XIV INSECTA 211 



sometimes accompanied by the alternate closing and opening 

 of the spiracles; but not much is actually known, from 

 experiment, of the interchange of gases that occurs. 



In larval insects, the spiracles are often closed, and the 

 tracheae may be continued into external, thin-walled, tubular 

 processes known as tracheal gills (p. 318). 



The wings, of which there are usually two pairs 

 ^sfcts* in the adult, consist of two thin, membranous 



plates, which adhere together and are united all 

 round the free margin. Each wing has, running through it, 

 harder horny lines known as the "veins," "nerves," or 

 " nervures " of the wing. The arrangement of these varies 

 in the different orders of insects, but is fairly constant within 

 each order, and therefore this character is made use of in 

 classifying them. The horny nervures ar© hollow tubes, 

 inside which blood-cells and tracheae have been demonstrated. 

 The nervures always enter the base of the wing as two or 

 three large stems, which branch and branch agajn, until they 

 may form as intricate a pattern as that seen in the Dragon-fly. 

 The variations in structure and development of the wings 

 will be studied in further detail when we consider the different 

 orders of insects. 



The metamorphosis during the development of 

 rahosis ™°®*' insects is very striking. From the egg there 



hatches a little larva, which is, in many cases, very 

 unlike the adult. This is specially so where the food and 

 habitat differ in the different stages of development. In 

 such cases the larval form, which is always wingless and 

 may also be legless as in the grubs of bees, or maggots of flies, is 

 retained until growth is complete, the growth being accom- 

 panied by a series of skin moults. Then the full-grown 

 larva may enter upon a resting, quiescent stage known as the 

 pupal stage, or as the chrysalis in the case of a butterfly. In this 

 stage the creature is usually motionless and ceases to feed, 

 whilst certain important internal changes take place, leading 

 to the perfecting of the special adult organs. When this is 

 complete, the skin is cast for the last time, and the winged, 

 adult creature,- or imago, emerges. Such a series of changes, 

 including a quiescent pupal stage, is known as a "complete 

 metamorphosis." In some insects there is no pupal stage, 

 the change from larva to imago taking place gradually through- 



