200 AN INTRODUCTION TO ZOOLOGY. 



wings become draggled. Some flying insects have 

 dilated vesicles on the tracheae, which make the 

 insect lighter, since they are filled with air, and thus 

 help it to sustain itself in the air. Some of the insect 



Fig. 56. — TrachejE of an insect, magnified. (Fxom Glaus and Sedgwick, after 

 Leyuig.) 



larvae that live in water have leaf-like appendages, 

 called tracheal gills, for respiring water (larvae of May- 

 flies and Spring-flies) ; others have respiratory plates 

 in the rectum (Dragon-fly). 



The Thysanura have been divided into the Collem- 

 hola, or Spring- tails, the CampodidcR, and the Lepis- 

 midce, or Bristle-tails. They infest greenhouse frames, 

 and the former have their tails tucked underneath 

 when at rest. Lepisma, so called from its minute 

 scales, which give it a silvery-bronze appearance, does 

 not jump, but Machilis does, a larger kind which lives 

 near the sea-shore. The latter, in my experience, has 

 a predilection for old boots which have been soaked 



