76 AKTICULATA. 



CLASS INSECT A (HEX APOD A). 



The name Insecta is used by some authors to include the 

 Myriapods and Arachnids, the class as here restricted being 

 termed Hexapoda. 



True insects are distinguished as follows : The body always 

 has three distinct divisions, head, thorax, abdomen ; the thorax 

 bears all the locomotive oi'gans, which are never more than three 

 pairs of legs and two pairs of wings. Respiration is by means 

 of air-tubes (tracheae). 



The number of known species of insects is about 190,000, in- 

 cluding 90,000 species of Coleoptera, and about 25,000 each of 

 Hymenoptera and Lepidoptera, and 24,000 of Diptera. 



The oldest insect fossils are from the Devonian of New 

 Brunswick. These have neuropterous wings with orthopterous 

 stridulating organs. Nearly all orders are known from the Car- 

 boniferous, nineteen species being found in the American Coal- 

 Measures. Over 140 species are known from the European Juras- 

 sic, chiefly Beetles. The earliest Butterfly is from the Stonesfleld 

 slates. In Eocene fresh-nv^ater shales at Florissant, Colorado, and 

 of Green River, Wyoming, have been found the richest insect- 

 beds. The shales are black with drifted remains, and 1,000 

 species are recognizable (Scudder). Similar deposits are known 

 in European Miocene, which have yielded 1550 species, over 

 900 of them from the bed at CEningen. Hymenopters and 

 Coleopters are most abundant, and the species indicate a tropical 

 climate. Extinct species are common in amber, but the species 

 in copal and the later resins are living forms. 



