DISTRIBUTION OF INSECTS. 279 



ment, but there can be no question that the first appearance of 

 these animals long anteceded the Cambrian period. The total 

 number of eurypteroid forms occurring in the American deposits 

 is twenty-three (representing the genera Eurypterus, Dolichopterus, 

 Pterygotus, and Stylonurus), six of which (Eurypteri) are Car- 

 boniferous, two Devonian, and the remainder Upper Silurian. 101 

 The oldest known limuloid form is the Neolimulus falcatus, from 

 the Upper Silurian rocks of Lanarkshire, whose early appearance 

 would seem to indicate that while the family to which it belongs 

 (BellinuridEe) may stand in its relations intermediately between the 

 eurypterids and the king-crabs, its actual origination may be 

 traceable, at least in part, to direct modification from the trilo- 

 bitic type. 



INSECTS. 



The number of recognised species of insects is generally con- 

 ceded to be upwards of 100,000, and by some authors is placed as 

 high as 150,000, but it is very questionable whether these represent 

 more than one-tenth of the number actually inhabiting the earth's 

 surface. Probably not less than one-half of the indicated forms 

 belong to the order Coleoptera, or beetles, which is by far the most 

 numerously represented of all the orders. The Lepidoptera, or but- 

 terflies, have thus far yielded some 15,000 species or about one- 

 thirteenth of the total number (200,000) estimated by Speyer for 

 the world at large and an equal number may, perhaps, with a cer- 

 tain amount of accuracy, be credited to the Hymenoptera (bees, 

 wasps, and ants), the Hemiptera (bugs), and Diptera (flies). The 

 Orthoptera, or straight-winged insects, which include the locusts, 

 grasshoppers, &c., are considerably less numerous, while the spe- 

 cies of netted-veined forms (Neuroptera) probably do not much ex- 

 ceed 2,000, or perhaps do not even reach this figure. 



Our knowledge of the general insect fauna of the globe is still 

 too limited to allow of any satisfactory conclusion being drawn as 

 to the geographical distribution of the class as a whole ; enough 

 is known, however, to permit it being stated that practically every 

 portion of the earth's surface harbours a more or less extensive 

 insect fauna, so that the distribution of this class of animals may 

 be said to be universal. While most numerously developed in 

 the warmer or tropical areas, insects are by no means rare in the 

 region of high latitudes, and, indeed, in some of the most north- 



