HEXAPODA OR INSECTS PROPER. 287 



Insgcts constitute by far the largest class in the Animal 

 Kingdom. About 200,000 species of insects are already 

 known, and the whole number may be safely estimated 

 as high as 300,000 species. The species are mostly small 

 many are microscopic in size ; but some kinds in the 

 warm regions are several inches, even a foot, in length. 

 The average length, however, is probably much less than 

 one inch. This vast class comprises three Orders : 



1. HEXAPODA or Six-footed Insects or Insects proper ; with body 



exhibiting three distinct regions ; generally two pairs of wings 

 in some kinds the wings are wholly wanting, and the Flies 

 and their allies have only one pair of wings three pairs 

 of thoracic legs ; both compound and simple eyes ; and in 

 passing from the egg-state to maturity they undergo a more 

 or less complete metamorphosis. 



2. ARACHN1DA or Spiders, with the body exhibiting only two dis- 



tinct regions, a cephalo-thorax and an abdomen ; no wings ; 

 four pairs of thoracic legs; in many cases, three pairs of 

 jointed abdominal appendages; simple eyes; no antennae^ 

 and in passing from the egg state to maturity they undergo 

 no metamorphosis. 



3. MYRIAPODA, as " Galley-worms, " Centipedes, etc.; with the 



body worm-like, exhibiting no grouping of the segments into 

 regions in the adult state ; no wings ; " feet " numerous ; 

 head free ; antennae present ; eyes simple ; no metamorpho- 

 sis, except that the yolk-sac is present a short time after 

 hatching. 



SUB-SECTION II. 

 THE ORDER OP HEXAPODA OR INSECTS PROPER. 



THE Insects proper, or the best representatives of the 



great group of the Insecta in general, are insects which 



have their body divided into three plainly marked regions 



the head, thorax, and hind body or abdomen.' The 



