THE CLASSIFICATION OF INSECTS 73 



gardeners and agriculturists as " blight " or " green -fly." 

 Many species carry a pair of abdominal tubes which 

 secrete a waxy substance that first appears as oil-like 

 globules. It was at one time supposed that the sweet 

 liquid, called " honey-dew," was produced by these tubes ; 

 but it is now known that this is derived from the alimen- 

 tary canal. Typical aphides have two pairs of delicate 

 wings, with very few nervures ; but in the case of most 

 species wingless individuals are numerous. In the allied 

 family Psyllidce, which comprises the so-called "jumpers," 

 or " springing plant-lice," the fore-wings are usually firmer 

 in texture than the hind-wings. A well-known species is 

 the apple sucker (Psylla mail) which often does much 

 mischief in orchards. 



The scale insects and "mealy bugs" (Coccidce) are 

 usually minute, but many of them are excessively in- 

 jurious to cultivated plants. They are characterised by a 

 great dissimilarity of the sexes, the adult males having 

 long antenna? and a pair of well-developed wings, while 

 the females are sluggish, grub-like creatures. Many 

 species secrete shell -like scales ; others, such as the felted 

 beech coccus (Cryptococcns fagi), a waxy substance 

 beneath which the insects live gregariously. Perhaps the 

 best known member of this family is the mussel scale, or 

 bark louse (Mytilaspis pomorum), which is found in all 

 parts of the world where the apple is cultivated. The 

 brown scale (Lecanium) and the cottony cushion scale 

 {Pluvhiaria) are also well known, both species being found 

 on the twigs of currant bushes. Mealy bugs constitute 

 the genus Dactylopius, while Coccus cacti, a native of 

 Mexico and Central America, is the cochineal insect of 

 commerce. 



