THE STRUCTURE AND LIFE OF INSECTS 247 



the pressure to the walls of the air-tubes, and drives the 

 air into the ultimate recesses. 



Examination of the tissues of an insect's body. shows 

 that they are traversed and overlaid by air-tubes, which 

 branch continually, until they become extremely fine. 

 A thread, wound spirally round every tube, acts like the iron 

 wire often used to line a flexible gas-pipe. In both cases 

 the spiral thread prevents the tube from " kinking " when 

 sharply bent. 



INSECT-TRANSFORMATIONS. 



One of the best-known and most interesting peculiarities 

 of insects is the transformation which so many of them 

 undergo. Most of them pass the chief part of their lives 

 as larvae or grubs, and do all their feeding and growing 

 in this stage. Then they turn to flies and lay their eggs. 

 Many, but not all, pass through a resting-stage just before 

 they acquire wings. Two questions call for consideration 

 at this point. Why should so many insects get wings 

 before they lay their eggs ? Why does a resting-stage 

 so often precede the winged stage ? 



Wings are necessary, I believe, to all insects which are 

 very particular about the place where they lay their eggs. 

 Suppose that a particular caterpillar will feed only on 

 the leaves of buckthorn. If the female moth lays all 

 her eggs on the tree where she herself was reared, that 

 tree will soon be overstocked, while there may be plenty 

 of other trees of the same species which are untouched. 

 It would evidently be far safer, if many generations are 

 to be reared in succession, that the eggs should be laid a 

 few together on a number of trees. Now a creeping insect 

 could not manage this ; it would exhaust itself to no pur- 

 pose in seeking fresh plants. But if the egg-laying moth 

 can fly, and if it is furnished with acute senses, it can make 

 its way to plant after plant, and distribute the eggs widely. 



It will be a work of time to lay eggs in a number of 

 different places, and the moth which undertakes the task 



