278 PRACTICAL LESSONS IN SCIENCE. 



insects are more powerful, all things considered, than &ny other 

 form of life. 



Insects are supplied with glands somewhat analogous to the 

 cutaneous glands of higher animals, which secrete substances 

 often of the most intense and penetrating odor, which seeroe to 

 be used as a means of protection. Then there are glands that 

 secrete wax, and those that secrete a sebaceous liquid that serves 

 to lubricate the joints; and in some larvae there are spinning 

 glands which serve for the production of webs, pupa cases, etc. 

 And many insects have poison glands in connection with stinging 

 organs. 



The sounds made by insects are produced by the friction of one 

 part of the external skeleton on another, as the legs against 

 wings, or wings against wings, etc., by the passage of air through 

 the thoracic stigmata, by the rapid movement of the wings, and 

 doubtless in other ways not understood. Insects emit odors and 

 doubtless have a sense of smell, but observers are by no means 

 agreed as to its nature or location. So it seems as if there must 

 be a sense of hearing. Among the grasshoppers and crickets 

 there seems to be an auditory apparatus in connection with the 

 anterior legs, but in reality little is known about the sense or- 

 gans among insects beyond what is known of the organs of sight. 

 Among insects reproduction is sexual. In some cases the egg 

 develops into a perfect insect without metamorphosis, but in 

 most cases the embryo passes through a well-marked metamor- 

 phosis, often requiring considerable time before the perfect stage 

 is reached. 



After the egg is fertilized it gradually develops into a worm-like 

 form called the larva, known commonly as caterpillar, grub or 

 maggot. As soon as hatched the larva eats voraciously, grow 

 ing rapidly, moulting from time to time, storing up fat from which 

 to form the tissues of its future body. At length it stops eating, 

 makes a cocoon and, slowly changing form, loses its worm-like 

 body and develops the head, thorax and abdomen of the perfect 

 insect. Sometimes this resting or pupa stage is very brief ; some- 

 times it lasts for a considerable time. 



