Insects and their world 



Onychophora (Peripatus) are strange, wormlike creatures, which 

 are intermediate between Arthropods and worms. 



Origin and Evolution of the Insects 



By analogy with other groups of animals we should expect insects to 

 have originated from a 'generalised', or 'primitive' ancestor, and to 

 have evolved in the course of time into 'specialised' or 'advanced' types. 

 A generahsed form of a segmented animal would be one in which the 

 body consisted of a large number of segments, nearly all of which were 

 alike, and bore similar appendages. All existing insects, with the seg- 

 ments grouped into head, thorax and abdomen, are more highly evolved 

 than this. 



A number of different theories of insect evolution have been sug- 

 gested, some looking for their origin among the extinct Trilobites, and 

 some among primitive Crustacea. The theory most widely accepted at 

 present is that the ancestor of the insects belonged to the Myriapoda, 

 and bore some resemblance to a centipede or a millipede. It was more 

 primitive than either of these, and most probably resembled a section 

 of the Myriapoda called the Symphyla, which have a single pair of legs 

 on nearly every segment. 



There are several groups of insects today that are only a small step 

 removed from this ancestral type. These are called the Sub-class 

 Apterygota, or truly wingless insects, of which the most familiar ex- 

 amples are the silverfish and firebrat. Both are common domestic forms 

 which frequent the habitations of man. If you examine one of these under 

 a microscope, you will find that it has long antennae, three large seg- 

 ments in the thorax, each with a pair of segmented legs, and an abdomen 

 of eleven segments, the first nine of which each have a pair of small leg- 

 Hke appendages on the underside, or venter. These insects show no 

 trace of wings, and are said to be 'primitively wingless': it is beheved 

 that none of their ancestors had wings either. 



The great majority of hving insects belong to the Sub-class 

 Pterygota, or winged insects. Some insects that have no wings are 

 nevertheless placed in the Pterygota, because we beUeve that they are 

 descended from winged ancestors, and that they have lost the power of 

 flight, and ultimately even the wings themselves, by a long process of 

 degeneration. The reasons for this beHef are firstly that their nearest 

 relatives are a fully- winged group ; and secondly that either in the adult 

 insea, or at some stage in its development, traces can be seen of struc- 

 tures associated with the wings and with flight. The fleas, for example, 



4 



