308 TEXTBOOK OF ZOOLOGY 



Body Wall 



Another distinctive feature found in the arthropods is the chitin- 

 ous body wall, which provides the only rigid support for the body. 

 The exoskeleton consists of three layers known as the cuticula or 

 outer layer which is impregnated, more or less, with calcareous mat- 

 ter, the hypodermis or intermediate layer, and the 'basement mem- 

 hrane. The hypodermis has its origin in the ectoderm and is the 

 active growing layer of the body wall. Chitin is a substance found in 

 many parts of the insect body, but it especially serves to give firm- 

 ness to the cuticula. Chitin is not destroyed by caustic potash. It 

 is a most interesting organic substance, resembling horn in some 

 physical ways. 



All the tubercles, spines, setae, and scales of the body wall are 

 formed by the cuticula. These structures are of importance in the 

 identification of insects. 



Metamorphosis 



Metamorphosis includes the alterations which an insect under- 

 goes after hatching from the egg, and which alters, extensively, the 

 general form and life of the individual. All the changes which are 

 undergone by a butterfly in passing from egg to adult — each change 

 from egg to larva, from larva to pupa, and from pupa to adult — con- 

 stitute metamorphosis. 



There are four types of development or metamorphosis : first, 

 ametabolous or development without metamorphosis; second, pauro- 

 metaholotis or gradual metamorphosis ; third, hemimetaholous or incom- 

 plete, and fourth, holometaholous or complete metamorphosis. The 

 ametabolous insects are the Thj-sanura, Collembola, Mallophaga, and 

 Pediculidae, which after hatching, grow through a number of instars, 

 remaining practically the same form as the adult insect during all the 

 development. This is development without metamorphosis. In the 

 following orders: Orthoptera, Hemiptera, Homoptera, Isoptera, 

 Thysanoptera, and Dermaptera, there is a type of development in 

 which the nymphs gradually increase in size and the rudimentary 

 wings and genital appendages become adult structures. This is 

 known as paurometabolous development. 



In the Odonata, Ephemerida, and Plecoptera, the newly hatched 

 naiads pass through an incomplete metamorphosis. All of the 



