What insects are, and how they are classified 



all the insects that belong to one Order are built to the same general 

 plan, though of course with much variation in details. The bigger 

 Orders, like those mentioned above, are fairly easy to recognise, but if 

 we are to classify all the rare and obscure insects we have to have about 

 thirty Orders, and not all entomologists agree how many there should 

 be. The list that follows is given as a convenient way of dividing up the 

 insects for discussion, but slightly different Hsts of Orders will be found 

 in some textbooks. 



In the same way, the estimates of the number of known species of 

 each Order are given so that the reader can compare the sizes of the 

 Orders, and can see which of them are best represented in the North 

 American fauna. The figures are a long way from being final, since many 

 new species are being discovered all the time. 



Sub-class Apterygota 



1. Thysanura (North America 20; world 350) 



Silverfish; bristle-tails; firebrat. These are the glistening, grey, slender 

 insects, without wings, that are seen in the kitchen, and which scurry 

 away when the light is put on. Out of doors they hide under leaves, 

 rotting wood, stones, and so on, as well as amongst rocks on the seashore. 

 They feed on starchy materials. 



2. Diplura (North America 30; world 400) 



These are very similar to the Thysanura, and used to be included in 

 that Order. They are slender, wingless insects, with three pairs of seg- 

 mented legs on the thorax. Their chief interest is that the genus Campodea 

 is taken as a pattern for this type of active, wingless insect, and larvae of 

 other Orders that have this shape are called campodeiform larvae (Fig. 33a). 



3. Protura (North America 30; world 100) 



Very tiny insects, about 1/12 inch long, collected from rotting wood 

 or forest floor litter, but otherwise seldom seen. 



4. CoUembola (North America 325; world 2000) 



Spring-tails. Small, or very small, found in all sorts of moist places. In 

 soil they can be seen in very large numbers as tiny, pale creatures, capable 

 of jumping a long way. Some authorities consider that the CoUembola 

 are not true insects. 



Sub-class Pterygota 



Division: Exopterygota 

 Orders 5, 6 and 7 have in the past generally been merged together as 

 Orthoptera (North America iioo; world 22,000), but there is now a 

 growing practice of separating them. Order 8 forms a link between them, 

 and gives an indication of how they may have evolved from a common 

 ancestor. 



