6 BULLETIN 66, UNITED STATES NATIONAL MUSEUM. 



nation of these parts, proposed in concrete form by John B. Smith 

 (1899). 



He presents a not altogether new table of the orders, which more 

 normally represents the morphological relationships of the orders 

 than any classification yet seen by the writer. In this table he 

 divides the mandibulate orders into three parallel groups: 



1 . Prothorax mobile ; head not free. 



2. Prothorax immobile, well developed; head free, not on distinct 

 neck. 



3. Prothorax immobile, reduced; head on distinct neck. 



The Coleoptera belong in the first, and the Hymenoptera and 

 Diptera in the third. 



Probably a more normal grouping of the essentially mandibulate 

 orders would be as follows : 



EXOPTERYGOTA, HEMIMETABOLA. 



1. All thoracic segments subequal, not agglutinate; wings arising 

 externally; head free, not on a distinct neck. 



2. Thoracic segments subequal, meso- and metathorax agglutinate, 

 prothorax free ; wings arising externally; head not on distinct neck. 



3. Thoracic segments unequal, agglutinate, prothorax reduced; 

 wings arising externally, head five, on distinct neck. 



ENDOPTERYGOTA, HOLOMETABOLA. 



4. Thoracic segments subequal, meso- and metathorax agglutinate, 

 prothorax free ; wings arising internally; head not on distinct neck. 



5. Thoracic segments unequal, not agglutinate, prothorax reduced, 

 metathorax greatly developed; wings arising internally; head free, 

 on distinct neck. 



6. Thoracic segments unequal, agglutinate, prothorax reduced, 

 metathorax greatly developed; wings arising internally; head free, 

 on distinct neck. 



In the first of these groups fall Isoptera, Mallophaga, Corrodentia. 

 In the second fall Orthoptera and Plecoptera, also Thysanoptera. 

 In the third fall Odonata and Ephemeroptera. In the fourth falls 

 Coleoptera ; in the fifth the Strepsiptera ; in the sixth the Neuroptera, 

 Trichoptera, Mecoptera, Lepidoptera, Diptera, and Hymenoptera. 

 The orders are here used in the Comstockian sense, although the 

 writer is disposed to regard the groupings of Handlirsch and Boerner 

 as more nearly correct. It is not the function of this article to erect 

 a classification of the Hexapoda. 



In preparing this grouping, the division by Sharp of the pterygote 

 insects into Exopterygota and Endopterygota (Sharp, 1899) sug- 

 gested a better outline than could have been gained from Smith alone. 



