INSECTS 289 



clothing, and have been proved to transmit typhus and 

 other fevers. They may be destroyed by applying 

 paraffin or turpentine to the body and clothing, or by 

 scalding the latter. Phthirius inguinalis, the Crab Louse 

 (Fig. 197), frequents the hairs about the pubic region of 

 man, and is conveyed by personal contact. 



The Plantlice, of which the well-known Greenfly {Aphis) 

 is an example, are not only of much practical importance 

 as pests in garden's, farms, and orchards, but also of 

 considerable zoological interest in more ways than one. 

 They are animals parasitic upon plants ; they are generally 

 wingless but spread and often fertilised by means of winged 



FIG. 196. — The Turnip-leaf Plantlouse (Aphis rapm). — 

 After Curtis. 



2 and 4 winged and wingless parthenogenetic females ; 

 1 and 3 natural size of the same. 



individuals, they are in many of their generations viviparous 

 (p. 448), and in most they are parthenogenetic (p. 190). 

 Throughout the summer the females reproduce in a manner 

 which is both viviparous and parthenogenetic, and, as the 

 young are all females and all capable of producing offspring 

 in a few days, the plantlice multiply very rapidly. Sooner 

 or later, generally towards the autumn, there appears a 

 generation which contains males as well as females. These 

 pair, and the female lays a fertilised egg which survives the 

 winter and gives rise in the spring to a parthenogenetic 

 female again. Most of the generations are wingless, but 

 from time to time there appears a generation of winged 

 females by which the animal is spread ; usually, at least, 

 this happens when the plant is ceasing to afford a good 

 supply of food. The males are generally winged. Food is 



