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 Lepidop- 

 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 Mjriopods. 



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. 



Sub-class I. Segments grouped into three distinct re- ^ 



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

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

 jointed abdominal appendages. A more or less | sects). 

 complete metamorphosis, . . . . .J 



4S;ife-cfa«.v //.Segments grouped into two regions, &\ 



false cephalothoraxt and an abdomen; no antennae; I * t, . <-TIXTTT^ . 

 eyes simple ; wuigless ; lour pau-.s ot thoracic legs ; )■ ('Sniders") 

 three pairs of jointed abdominal appendages (spin- I ^ ^ -'■ 



nerets) often present. No metamorphosis, . . J 



Sub-dass rrr.BoLiy cylindrical, worm-like. Segments - 

 not grouped into regions. Head free; eyes sim- 

 ple; antennae present; wingless; numerous ab- 

 dominal legs present; yelk-sac present for a 

 short period after hatching. No metamorphosis. 



Myriopoda 

 (Centipedes). 



The Orders of Six-footed Insects J (Hexapoda). 



Metabola. The body usually cylindrical; p roth o rax ^ 



small; mouth-parts more generally haustellate j Hymenoptera 



(formed for sucking) ; metamorphosis complete ; )■ Lepidoptera. 



pupa inactive ; larva usually cylindrical, very j Diptera. 



unlike the adult, j 



Eeterometabola. The body usually flattened ; pro- "] Coleoptera. 



thorax large and squarish; mouth-parts usually Hemiptera. 



adapted for biting; metamorphosis in a large }- Orthoptera. 



number incomplete; pupa often inactive; larva j Neuroptera. 



flattened, often resembling the adult, . . J Thysanura. 



* The number of wingless forms is comparatively few. The Diptera have but 

 one pair. 



fTlie so-called " cephalothorax " of Spiders is not like that region in the Crabs, 

 the head being much Ireer from the thorax. 



X Leuckart'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 



