologists 

 have been too diffident in the matter of making intimate acquain- 

 tances with arthropods other than insects. A formal letter of in- 

 troduction to them, therefore, may not be out of place here, especially 

 since the members of the arthropod classes will not be familiar to 

 all readers under their scientific names. 



The Arthropoda are the " jointed-legged " animals, so named not 

 because other animals do not have jointed legs but because the legs 

 of arthropods are so conspicuously jointed. The familiar arthropods 

 are the crabs, lobsters, crayfish, spiders, millipedes, centipedes, and 

 insects. Associated with these forms, however, are many obscure 

 relatives known for the most part only to zoologists. The following 

 table will show briefly the way in which the Arthropoda may be 

 classified according to their structural characters. 



CLASSIFICATION OF THE ARTHROPODA 



I. Chelicerata. — ^Arthropods in which the principal feeding 

 organs, or chelicerae, are appendages of the first postantennal somite, 

 and generally have the form of a pair of small pincers. The legs 

 often have a segment, the patella, interpolated between the femur 

 and the tibia. 



Eurypterida (extinct fossil forms). 

 Xiphosura (liorseshoe crabs). 

 Pycnogonida (spiderlike, marine arthropods). 

 Araclmida (spiders, scorpions, ticks, mites). 



II. Mandibulata. — Arthropods in which the principal feeding 

 organs, the mandibles, are appendages of the second postantennal 

 somite, and have typically a jawlike form. A patellar segment 

 is never present in the leg. 



Crustacea (shrimps, crayfish, lobsters, crabs, sowbugs). 



Symphyla (small, ccntipedelike relatives of the diplopods). 



Pauropoda (relatives of the the diplopods having only a few legs). 



Diplopoda ("thousand-legs," or millipedes). 



Chilopoda (centipedes). 



Hexapoda (proturans and insects). 



The insects comprise two principal groups, the Apterygota, or 

 primitive wingless insects, and the Pterygota, including all winged 



