INSECTA 275 



the abdomen. This may be used for depositing eggs or for defense. 

 The brain is highly developed. Some larvae feed on leaves, others are 

 parasitic in the tissues of other insects or of plants while others are 

 fed by the adults with either animal or vegetable food. The larvae 

 usually spin a cocoon in which the pupa stage is passed. Some of the 

 most important forms are: The gall wasps which deposit the eggs in 

 the tissues of plants whereby a gall develops and forms a shelter and 

 source of food for the larvae; the ichneumon flies which sting the 

 larvae of other insects and deposit their eggs there, the larvae then 

 developing as internal parasites; the ants with their complex social organi- 

 zation, polymorphism, consisting of three or four types of individuals, 

 and division of labor, keeping of slaves, cultivation of plants and fostering 

 of aphids for economic purposes; the common solitary wasps and social 

 wasps with the more or less artfully constructed nests of mud or "paper"; 

 the social and polymorphic honey bee and the bumble bees with their 

 combs and honey. This interesting order merits a special treatise. 



591. Order 17. — The Rhynchofa or bugs. These insects are provided 

 with a protruding snout and piercing mouth parts. Metamorphosis 

 occurs in some cases, in variable degree. The wings are sometimes 

 wanting but there are usually two pairs. The anterior pair may be 

 partly horny. They are all ectoparasitic on other animals or plants. 

 Included in this group are the bed bugs, the plant bugs, such as the 

 squash bug, chinch bug and cicada, the water bugs, water-boatmen, 

 water-striders and electric-light bug, and the ''plant lice" and scale 

 insects. 



592. Phylum VIII. Mollusca. — The Molluscs are the highest 

 group of unsegmented animals. The group is pretty well 

 defined but there is a great difference in the scale of organization 

 between the lowest and highest orders. The most marked 

 anatomical character of the phylum is the mantle, which is a 

 single or paired fold of the integument of the dorsal side of the 

 body. This mantle is usually of sufficient extent to entirely 

 enclose the body of the animal, and on its external surface it 

 secretes a hard shell composed of horny and calcareous matter 

 deposited in layers. There are no paired appendages. A 

 ventral muscular portion of the body is called the foot and 

 sometimes serves for locomotion. The nervous system con- 



