INSECTS AND DISEASE 



411 



laid at a time, and half a dozen such masses within the life 

 of one fly. The eggs hatch in eight to twenty-four hours, 

 and the whole development may be completed in a 

 fortnight. This time may be shortened or lengthened 

 considerably according to the climatic conditions of 

 temperature, etc. 



The wingless fleas and lice, and some of the bugs 

 (Hemiptera) are also concerned in the spread of disease, 

 in most cases mechanically. Germs voided in the insect's 

 excrement may be rubbed in by scratching, or the insect 

 itself may be crushed. Thus Xenopsylla cheopis, the 

 rat-flea of warmer countries, transmits bubonic plague. 

 The chigoe or " jigger " {Dermatophilus penetrans) is a 



Fig. 228. — Mosquito.— After Nuttall and Shipley. 



curiously modified tropical flea whose fertilised females 

 burrow into the skin of the feet of man, dogs, and other 

 animals. There the abdomen of the insect swells to the 

 size of a pea, the eggs being eventually passed out to the 

 ^exterior. " European " relapsing Tever, typhus fever, and 

 trench fever are all transmitted by the body-louse, Pediculus 

 humanus. One of the South American " assassin " bugs 

 — Triatoma — carries Trypanosoma cruzi — ^discovered in 

 1907 by Chagas, and causing Chagas' disease — a wasting 

 disease of mind and body. There is some evidence to 

 show that a reservoir of this trypanosome comparable to 

 the game reservoirs of T. hrucei in Africa exists in various 

 species of armadillo. 



