HOMOPTERA 



413 



in length; in some species it is longer than the body. The antennae 

 consist of from three to six segments; the last segment is usually 

 provided with a narrowed prolongation (Fig. 488, aa). The first two 

 segments of the antennae are always short, but the other segments 

 show a great specific variation in length and are therefore very 

 useful as systematic characters. Excepting the first two, the seg- 

 ments of the antennas are usually provided with sense-organs, the 

 sensoria, which vary in niunber and shape in different species and are 



||* i^'''"' ■■'•''■< '/'I'lii'i 



^"^^^^^GgaarTTTy^-^ 



CUZ'. 



Fig. 488. — The melon aphis, Aphis gossypi: a, winged agamic female; aa, en- 

 larged antenna of same; ah, winged agamic female, with wings closed, sucking 

 juice from leaf; b, young nymph; c, last nymphal instar of winged form; d, 

 wingless agamic female. (Prom Chittenden.) 



much used in the classification of these insects. On the back of the 

 sixth abdominal segment there is, in many species, a pair of tubes, 

 the cornicles, through which a wax -like materialis excreted. In some 

 genera these organs are merely perforated tubercles, while in still 

 other genera they are wanting. It was formerly believed that the 

 honey -dew excreted by aphids came from the cornicles; for this 

 reason they are termed the honey-tubes in many of the older books. 

 The honey-dew of aphids is excreted from the posterior end of the 

 alimentary canal. It is sometimes produced in such quantities that 

 it forms a glistening coating on the leaves of the branches below the 

 aphids, and stone walks beneath shade-trees are often densely spotted 



