192 INJURIOUS AND USEFUL INSECTS 



Here come the various families of tree-bugs, water-bugs 

 (pond-skaters, etc.), and bed-bugs. 



Sub-order II. — Homoptera (with similar wings). 



There are usually two pairs of membranous wings, one or both 

 being sometimes wanting ; when at rest they are usually erected 

 over the back. 



To this sub-order belong the cicadas, lantern-flies, cuckoo- 

 spits, aphids (see p. 153), and coccids or scale-insects (see 

 p. 162). 



The genus Aphis (see Lesson 31) is characterised by the 

 long, apparently seven-jointed antennae (often longer than 

 the body), and in most species by the two abdominal tubes. 

 The rose aphis (A. rosce), the corn aphis {A. cerealis), the hop 

 aphis {A. humuli), the plum aphis {A. pruni), the apple aphis 

 {A. mall), the cabbage aphis {A. brassica), and the bean aphis 

 (A.fabcB), are all common and mischievous. 



In the rest of the family Aphidas the antennae are shorter, 

 and only five to six-jointed ; the abdominal tubes are wanting, 

 or reduced to warts, and a cottony exudation from the body 

 is general. American blight (Schizoneura lanigera), the 

 Lachnus of pines and other trees, and the Pemphigus of 

 poplars are examples. In Chermes the legs are short, and 

 the antenna have often only three joints. In Phylloxera the 

 legs are short, the antennae three-jointed, and the wings, unlike 

 those of all other aphids, lie flat over the back. Chermes and 

 Phylloxera, like some other aphids, make galls. Chermes 

 forms a cottony exudation, which is not found on Phylloxera. 

 These two genera are often placed in a separate family, 

 Phylloxeridae. 



Aphids are known sometimes to migrate from one food- 

 plant to another, and to set up a fresh cycle of generations 

 at each migration. There may be no botanical affinity between 

 the two food-plants ; thus the aphis of the apple may migrate 

 to corn or other grasses, and back again to the apple. 

 The migrating forms are winged; the true sexual forms, if 

 female, appear to be always wingless, but the males are 

 sometimes winged, and sometimes wingless. 



In Chermes several cases of a complicated migration have 

 been investigated. This is a gall-making aphid, and its galls, 

 which have a remarkable resemblance to natural cones, are 



