THE ORGANISM AND ITS ENVIRONMENT 355 



the mouth parts may be accompanied by the expulsion of the con- 

 tents of mouth glands, as in the case of the bite of the mosquito. 



In addition to the familiar stings and bites insects have other ad- 

 ditional weapons of active defense. In the more southerly sections 

 of the United States one may occasionally pick up fairly large dark 

 beetles, with yellow ventral surfaces. They are to be handled with 

 caution for this particular type has in the rear end of the body two 

 small glands that squirt a vile-smelling liquid when the animal is 

 disturbed. A related form has a refinement of this type of weapon. 

 When the discharged liquid comes in contact with air it explodes 

 with a small report and becomes a smoke-like vapor, behind which 

 the beetle escapes. Beetles that have this sort of defense mechanism 

 are known as bombardier beetles. It is said that there are twenty- 

 five diflferent species of bombardiers in the United States. 



The cephalopod molluscs (p. 112) also employ screens as a means 

 of confusing their enemies. The squid has a small gland with a sac 

 or reservoir attached to the intestine near the anus. When disturbed 

 the inky contents of this reservoir are expelled into the surrounding 

 water, eflfectively masking the retreat of the animal. 



The poisonous snakes have fangs that are pierced by canals much 

 in the manner of a hypodermic needle. In the tissue at the bases of 

 the fangs are poison glands and sacs containing the poison. When 

 the snake has its mouth closed these fangs are folded backward into 

 the roof of the mouth; when it strikes the act of opening the jaws 

 pushes the fangs upright and pressure is brought to bear upon the 

 poison sacs, which eject their contents along the canal in the fang 

 into the body of the victim. 



It would be possible to devote a great deal of space to descrip- 

 tions of defense mechanisms, but sufficient has been said to show 

 the general nature of passive and active defenses among animals. A 

 moment's consideration of the defensive equipment of the human 

 body will bring one to the conclusion that it is relatively poorly 

 fitted with mechanisms for either passive or active defense. There 



