104 THE CLASS OF INSECTS. 



Gerstaecker's estimate. He reckons that there are at least 

 25,000 species of Hymenoptera, from 22,000 to 24,000 Lepiclop- 

 tera, about 24,000 Diptera, and 90,000 Coleoptera ; the number 

 of the other suborders cannot be easily estimated. Besides 

 these there are about 4,600 Arachnida, and 800 Myriopods. 



Grouping of Insects into Orders and Suborders. Be- 

 fore beginning an "account of the Six-footed Insects, we 

 present the following tabular view of the Classification of In- 

 sects. The idea that the Myriopods, Spiders, and Six-footed 

 Insects formed orders and not classes was first proposed by R. 

 Leuckart in 1848, and afterwards supported by Agassiz and 

 Dana. The arrangements proposed by these and other authors 

 are put in tabular form on page 106. 



The Class of Insects. 



Order I. — Segments grouped into three distmct re- ^ 



gions ; eyes compound and simple ; two pairs of j Hexapoda 

 wings:* three pairs of thoracic legs; one pair of )»(Six-footed In- 

 jointed abdominal appendages. A more or less sects). 

 complete metamorphosis, J 



Order 11. — Segments grouped into two regions, a^ 

 false cephalothorax f and an abdomen ; no antenna3 ; I 

 eyes simple ; wingless ; four pairs of thoracic legs ; J» 

 three pairs of jointed abdominal appendages (spin- j 

 nerets) often present. No metamorphosis, . . j 



Order III. — Body cylindrical, worm-like. Segments "j 



not grouped into regions. Head free ; eyes sim- | Myrtopoda 

 pie; auteuna3 present; wingless; numerous ab- V cceutiDedes') 

 dominal legs present; yelk-sac present for a | '^ ^ '" 



short period after hatching. No metamorphosis. J 



The Order of Six-footed Insects J {Hexapoda). 



Iletahola. The body usually cylindrical; prothorax ") 



small ; mouth-parts more generally haustellate j Hymenoptera. 

 (^ formed for sucking) ; metamorphosis complete ; V Lepidoptera. 

 pupa inactive ; larva usually cylindrical, very [ Dipteka. 

 unlike the adult, J 



Heterometabola. The body usually flattened; pro- S Colfoptera. 

 thorax large and squarish; mouth-parts usually j TJi.-,jipr.oi.. ' 

 adapted for biting; metamorphosis in a large r" OiJtuoptfr'a 

 number incomplete; pupa often inactive; larva Nfttroptera' 

 flattened, often resembling the adult, . .J * 



*The mimbei' of M'ingless forms is coniparativcl}'' few. The Diptera have but 

 one pair. 



tThe so-called " cei)halothorax" of Spiders is not like that region in the Crabs, 

 the head being much freer from the thorax. 



JLeuckart's classification is an advance on others in his considering the Hexa- 

 poda, Arachnida, and Myriapoda as orders instead of classes, but he says nothing 



Arachnida 



(Spiders). 



