HOMOPTERA 119 



In the immature stages, these insects are scale-like in form and often 

 resemble somewhat certain species of the genus Lecanium of the family 

 Coccidae. Except during the first stadium, the larva? remain quiescent 

 upon the leaves of the infested plant and in most species are surrounded 

 or covered by a waxy excretion. In Figure 198 there 

 is represented one of the many forms of this excre- 

 tion. 



The members of this family feed exclusively on 

 the leaves of the host-plants. With few exceptions 

 they are not of economic importance ; and also with 

 few exceptions, the injurious species are not widely 

 distributed over the world as are many aphids 

 and coccids. They are most abundant in tropical 

 and semi-tropical regions. FlG - iq8 - — Ale y° des - 



The greenhouse white fly, Aster ochiton vapor ariorum, is an important 

 pest of the greenhouse. It infests very many species of plants that are 

 grown under glass; and sometimes it is a serious pest in the open on 

 tomato and other plants that are set out after the weather is warm. 



The citrus white fly, Dialeurodes citri, is a well-known pest in the 

 orange-growing sections of our country, and is also found in greenhouses 

 in the North. It infests all citrus fruits grown in this country and is 

 found on several other plants. 



Family Coccidae 

 The Scale-insects or Bark-lice, Mealybugs, and others 



The family Coccidae includes the scale-insects or bark-lice, the mealy- 

 bugs, and certain other insects for which there are no popular names. 

 To this family belong many of the most serious pests of horticulturists; 

 scarcely any kind of fruit is free from their attacks; and certain species 

 of scale-insects and of mealybugs are constant pests in greenhouses. 

 Most of the species live on the leaves and stems of plants; but some 

 species infest the roots of the host-plants. The great majority of the 

 species remain fixed upon their host during a part of their life-cycle, and 

 can thus be transported long distances while yet alive, on fruit or on nurs- 

 ery stock; this has resulted in many species becoming world-wide in dis- 

 tribution. 



Happily some scale insects are useful to man. The lac-insect, Tachar- 

 dia lacca, furnishes the stick-lac of commerce, from which shellac is 

 made. Formerly, a red dye was obtained from the dried bodies of the 

 cochineal insect, Coccus cacti, but recently this dye has been largely 

 supplanted by the coal-tar dyes. China-wax is an excretion of the pe-la 

 insect, Ericerus pe-la, and was formerly used in China in making candles. 



In the adult state, the two sexes of coccids differ greatly in form. The 

 males are usually winged (Fig. 199, ib); in a few species they are either 

 wingless or have vestigial wings. The fore wings are usually large, com- 

 pared with the size of the body; the hind wings are always greatly 

 reduced in size; usually they are a pair of club-shaped halteres, but in a 

 few forms they are more or less wing-like. Each hind wing is furnished 

 with a bristle, which is hooked at the end and fits into a pocket or fold 



