HEMIPTERA. 157 



which are of small size. In our largest species the body 

 measures only about one-fourth inch in length, and usually 

 these insects are very much smaller. 



The body is usually more or less pear-shaped. The 

 winged forms have two pairs of delicate, transparent wings. 

 These are furnished with a few simple veins, but the vena- 

 tion is more extended than in either of the two following 

 families. The first pair of wings is larger than the other, 

 and the two wings of each side are usually connected by a 

 compound hooklet. The beak is three-jointed, and varies 

 greatly in length ; sometimes it is longer than the body. 

 The compound eyes are prominent, and ocelli are also 

 usually present. The antennae are from three to seven 

 jointed. On the back of the sixth abdominal segment there 

 is, in many species, a pair of tubes, through which a wax- 

 like material is excreted. In some genera these organs are 

 merely perforated tubercles, while in still other genera they 



are wanting. 



The substance known as honey-dew is excreted from 

 the posterior end of the alimentary canal. It is sometimes 

 produced in such quantities that it forms a glistening coat- 

 ing on the leaves of the branches below the plant-lice, 

 and stone walks beneath shade-trees are often densely 

 spotted with it. This honey-dew is fed upon by bees, wasps, 

 and ants. The bees and wasps take the food where they 

 find it, paying little if any attention to its source ; but the 

 ants recognize in the plant-lice useful auxiliaries, and often 

 care for them as men care for their herds. This curious re- 

 lationship will be discussed farther under the head of Ants. 



It is easy to see what benefit ants derive from this asso- 

 ciation with plant-lice, and how they should learn that it is 

 worth while for them to care for their herds of honey pro- 

 ducing cattle. Little has been done, however, to point out 

 the great benefit that accrues to the plant-lice from this re- 

 lationship. It seems fair to assume that the plant-lice are 

 greatly benefited, else why has the highly specialized appa- 



