INTRODUCTION. 13 



base of the fresb leaves of the new shoots. The puuctiires made iu the 

 willow by saw-flies of the geuus Euura result in the formation of galls 

 or tumors within which the larvse live. 



The larv» strongly resemble caterpillars, hence they are sometimes 

 called " false caterpillars;" but they have from six to eight pairs of ab- 

 dominal legs, whereas caterpillars have only five pairs. Many kinds 

 (Nematus, etc.) curl the hind body spirally when feeding or at rest. 

 They are usually green, of the color of the leaves upou which they feed, 

 with lines and markings of various colors. They usually molt four 

 times, the last change being the most marked. Most of the larvae se- 

 crete silk and spin a tough oval, cylindrical cocoon, in which they 

 hybernate iu the larva and often in the pupa state. 



Ants and bees. — Ants have not been noticed in the United States to 

 injure trees, but iu the tropics species of CEcodoma, or leaf-bearing ants, 

 are very destructive to trees; it is possible that there are species in the 

 Gulf States which may in part defoliate trees. 



Bees are of great use in setting the fruit of trees ; little has been ob- 

 served on this point in this country, but without doubt the visits of in- 

 numerable bees to linden trees are of service in " setting " the seed of 

 that tree. 



Mr. Lugger* mentions the fact that the seeds of the rock maple, so 

 numerous in the grounds of the Smithsonian Institution, Washington, 

 D. C, were in 1886 uniformly sterile. He attributed this phenomenon 

 to the inclement weather prevailing during the flowering season, which 

 prevented bees from visiting the flowers. 



Plant lice.— Wh'ila many Hemiptera, such as the bugs, destroy many 

 caterpillars, particularly span-worms and leaf-rollers, some of the most 

 annoying and destructive of our forest insects belong to this order. 

 They all take their food by piercing the succulent leaves and stems, or 

 twigs of trees, shrubs, or herbs, often causing them, as in the elm aphis, 

 to crumple up. The species of Psyllidce are very common on the leaves 

 of hard-wood trees, either hopping over the surface or living iu leaf- 

 galls which are the results of their punctures. 



The following account of Aphides or plant-lice is adapted from the 

 writer's "Guide to the Study of Insects:" 



The plant-lice have greenish, flask-shaped bodies, covered with a soft, powdery, 

 bloom ; their antenme are five to seven-jointed, with a three-jointed beak, and legs 

 with two-joiuted tarsi. The males and females are winged, and also the last brood 

 of asexual individuals, while the early summer brood are wingless. The abdomen is 

 thick and rounded, and in Aphis and Lachnus provided with two "honey tubes" for 

 the passage of a sweet fluid secreted from the stomach. 



In the early autumn the colonies of plant-lice are composed of both male and female 

 individuals; these pair, the males then die, and the females begin to deposit their 

 eggs, after which they also die. Early in the spring, as soon as the leaves begin to 

 unfold, the eggs are hatched, and the young lice begin to suck the sap, and soon be- 

 gin to bring forth young, which develop by a budding process within the body of the 



* Entomologica Americana, ii, 89. 



